Revised systematics of Palaeozoic ‘horseshoe crabs’ and the myth of monophyletic Xiphosura

Article (PDF Available)inZoological Journal of the Linnean Society 167(1):1-27 · January 2013with 821 Reads
DOI: 10.1111/j.1096-3642.2012.00874.x
The monophyly of the class Xiphosura is critically re-examined. For the first time a phylogenetic analysis of a number of synziphosurine and xiphosurid taxa is performed together with representatives of the other chelicerate orders also included as ingroup taxa. Xiphosura as currently defined is shown to be paraphyletic, and a revised classification is presented. Previous characteristics used to unite the xiphosurids (possessing a fused thoracetron) and a paraphyletic grade of synziphosurines (retaining freely articulating opisthosomal tergites) include the presence of a cardiac lobe, ophthalmic ridges, an axial region of the opisthosoma, and a reduced first opisthosomal segment. All of these characteristics are, however, here shown to be present in other chelicerate groups, leaving Xiphosura without any defining synapomorphies. A number of other characters, including the form of the chelicerae and appendage VII, indicate that xiphosurans may be paraphyletic with respect to a clade consisting of chasmataspidids, eurypterids, and arachnids. What ramifications this has for the evolution of basal chelicerates is briefly discussed, and it is recognized that most of the currently known ‘synziphosurine’ taxa represent offshoots from the main chelicerate lineage with ghost ranges extending into at least the Middle Ordovician.
Figures - uploaded by James Lamsdell
Author content
All content in this area was uploaded by James Lamsdell
Revised systematics of Palaeozoic ‘horseshoe crabs’ and
the myth of monophyletic Xiphosura
Department of Geology and Paleontological Institute, University of Kansas, 1475 Jayhawk
Boulevard, Lawrence, KS 66045, USA
Received 28 May 2012; revised 18 September 2012; accepted for publication 19 September 2012
The monophyly of the class Xiphosura is critically re-examined. For the first time a phylogenetic analysis of a
number of synziphosurine and xiphosurid taxa is performed together with representatives of the other chelicerate
orders also included as ingroup taxa. Xiphosura as currently defined is shown to be paraphyletic, and a revised
classification is presented. Previous characteristics used to unite the xiphosurids (possessing a fused thoracetron)
and a paraphyletic grade of synziphosurines (retaining freely articulating opisthosomal tergites) include the
presence of a cardiac lobe, ophthalmic ridges, an axial region of the opisthosoma, and a reduced first opisthosomal
segment. All of these characteristics are, however, here shown to be present in other chelicerate groups, leaving
Xiphosura without any defining synapomorphies. A number of other characters, including the form of the chelicerae
and appendage VII, indicate that xiphosurans may be paraphyletic with respect to a clade consisting of
chasmataspidids, eurypterids, and arachnids. What ramifications this has for the evolution of basal chelicerates is
briefly discussed, and it is recognized that most of the currently known ‘synziphosurine’ taxa represent offshoots
from the main chelicerate lineage with ghost ranges extending into at least the Middle Ordovician.
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27.
doi: 10.1111/j.1096-3642.2012.00874.x
ADDITIONAL KEYWORDS: Dekatriata – Euchelicerata – ghost ranges – paraphyly – phylogenetics –
Prosomapoda – synziphosurines – tagmosis – Xiphosura – Xiphosurida.
Few organisms alive today evoke an awareness of the
primeval as does the horseshoe crab. Often only seen
when shuffling awkwardly onto land to reproduce, yet
surprisingly graceful when observed within their usual
marine habitat, horseshoe crabs are striking in both
their apparent dissimilarity from any of the four
familiar extant arthropod groups and their sheer size.
Large size among arthropods is something often
evoked as a relic of the distant past, from an age when
giant arthropods ruled the Carboniferous, and with a
maximum length of just over half a metre horseshoe
crabs certainly appear to be remnants of grander
times. The four surviving species of the class
Xiphosura are actually descendants of more diminu-
tive ancestors however, with no currently known fossil
species approaching the size of Recent xiphosurans
until possible representatives of the extant genus
Tachypleus Leach, 1819 appear in the Middle Triassic
(Diedrich, 2011). Xiphosurans have existed for some
480 Myr, with the earliest unequivocal representatives
found from the Lower Ordovician of Morocco (Van Roy
et al., 2010). As a group they appear to have had a
relatively low species diversity since the late Palaeo-
zoic, although during the Silurian and Devonian there
were a plethora of distinct morphologies (see Størmer,
1955). It has been suggested that the post-Palaeozoic
limuloids represent a good example of bradytely
(Fisher, 1984). Their apparent morphological con-
servatism has resulted in their being branded as ‘living
fossils’, a term which has been rightly criticized as both
inaccurate and misleading (Schopf, 1984).
Zoological Journal of the Linnean Society, 2013, 167, 1–27. With 10 figures
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27 1
Once considered to be relatives of crustaceans, these
creatures have an important place in the history of
arthropod research, with the formalization that xipho-
surans are chelicerates by Lankester (1881) setting
the foundations for the modern interpretation of
arthropod relationships. Xiphosurans are now consid-
ered to be relatively basal euchelicerates (Dunlop,
2010) with a distinction between those species in
which the opisthosoma has become fused into a tho-
racetron (Xiphosurida) and those that retain freely
articulating opisthosomal tergites (‘synziphosurines’),
both originally considered monophyletic suborders
(Zittel, 1885). A number of other groups have also in
the past been considered members of the Xiphosura,
namely aglaspidids, chasmataspidids, strabopids,
and paleomerids. Aglaspidids (see Raasch, 1939) were
included by Størmer (1944) but removed when they
were shown not to possess chelicerae (Briggs, Bruton
& Whittington, 1979), while chasmataspidids were
initially described as aberrant xiphosurans (Caster
& Brooks, 1956) and, although they were suggested
to have closer affinities to eurypterids by Eldredge
(1974), were still retained within Xiphosura until
finally being excluded by Anderson & Selden (1997).
Strabopids (Beecher, 1901) and paleomerids (Størmer,
1956), shown to be synonyms by Tetlie & Moore
(2004), were also originally assigned to Xiphosura
by Størmer (1944) as part of an aglaspidid complex,
but these were excluded by Bergström (1971). One
other problematic taxon, Lemoneites Flower, 1969,
was originally considered an aglaspidid before being
assigned to Xiphosura incertae sedis by Eldredge
(1974) and finally excluded from Xiphosura by Ander-
son & Selden (1997), eventually to be revealed as
a glyptocystitid echinoderm (Moore & Braddy, 2005).
Xiphosura therefore at present encompasses solely
Xiphosurida and the synziphosurines.
The monophyly of Xiphosura (xiphosurids plus
synziphosurines) has not been questioned since syn-
ziphosurines were united with xiphosurids by Zittel
(1885); although Packard (1886) follows Woodward
(1867) in suggesting that synziphosurines showed
closer affinities to eurypterids, it is clear that by
the turn of the 20th century Zittel’s classification
had been universally adopted (e.g. Laurie, 1893;
Clarke & Ruedemann, 1912; Woodward, 1913). Origi-
nally united primarily through general similarity
rather then specific synapomorphies, Xiphosura rep-
resents a hang-up of pre-phylogenetic thinking that
has carried through into early analyses of chelicerate
relationships. Supposed synapomorphies were coded
as such based solely on their apparent uniqueness,
with no attempt to test for homologous structures in
non-xiphosuran arthropods. It is important that the
characters used to define monophyletic groups are
critically evaluated without an a priori bias for or
against monophyly, as has been done for trilobites
(Ramsköld & Edgecombe, 1991). Assumptions derived
from historical classification still influence modern
studies, and until such assumptions are critically
evaluated the validity of groups based upon them is
questionable. The only non-xiphosuran taxa included
in previous phylogenetic analyses of Xiphosura was
an outgroup (Anderson & Selden, 1997), or the entire
group was represented by a single taxon in higher
level analyses of the Arthropoda (Briggs & Fortey,
1989; Briggs, Fortey & Wills, 1992; Dunlop & Braddy,
2001; Cotton & Braddy, 2004; Hendricks & Lieber-
man, 2008). A monophyletic Xiphosura was recovered
in the analyses of Dunlop & Selden (1997) and Shultz
(2007), but Shultz used Xiphosura as his outgroup
and so essentially forced their monophyly. Notably,
Wills et al. (1998) included a synziphosurine (Wein-
bergina Richter & Richter, 1929) and a xiphosurid
(Tachypleus) in their analysis and retrieved a para-
phyletic Xiphosura, an intriguing result.
Furthermore, in the only comprehensive cladistic
treatment of Xiphosura to date synziphosurines were
shown to not represent a natural group, instead
forming a paraphyletic grade that was interpreted
as being the stem lineage to Xiphosurida (Anderson
& Selden, 1997). At the time the oldest xiphosurid
was known from the Carboniferous with synzipho-
surines known from the Silurian to Devonian, with
the expected stratigraphic distribution from stem
lineage to crown group. This congruence between
stratigraphic and phylogenetic sequence was upset,
however, by the discovery of an unequivocal xipho-
surid from the Upper Ordovician of Manitoba,
Canada (Rudkin, Young & Nowlan, 2008), followed
by further xiphosurid reports from the Lower Ordo-
vician of Morocco (Van Roy et al., 2010). The strati-
graphic range of the synziphosurines has also been
expanded, with the youngest representative now
known from the Carboniferous (Moore, McKenzie &
Lieberman, 2007) while the oldest occurs in strata
slightly younger than that of the undescribed xipho-
surid in Morocco (Van Roy et al., 2010). It is clear
that the synziphosurines described to date are at
best offshoots of the xiphosurid stem and not
the direct ancestors of xiphosurids as inferred by
Anderson & Selden (1997) for a few taxa. While the
retention of plesiomorphic conditions in taxa is not
unusual it should be remembered that these off-
shoots will have continued following their own evo-
lutionary development, no matter how imperceptible,
and autapomorphies, convergences and parallelisms
can complicate attempts to resolve their affinities.
Hence, character polarity is key to resolving the
relationships of groups with a high proportion of
ghost ranges and it is because of this that outgroup
selection is of utmost importance for any analysis.
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
Unfortunately, as we shall see for Xiphosura, this
can be a particularly complex issue.
The monophyly of Xiphosura therefore remains
to be fully tested. With the discovery of Ordovician
xiphosurids the previous hypotheses of the group’s
evolutionary history (e.g. Bergström, 1975; Fisher,
1984; Selden & Siveter, 1987; Anderson & Selden,
1997) are all shown to be in need of revision. With
the knowledge that there are large gaps in the xipho-
suran fossil record it is now opportune to re-evaluate
the relationships of the known species without
allowing their stratigraphic position to overly influ-
ence their hypothesized affinities. Many species
have remained largely neglected over the last century,
however, and would benefit from restudy. The Meso-
zoic Limulina, in particular, need revision, so a phy-
logenetic treatment of all species is not possible,
but these species are less critical to working out the
origins of the group. Instead, the present work uti-
lizes a number of well-known or recently redescribed
taxa to elucidate the exact relationship of the synzi-
phosurines to the xiphosurids within the context
of the higher euchelicerates. This includes a critical
re-evaluation of the homology statements employed
by previous workers and the polarization of these
Throughout this discussion a number of aspects of
generalized arthropod morphology will be dealt with,
and these are briefly defined here. The fundamen-
tal division of the arthropod body is the somite, or
metamere (Lankester, 1904). The first somite is
here termed the ocular/protocerebral region (Scholtz,
1995), which is considered equivalent to the concept
of the arthropod acron (see Scholtz & Edgecombe,
2006), and is numbered 0. The subsequent somites
are numbered using roman numerals from anterior to
posterior; in xiphosurans this ranges from I to XVII.
The convention adopted by Selden & Siveter (1987) of
separating segments from somites is adopted here; a
segment refers to an externally differentiated unit
that may comprise multiple somites, and is often used
when segmentation is obvious but somite count is
uncertain, as can be the case for xiphosurans. Seg-
ments are labelled using arabic numerals, with the
first post-cephalic segment numbered 1. The dorsal
sclerite of each segment is termed the tergite, while
the ventral sclerite is the sternite. Somites frequently
undergo differentiation as part of specialization of
limb form and function. The arthropod body is divided
into regions of serial somites exhibiting a character-
istic differention and each of these regions are termed
tagmata. The term pseudotagma is applied to a divi-
sion of the body that appears to be a distinct unit but
does not represent a true tagma (van der Hammen,
The definition of what constitutes a tagma has
varied throughout its use. Lankester (1904) defined a
tagma as a definite region of similar modification of
the somites and their appendages, differing in their
modification from that observed in regions preceding
and succeeding them – modification was defined as a
difference in any of the four ‘meromes’ that comprise
each somite, in arthropods namely the tergite, ster-
nite, muscular fibres and the appendages. Lankester
(1904) noted (and implicitly criticized) a tendency
to define tagma based exclusively on the form of the
tergites and appendages, and this is a practice that
has been largely maintained to this day. Subse-
quently, some authors have considered tagmosis to be
defined by specialization of the appendages (Flessa,
Powers & Cisne, 1975), while others view tagmosis
to be primarily a property of the dorsal exoskeleton.
van der Hammen (1980: 155) described a tagma as a
‘division of the body, composed of a series of more or
less similar segments or metameres, and constituting
a distinct unity characterised by its own individual-
ity’. In the same publication the term pseudotagma
was simply defined as a ‘division of the body, consti-
tuting a more or less distinct unity with its own
individuality, but not representing a true tagma’ (van
der Hammen, 1980: 131). These conflicting (and often
vague) definitions of what constitutes a tagma has
resulted in a fair degree of subjectivity in defining an
arthropod’s tagmosis, although in more recent work
on trilobites a somewhat more strict definition of
a tagma was used: a discrete morphological entity
which is distinct in at least the dorsal exoskeleton
from its first appearance in ontogeny so that its
boundaries with adjacent regions are never crossed by
newly recruited or released segments (Minelli, Fusco
& Hughes, 2003), a definition already used in many
neontological works. Yet this definition does not cover
conditions where the dorsal evidence for tagmata does
not correspond to the ventral (primarily appendicu-
lar) evidence, as appears to be the case in a number
of arthropods including trilobites, xiphosurans, Acari,
and other chelicerate groups. It also fails to differen-
tiate tagmata from pseudotagmata. The concept of
pseudotagmata appears to stem from the work of van
der Hammen (1963) on the subdivisions of parasiti-
form Acari and was then extended for use in the other
chelicerate groups (van der Hammen, 1986a, b) but
has not found widespread use among other arthropod
workers. It seems that there is a great utility in the
pseudotagma concept that has gone largely unrecog-
nized but has the potential to aid in providing a
clearer definition for arthropod tagmata. Tagmata are
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
most comprehensible as regions of functional specia-
lization, which in arthropods is predominantly
mediated through modification or suppression of
the appendages. The definition for a tagma adopted
herein is therefore a distinct and discrete morphologi-
cal region that comprises a series of equivalently
modified appendages that constitute a unit of specific
form (as in the case of the malacostracan abdomen)
or sometimes function (as in the hexapod head). The
first or last appendages of a tagma may, however, be
dramatically differentiated from the others; examples
include the chelicera in chelicerates (first appendage
of the prosoma), antennula in crustaceans, myria-
pods, and hexapods (first appendage of the cephalon),
the chilaria in xiphosurans (first appendage of the
opisthosoma), and the uropods in decapod crusta-
ceans (last appendage of the abdomen). Further to
this, the boundaries of a tagma should never be
crossed by newly recruited or released segments
throughout the organism’s ontogeny. Using this defi-
nition, the trilobite pygidium would not constitute a
tagma as segments of the thorax develop from the
anterior end of the pygidium; instead, it would con-
stitute a pseudotagma – see discussion below. While
this definition may appear to discount the differen-
tiation produced by the fusion of tergites, Lankester
(1904) effectively argued that such fusion was
essentially superficial and easily acquired through a
disposition of chitinous cuticle of equal thickness
across an area instead of the usual thinning at the
segment boundary that results in a discrete tergite.
Such fusions do, however, fall within the realm of
Herein the term pseudotagma is used to refer to
units defined by differentiation of the tergites or ster-
nites without an associated change in form or func-
tion of the appendages. This definition is consistent
with its use in Acari for defining the gnathosoma
and idiosoma, or proterosoma and hysterosoma,
which is based on a dorsal division that is independ-
ent of appendage structure (Fig. 1A). For this reason,
too, the trilobite pygidium represents a pseudotagma
(Fig. 1B). Pseudotagmata can be further subdivided
into functional pseudotagmata and non-functional
pseudotagmata; functional pseudotagmata impose
some type of functional constraint on the organism,
and fused tergite series enforce such a constraint. The
solifuge cephalosoma (demarcated dorsally by the
propeltidium), which like the acariform proterosoma
is also differentiated ventrally by the sejugal furrow
(see Dunlop, Krüger & Alberti, 2012), is also defined
as a functional pseudotagma as there is no differen-
tiation in the form of the appendages of the prosoma
(Fig. 1C). Furthermore, if the cephalic appendages in
trilobites are in fact undifferentiated from the trunk
appendages (Hughes, 2003a; Minelli et al., 2003) then
the trilobite cephalon also represents a functional
pseudotagma and trilobites therefore possess only
a single tagma (Fig. 1B). Non-functional pseudo-
tagmata, meanwhile, are the most subjective type
of arthropod body division and are largely limited
to changes in tergite or sternite dimensions, most
usually width, or are defined by alterations in cuticle
ornamentation or the presence or absence of epimera.
Examples of non-functional pseudotagmata are the
divisions of the olenelline trilobite thorax proposed
by Lauterbach (1980, 1983, 1989), demarcated by the
third macropleural segment, and the xiphosuran
preabdomen and postabdomen, which are discussed
further below.
The terminology for aspects of xiphosuran morphol-
ogy largely follows Siveter & Selden (1987) and
Selden & Siveter (1987). The prosoma comprises the
entirety of the anterior tagma, including the dorsal
carapace and the prosomal appendages. The prosomal
appendages are denoted by roman numerals I–VI,
appendage I being the chelicerae. In limulids the
anterior cephalic region is termed the cephalothorax
by Shultz (2001) as a number of opisthosomal somites
have been at least partially incorporated into the
prosoma (Fig. 1D). The carapace/cephalothorax itself
bears a number of structures; the cardiac lobe is
located at the posterior of the carapace and is axially
inflated, while the ophthalmic ridges extend either
side of the cardiac lobe and dorsally shade the lateral
compound eyes. Anterior to the cardiac lobe the
median ocelli, simple eyes, can be identified in a
number of taxa. Some species bear extraophthalmic
ridges, transverse ridges that are arrayed on the
Figure 1. Schematic diagrams of a number of different arthropods showing examples of functional pseudotagmata
compared with true tagmata. Somites are labelled 0–XVII; ‘tl’ indicates the telson, ‘m’ the mouth, ‘a’ the anus, and ‘g’ the
gonopores. A, the oribatid mite Epilohmannia cylindrica (Berlese, 1904). B, the trilobite Olenoides serratus (Rominger,
1887). C, the solifuge Galeodes armeniacus Birula, 1929. Solifuge arachnids possess opercula on the first three
opisthosomal segments associated with the genital ducts and repiratory spiracles. These opercula could be considered
homologous to the opercula of the thelyphonid Mastigoproctus giganteus (Lucas, 1835) which Shultz (1993) showed to be
opisthosomal appendages that had become completely sutured to the ventral body wall. D, the limulid xiphosurid Limulus
polyphemus Linnaeus, 1758.
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
surface of the carapace outside of the ophthalmic
ridges. Interophthalmic ridges occur within the area
demarcated by the ophthalmic ridges, and may cor-
respond to internal apodemes for the attachment of
extrinsic limb musculature, indicating arrangement
of the prosomal appendages.
The term opisthosoma refers to the post-prosomal
segments, and is equivalent to the trunk region of
other arthropods (see Hughes, 2003a, b). The opistho-
soma itself is not considered a single tagma, contrary
to most recent treatments. Lankester (1904) consid-
ered chelicerates to possess three tagmata based on
appendage differentiation: the prosoma, mesosoma,
and metasoma. The prosoma comprises somites I–VI,
the mesosoma somites VII–XIII, and the metasoma
somites XIV–XVII (in xiphosurans, eurypterids, scor-
pions, and probably chasmataspidids the metasoma
is formed from somites XIV–XIX). van der Hammen
(1986a) termed the mesosoma and metasoma pseudo-
tagmata and this has been followed by subsequent
authors including Selden & Siveter (1987) and
Rudkin et al. (2008), although van der Hammen con-
sidered the mesosoma to consist of somites VII–XIV
and the metasoma somites XV–XIX (in scorpions).
This in fact corresponds to the preabdomen and
postabdomen; these are actually non-functional pseu-
dotagmata defined by a dorsal constriction of the
tergites (Fig. 2). In xiphosurans the postabdomen
comprises somites XV–XVII, while eurypterids fre-
quently correspond to the condition in scorpions but
sometimes have a preabdomen and postabdomen that
corresponds to the true tagmata of mesosoma and
metasoma. In chasmataspidids the situation is some-
what different, having somites VII–X fused together
into a buckler and somites XI–XIX forming a freely
articulating postabdomen. These are, however, con-
sidered to be functional pseudotagmata, as the fusion
of the buckler places a functional constraint on the
Somite VII is considered to be opisthosomal in
origin, as is the conventional view (see Snodgrass,
1952), and not prosomal as suggested by Stürmer &
Bergström (1981) (see also Haug et al., 2012a for
discussion on this subject). Evidence for this is
shown in the dorsal expression of somite VII as a fully
sclerotized tergite in synziphosurines (Fig. 3A) and
chasmataspidids (Fig. 3B) and in the way the tergite
remains attached to the opisthosoma after disarti-
culation of the carapace (Fig. 4A, B). This tergite is
termed the microtergite in chasmataspidids, but this
refers specifically to the heavily reduced condition
seen in these taxa and the term pre-opercular tergite
is used herein while microtergite is applied as a
condition which the pre-opercular tergite can attain.
The tergite of somite VIII may be called the opercular
tergite and is hypertrophied in some synziphosurines.
Embryological studies have shown that in limulids
the tergite of somite VII and part of the tergite of
somite VIII are incorporated into the prosoma (Scholl,
1977; Sekiguchi, Yamamichi & Costlow, 1982) with
the lateral portions of the opercular tergite distinctly
set off from the main body of the tergite, frequently
deflected dorsally towards the outer margin, and
these regions are termed free lobes. The term thora-
cetron is used for the fully fused dorsal opisthosomal
shield in xiphosurids.
The opisthosomal appendages in aquatic chelicer-
ates are modified into flattened opercula that bear the
respiratory organs, termed book gills. In xiphosurans
the opercula are not medially fused, although in
modern xiphosurids they are connected by a thin
membrane which would be unlikely to fossilize, and so
its presence in extinct taxa is equivocal. The opercu-
lum of somite VIII is the genital operculum and bears
Figure 2. Schematic of a generalized synziphosurine
arthropod, showing the distinction between the preab-
dominal and postabdominal non-functional pseudotag-
mata and the true tagmata of the prosoma, mesosoma, and
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
Figure 3. A, Bunodes lunula d’Eichwald, 1854 from the late Silurian (Ludlow) of Oesel, Estonia. Specimen ELM
G1:262:2, clearly showing the partially reduced pre-opercula tergite of somite VII (arrowed) between the downturned
carapace and the hypertrophied tergite of somite VIII. Scale bar =10 mm. B, ‘Eurypterusstoermeri Novojilov, 1959, a
chasmataspidid from the early Devonian (Lochkovian) of Siberia, Russia. Specimen PIN 1138-1, exhibiting the micro-
tergite of somite VII (arrowed) positioned between the carapace and the buckler. The microtergite curves anteriorly
towards its lateral edges, and is particularly noticeable on the left-hand side. The occurrence of ridges associated with
the lateral eye on the carapace is also labelled. Image courtesy of Dave Marshall. Scale bar =2 mm.
Figure 4. A, Pasternakevia podolica Selden & Drygant, 1987 from the late Silurian (Ludlow) of Zalissia, Ukraine.
Specimen ISEA I -F/MP/3/1499/08, isolated opsithosoma with articulated microtergite of somite VII (arrowed). Image
courtesy of Ewa Krzemin´ ska. Scale bar =10 mm. B, undescribed chasmataspidid from the Lower Devonian (Emsian) of
Siberia, Russia. Specimen PIN 5116-6, disarticulated buckler with microtergite of somite VII (arrowed) still firmly
attached to its anterior margin. Image courtesy of Dave Marshall. Scale bar =2 mm.
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
the paired gonopores; in modern xiphosurids the
genital operculum is devoid of respiratory structures,
but they are present on the genital operculum of
Offacolus Orr et al., 2000 and the condition is unclear
in Weinbergina. The appendages of somite VII are
functionally incorporated into the prosoma and may be
retained as fully pediform walking limbs or reduced to
chilaria, which act in a masticatory function.
The institutional abbreviations for xiphosuran
specimens figured in this study are as follows:
ELM, Estonian Museum of Natural History, Tallinn,
Estonia; ISEA, Museum of the Institute of Systemat-
ics and Evolution of Animals, Kraków, Poland; MM,
Manitoba Museum, Winnipeg, Canada; PIN, Paleon-
tological Institute, Moscow, Russia; PWL, Landessa-
mmlung für Naturkunde Rheinland-Pfalz, Mainz,
Germany; SMF, Naturmuseum und Forschungsinsti-
tut Senckenberg, Frankfurt am Main, Germany.
Dunlop & Selden (1997) listed four synapomorphies
that united Xiphosura as a monophyletic taxon within
Chelicerata: the presence of ophthalmic ridges, a
cardiac lobe on the carapace, an axial region of the
opisthosoma, and a reduced pre-opercular segment.
However, as noted elsewhere (Lamsdell, 2011), at
least two of these ‘synapomorphies’ are actually found
in several other potentially closely related chelicerate
groups: a cardiac lobe is widespread in eurypterids
(see Selden, 1981) and present in chasmataspidids
(Dunlop, Anderson & Braddy, 2004) while a reduced
pre-opercular segment is present as a microtergite in
chasmataspidids (Dunlop et al., 2004) and potentially
in a heavily reduced form in eurypterids (Dunlop
& Webster, 1999). Furthermore, the division of the
trunk tergites into an axial and tergopleural region
is common in arthropods, being most obvious in
the three-lobed bodies of trilobites. The dorsal axial
region corresponds to the position of ventral limb
insertion, with the appendages arrayed so they attach
to the body just inside the lateral margin of the
axis (see Whittington & Almond, 1987 for a trilobite
example). Initially it would appear that the axis in
xiphosurans does not correspond to the opisthosomal
limb insertion as the trunk limbs have been modified
into laterally expanded opercular flaps that extend
across the entirety of the ventrum (Fig. 5A), whereas
the axis takes up just one-quarter of the thoracetron’s
width (Fig. 5B), although after removing the opercula
it is apparent that their insertion is limited to
the margin of the axial region (Fig. 5C, D). The axis
therefore is homologous to the condition found in
other arthropods and is not a suitable synapomorphy
for Xiphosura. Furthermore, an axial region similar
to that of xiphosurans is present in the euchelicerate
Offacolus kingi Orr et al., 2000, while a differentiated
axial region that may be homologous is observed in
Chasmataspis laurencii Caster & Brooks, 1956. Hints
of similar structures (albeit extremely poorly pre-
served) may exist in Diploaspis muelleri Poschmann,
Anderson & Dunlop, 2005 and Diploaspis casteri
Størmer, 1972. The presence of these structures
in other chelicerates rules out the possibility that
the xiphosuran axial region is a re-expression of a
characteristic not otherwise present in the chelicerate
ground plan.
Ophthalmic ridges, the final proposed synapomor-
phy, occupy a similar position to the palpebral lobes
of eurypterids and chasmataspidids and may in fact
be transformational homologues. Furthermore, in
a number of xiphosurans the ophthalmic ridges are
only weakly developed or absent altogether. While
this does not invalidate ophthalmic ridges as a
potential xiphosuran synapomorphy, the presence
of ridges associated with the palpebral lobes in
some chasmataspidids poses more of a problem. Best
known from Octoberaspis ushakovi Dunlop, 2002,
these structures consist of ridges extending from
the palpebral lobe along the length of the carapace
and appear almost identical to the ophthalmic ridge,
the only difference being the much more prominent
nature of the lateral eye (Dunlop, 2002: fig. 6A).
Although poorly preserved, ridges in the same region
of the carapace can potentially be discerned on the
only known specimen of Forfarella mitchelli Dunlop,
Anderson & Braddy, 1999 (Dunlop et al., 1999: fig. 1)
and ‘Eurypterusstoermeri Novojilov, 1959 (Fig. 3B),
a species in need of being redescribed as a chas-
mataspidid. The presence of ophthalmic ridges in
some chasmataspidid species also nullifies this char-
acter as a xiphosuran synapomorphy. This leaves
them currently with no recognized characters uniting
the group to the exclusion of other chelicerates. This
is not a unique occurrence; after the discovery that
chasmataspidids possessed both a metastoma and
a genital appendage (Dunlop, 2002; Tetlie & Braddy,
2004) eurypterids were left without a synapomorphy,
until the realization that the fusion of appendages
VIII and IX to form the genital operculum was a
unique characteristic (Lamsdell, 2011).
The monophyly of chasmataspidids has also been
seriously questioned, with Tetlie & Braddy (2004)
proposing a scenario in which chasmataspidids
form a paraphyletic grade towards eurypterids. This
analysis failed to account for a number of characters,
however, including the formation of the chasmata-
spidid buckler, and incorrectly plots the distributions
of some characters on the tree (the presence or oth-
erwise of deltoid plates and the anterior opercular
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
plate, specifically) as well as assuming that several
characteristics such as the chasmataspidid ventral
plate and the chelate prosomal appendage of Chas-
mataspis Caster & Brooks, 1956 are plesiomorphies.
Without a suitable outgroup taxon to polarize the
characters there is no way to tell whether these are
indeed plesiomorphic (although it is worth noting that
ventral plates are at present known solely from chas-
mataspidids; furthermore, chelate prosomal append-
ages are known from Offacolus kingi and xiphosurids
but are demonstrably absent from synziphosurines
when the appendages are preserved and so it appears
that this character has evolved more than once within
Chelicerata). A metastoma and genital appendage
may yet be shown to be present in Chasmataspis,
and if undescribed Cambrian resting traces figured
in Dunlop et al. (2004) are indeed produced by a
Chasmataspis-like animal then it clearly possessed
opercula at a minimum. Given the current evidence it
seems most likely that chasmataspidids also repre-
sent a monophyletic group defined by the possession
of a four-segmented buckler with ventral plate (the
two characters almost certainly being linked).
Other previously unconsidered characters may
serve to define a monophyletic Xiphosura. Opisthoso-
mal segment count is a tempting character, especially
given that an opisthosomal segment count of 13 is
consistent across chasmataspidids, eurypterids, and
scorpions, probably constituting the arachnid ground
plan (Dunlop & Webster, 1999). This results in a total
post-ocular (all segments except the protocerebral
region) somite count of 19. Many arachnids, however,
including Pantetrapulmonata, exhibit only 18 somites
(Shultz, 2007) but this could potentially be accounted
for by the complete suppression of somite VII. The
ground pattern for Xiphosura (and potentially Euche-
licerata) most likely comprises 11 opisthosomal seg-
ments, or 17 somites in total (Fig. 6) as evidenced by
a number of fossil taxa and the fact that the neural
ganglia for 17 somites are still identifiable in Limulus
Müller, 1785 (Scholl, 1977). In terms of unfused seg-
ments, Legrandella Eldredge, 1974 clearly shows 11
segments dorsally (Eldredge, 1974: figs 1, 3) while
Weinbergina shows ten tergites with a small microt-
ergite assumed to be present but recessed beneath
the carapace (Stürmer & Bergström, 1981). However,
Figure 5. Limulus polyphemus Linnaeus, 1758 from the Recent of North America. A, ventral view of opisthosoma
showing opercula. B, dorsal view of opisthosoma with prosoma and telson removed. C, schematic of opisthosoma in dorsal
view with apodemes (shallow pits indicating sites of muscle attachment) marked in black and the insertion points of
the opercula shown by grey ovals. D, ventral view of opisthosoma with prosoma, telson, and opercula removed. It can be
clearly seen that the opercula are not attached to the lateral regions of the opisthosoma. Scale bars =10 mm.
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
Figure 6. Schematic of the xiphosuran ground pattern. Somites are labelled 0–XVII; ‘tl’ indicates the telson, ‘gills’ the
position of gills on the five posterior opisthosomal appendages, ‘m’ the mouth, ‘a’ the anus, and ‘g’ the gonopores. The
appendages are shaded with the light ramus representing the endopod, the light grey the basipod, and the dark grey the
exopod. The appendage of somite VII is reduced in the majority of taxa into the chilaria. It is possible that appendage
VII may also have possessed a reduced exopod as in appendage VI given its intermediate position between appendage VI
and the biramous opisthosomal appendages, but there is currently no evidence for this. To the left the somite compositions
of the fused xiphosurid cephalothorax, thoracetron, and pretelson are shown, along with the varying combinations of
segments that can form the diplotergite in bunodids and pseudoniscids.
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
Anderson, Poschmann & Brauckmann (1998) consid-
ered Weinbergina to possess only ten opisthosomal
segments, a view adopted by Moore, Briggs & Bartels
(2005a). The appendages of Weinbergina show that
tergite VII cannot be the first dorsally visible opistho-
somal tergite as this segment bears opercula; the
pediform walking limbs of somite VII originate from
the prosomal region (Stürmer & Bergström, 1981:
fig. 6; Moore, Briggs & Bartels, 2005a: fig. 2), indicat-
ing that they have already been encephalized and
their corresponding tergite is either obscured by the
carapace fold or has also been incorporated into
the prosoma. Thus, both Weinbergina and Legran-
della have an 11-segmented opisthosoma. There is a
general trend, however, for segments to become fused,
the most extreme example being the xiphosurid tho-
racetron, while certain synziphosurines (some buno-
dids and pseudoniscids) also exhibit fusion of the two
posterior preabdominal segments. Furthermore, the
majority of synziphosurine taxa seem to possess only
ten opisthosomal segments [two of which are fused in
Cyamocephalus Currie, 1927 and Pseudoniscus Niesz-
kowski, 1859 (Anderson, 1999)], as do most xipho-
surids including Lunataspis aurora Rudkin, Young
& Nowlan, 2008. Limuloides Woodward, 1865 and
Bunodes d’Eichwald, 1854 also possess fused seg-
ments that at first appear to occupy the same position
as in Cyamocephalus and Pseudoniscus (Størmer,
1955), but Bunodes and Limuloides possess 11
opisthosomal segments (Bergström, 1975) and so the
tergites that have fused are not homologous (XIII
and XIV as opposed to XII and XIII). This suggests
that tergite fusion has occurred independently in a
number of xiphosuran groups. A further complication
of using opisthosomal segment count to define Xipho-
sura is the possibility that the total somite count of 17
represents a plesiomorphic condition. The megachei-
ran arthropod Yohoia tenuis Walcott, 1912 possesses a
13-segmented trunk and a head bearing the great
appendages and three postantennular appendages for
a total of 17 somatic segments (Haug et al., 2012b), a
somite count potentially shared with Haikoucaris
ercaiensis Chen, Waloszek & Maas, 2004, although
the number of cephalic appendages in this taxon is
unclear. Megacheirans are one of the potential sister
groups to Chelicerata, as first proposed by Størmer
(1944) [not Cotton & Braddy (2004) as suggested by
Haug et al. (2012b)] and retrieved through cladistic
analysis by Briggs & Fortey (1989), and it is therefore
possible that 17 somites is the plesiomorphic state
retained by xiphosurans from megacheiran ancestors.
Two other potential synapomorphies remain.
Axial nodes are found on a number of xiphosurids
and several synziphosurines including Weinbergina
(Stürmer & Bergström, 1981), Legrandella (Eldredge,
1974), Willwerathia Størmer, 1969 (Anderson et al.,
1998), and Limuloides (Woodward, 1872). Pseudonis-
cids, along with the remaining bunodids, lack axial
nodes (Anderson, 1999; Krzemin´ski, Krzemin´ ska &
Wojciechowski, 2010). The presence of axial nodes,
however, is probably the plesiomorphic condition and
therefore not a good clade-defining character, as they
are also known from Offacolus kingi, which is con-
sidered to be the most basal known euchelicerate
(Dunlop, 2006; this study). Finally, a three-segmented
postabdomen is present in all synziphosurines (except
perhaps Pasternakevia podolica Selden & Drygant,
1987) with three segments also visible in the fused
postabdomen of the xiphosurid Lunataspis aurora,
although the postabdomen has become fused into a
single segment in all other xiphosurids. The pattern
of tagmosis in Offacolus kingi is somewhat unclear;
Sutton et al. (2002) describe their specimens as pos-
sessing a three-segmented preabdomen (for which
they incorrectly use the term mesosoma) and a five-
segmented postabdomen (their metasoma). However,
the first postabdominal tergite is identical in form to
the preabdominal tergites and the terminal segment
is little more than a boss that forms the articulat-
ing base for the telson; a similar structure is seen
in eurypterids, xiphosurids, and ceratiocarids, and is
derived from the telson itself rather than the trunk.
This would appear to reduce the number of postab-
dominal segments to three, making this count plesio-
morphic with respect to xiphosurans. Accounting for
the ventral trunk appendages reveals a different situ-
ation, however. Offacolus possesses six pairs of trunk
appendages, one pair for each of the first six opistho-
somal tergites. This represents the mesosoma and
shows that a full, six-segmented mesosoma was part
of the euchelicerate ground plan. Sutton et al. (2002)
show that the posterior three opercula are reduced
and devoid of respiratory structures, although the
lack of respiratory structures is not a signifi-
cant enough difference to warrant the exclusion of
these segments from the mesosoma. The erroneous
mesosoma/metasoma division employed by Sutton
et al. (2002) stems from a failure to appreciate the
dichotomous nature of the mesosoma/metasoma and
preabdomen/postabdomen tagma (see Terminology),
and it is clear that the mesosoma of Offacolus consists
of six segments while the metasoma comprises just a
single segment.
The division into preabdomen and postabdomen
is less clear. Although the postabdomen generally
consists of fewer segments than the metasoma
(for example three or four metasomal segments in
xiphosurans and five or six metasomal segments in
eurypterids) it is actually defined solely on dorsal
taper and tergite differentiation. The postabdomen
of Offacolus is therefore defined as comprising three
fused segments, two of which bear opercula. This
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
condition of having opercula-bearing postabdominal
segments is not unique among Chelicerata; Upper
Cambrian trace fossils that show some morphologi-
cal affinities to Chasmataspis (Dunlop et al., 2004)
clearly possess six pairs of opercula while having a
three-segmented preabdomen and a nine-segmented
postabdomen. Further uncertainty regarding the
utility of a three-segmented postabdomen in defining
Xiphosura arises from Yohoia tenuis which also lacks
appendages on the three terminal trunk segments.
The tergites of these segments are also differentiated
from those that precede them, narrowing consecu-
tively and bearing shorter, more acute epimera. This
too may indicate that a postabdomen of three seg-
ments is the plesiomorphic condition.
In summation, a post-ocular somite count of 17
may in fact serve to unify Xiphosura to the exclusion
of all other euchelicerates, as might a postabdomen
consisting of three segments, but these could also
prove to be plesiomorphic depending on which taxa
are eventually shown to constitute the sister group
to Chelicerata. Thus all of the characters previously
proposed as uniting Xiphosura are either plesiomor-
phic or also present in other euchelicerates and at
present there are no convincing synapomorphies of
the group.
While this refutation of the evidence for xiphosuran
monophyly may seem compelling, one needs to take
into account all the available morphological data. To
thoroughly evaluate the signal of the current data a
phylogenetic analysis was performed incorporating 12
of the most completely preserved synziphosurines,
along with four xiphosurids, Offacolus kingi, and
representatives of Eurypterida, Chasmataspidida
and Arachnida. In total 27 euchelicerate taxa were
included in the analysis, along with a further ten
non-euchelicerate arthropods (Table 1), coded based
solely on characters either observable in directly
studied specimens or in photographs published in the
literature. Camera lucida drawings, reconstructions
and descriptive text were not used as sources for
taxon coding so as to avoid undue interpretation.
Pycnogonids are frequently considered the sister
group to Euchelicerata, together forming the subphy-
lum Chelicerata; however, it has been proposed that
pycnogonids represent the sister group to all other
euarthropods, which form a clade termed Cormogo-
nida (see Dunlop & Arango, 2005 for a review). For
this analysis pycnogonids were treated as ingroup
taxa, contra the Cormogonida hypothesis, as sup-
ported by a number of recent molecular analyses
(Regier et al., 2010; Rota-Stabelli et al., 2011). Three
species were included: the fossil taxa Haliestes dasos
Siveter, Sutton, Briggs & Siveter, 2004 and Palaeoi-
sopus problematicus Broili, 1928, and the extant
Pycnogonum litorale (Ström, 1762). If pycnogonids do
in fact represent the sister group to Euchelicerata,
however, then they would appear to be highly apo-
morphic and it then becomes necessary to resolve the
sister group to Chelicerata as a whole, although there
is still a degree of controversy surrounding this
matter. Previously, trilobites and trilobite-like arthro-
pods had been considered to form the chelicerate stem
Table 1. Taxa included in the phylogenetic analysis
including their higher-level taxonomic assignment
Species Assignment
Fuxianhuia protensa Stem euarthropod
Olenoides serratus Trilobita
Emeraldella brocki Xenopoda
Sidneyia inexpectans
Alalcomenaeus cambricus Megacheira
Leanchoilia illecebrosa
Yohoia tenuis
Haliestes dasos Pycnogonida
Palaeoisopus problematicus
Pycnogonum litorale
Offacolus kingi Euchelicerate
Bembicosoma pomphicus Synziphosurines
Bunodes lunula
Camanchia grovensis
Cyamocephalus loganensis
Kasibelinurus amicorum
Legrandella lombardi
Limuloides limuloides
Pasternakevia podolica
Pseudoniscus roosevelti
Venustulus waukeshaensis
Weinbergina opitzi
Willwerathia laticeps
Euproops anthrax Xiphosurida
Limulus polyphemus
Lunataspis aurora
Paleolimulus signatus
Chasmataspis laurencii chasmataspidids
Octoberaspis ushakovi
Eurypterus tetragono-
Parastylonurus ornatus
Rhenopterus diensti
Stoermeropterus conicus
Centruroides vittatus Arachnida (Scorpionida)
Palaeophonus nuncius
Galeodes armenicus Arachnida (Solifugae)
Mastigoproctus giganteus Arachnida (Thelyphonida)
Taxa shown in bold type are those for which original
specimens were investigated for the analysis.
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
lineage (Størmer, 1944), a concept termed Arach-
nomorpha. A number of arachnomorph phylogenies
have been carried out (e.g. Cotton & Braddy, 2004)
without seriously testing the monophyly of the group,
but Scholtz & Edgecombe (2005) explicitly rejected
Arachnomorpha as a clade, placing the majority
of trilobitomorph taxa in the mandibulate stem
lineage. The conclusions of Scholtz & Edgecombe
have, however, been ignored by a number of workers
who continue to perform phylogenetic analyses solely
inclusive of ‘arachnomorph’ taxa (e.g. Hendricks &
Lieberman, 2008). Therefore, three trilobitomorphs
were included in the analysis: the trilobite Olenoi-
des serratus (Rominger, 1887), Emeraldella brocki
Walcott, 1912, and Sidneyia inexpectans Walcott,
1911. Note that no mandibulates were included in the
analysis; however, given that trilobites have been
shown to resolve as part of a clade including man-
dibulates when both have been included in an analy-
sis (e.g. Stein & Selden, 2012), Olenoides is here
taken to represent Antennata, i.e. a clade including
Mandibulata and its stem lineage taxa. Finally, the
megacheiran arthropods Alalcomenaeus cambricus
Simonetta, 1970, Leanchoilia illecebrosa Hou, 1987
and Yohoia tenuis were included in the analysis.
Megacheirans are the most recent taxa to have been
suggested as a sister group to chelicerates (e.g. Chen
et al., 2004; Dunlop, 2006), although whether meg-
acheirans are monophyletic or paraphyletic varies
between authors. Two taxa that are not included in
the analysis but have previously been considered to
be allied in some manner to chelicerates are Aglaspis
spinifer Raasch, 1939 and Sanctacaris uncata Briggs
& Collins, 1988. The similarities between Aglaspis
and chelicerates are purely superficial, however; the
supposed chelicerae were shown to be multiseg-
mented and more similar to antenniform appendages
by Briggs et al. (1979), while the head incorporates no
more than four pairs of postentennula appendages
(Hesselbo, 1992). Furthermore it is unclear whether
the apparent uniramy of the appendages is preserva-
tional or genuine (it is worth noting that the trunk
limbs of xiphosurans retain both endopods and
exopods, and uniramy would preclude Aglaspis from
being a direct chelicerate ancestor). The possession of
a telson and the form of the smooth articulating
facets on each tergite are also euarthropod plesiomor-
phies. When aglaspidids have been included in recent
phylogenetic analyses their position has proven to be
rather inconstant, although they consistently resolve
as relatively basal arthropods somewhat removed
from chelicerates (Briggs & Fortey, 1989; Dunlop &
Selden, 1997; Cotton & Braddy, 2004) and Aglaspis is
here considered to show closer affinities to taxa such
as Emeraldella as suggested by Van Roy (2006) and
retrieved in the recent cladistic analysis by Ortega-
Hernández, Legg & Braddy (2012). Sanctacaris mean-
while was originally described as resolving within the
subphylum Chelicerata based on its supposed posses-
sion of at least six pairs of encephalized appendages,
a cardiac lobe, tagmata equivalent to the prosoma
and opisthosoma, and the anus being positioned at
the rear of the last trunk segment. The presently
available material of Sanctacaris, however, bears no
clear similarity to chelicerates; numerous phyloge-
netic analyses have consistently failed to resolve
it in any proximity to the chelicerate clade (Briggs
& Fortey, 1989; Briggs et al., 1992; Wills, Briggs &
Fortey, 1994). The prosomal/opisthosomal tagmosis is
essentially a restatement of having more than four
appendages in the head and, given that the opistho-
soma is here considered equivalent to the trunk of
other arthropods, is not a valid character to unite the
species with chelicerates. The nature of the head
appendages is not clear; it is possible that the rami
described represent branches of a single appendage,
or that some of the rami are exopods and others
endopods, and although Boxshall (2004) has offered
an interpretation of the appendages based on Offaco-
lus the published material does not permit easy
comparison. The ‘cardiac lobe’ does not resemble a
true cardiac lobe, which should be positioned at the
very posterior of the carapace, and the position of the
anus is known to vary among other fossil arthropods.
A number of characters, however, such as the clus-
tered nature of the cephalic appendages and the
morphology of the carapace, do invite some compari-
son with the Burgess Shale arthropods Habelia
optata Walcott, 1912 and Habelia (?) brevicaudata
Simonetta, 1964.
Fuxianhuia protensa Hou, 1987, considered to be a
derivative of the euarthropod stem lineage, was used
as outgroup for this analysis. Although phylogenetic
analyses have previously placed Fuxianhuia within
Euchelicerata (Wills, 1996), or as a component of a
paraphyletic Megacheira (Budd, 2002; Kühl, Briggs &
Rust, 2009), this is due to misinterpretation of the
animal’s morphology. These analyses consider a pair
of tubular structures located within the head of Fuxi-
anhuia to be subchelate appendages homologous
to the great appendages of megacheirans, although
these structures have also been interpreted as gut
diverticulae (Waloszek et al., 2005). The arguments
for these structures being gut diverticulae are sup-
ported by the discovery of large gut diverticulae
of similar morphology and position in Emeraldella
brocki (Stein & Selden, 2012: fig. 4A) and this, com-
bined with Fuxianhuia having a simple un-segmented
exopod lobe and un-differentiated endopod podo-
meres, suggests that Fuxianhuia is a member of
the euarthropod stem lineage with close affinities
to crown Euarthropoda, as initially suggested (Chen
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
et al., 1995; Edgecombe & Ramsköld, 1996). Even
phylogenetic analyses that maintain an appendicular
origin for the tubular structures in the head now
resolve Fuxianhuia as the sister taxon to Euarthro-
poda (Daley et al., 2009). As the most basic division of
the Euarthropoda appears to be between Antennata
and Chelicerata, both of which are represented as
ingroup taxa within the analysis, it is appropriate
that Fuxianhuia, as a stem-euarthropod with close
affinities to the crown group, be used as the outgroup.
In total 37 taxa were coded for 114 characters
(see the Supporting information); the matrix is
deposited in morphobank (O’Leary & Kaufman, 2012)
with the project code p724 and can be accessed
from Phylo-
genetic analysis was performed using random addi-
tion sequences followed by branch swapping (the mult
command) with 100 000 repetitions with all charac-
ters unordered and of equal weight in TNT (Goloboff,
Farris & Nixon, 2008; made available with the
sponsorship of the Willi Hennig Society). Jackknife
(Farris et al., 1996), Bootstrap (Felsenstein, 1985) and
Bremer support (Bremer, 1994) values were calcu-
lated in TNT and the consistency (CI), retention (RI)
and rescaled consistency indices (RCI) were calcu-
lated in Mesquite 2.73 (Maddison & Maddison, 2010).
The analysis resulted in 12 most-parsimonious trees
with a tree length of 283, a strict consensus of which
(showing branch support values) is in the Supporting
information and is summarized here (Figs 7, 8). The
strict consensus tree has an ensemble CI of 0.583,
ensemble RI of 0.770, and an ensemble RCI of 0.449.
Pycnogonids are resolved as the monophyletic sister
group to Euchelicerata (Fig. 7) which together form
Chelicerata, but Pycnogonida are highly autapomor-
phic and the reduced nature of the dorsal carapace
Figure 7. Summary cladogram of higher-level relationships retrieved from the phylogenetic analysis. Chelicerata
consists of Pycnogonida and Euchelicerata, with megacheirans forming a polytomy on the node below. Trilobites and
xenopods form a basal clade which is here considered to represent Antennata. For the full consensus tree see the
Supporting information.
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
and trunk region in these taxa limit their use in
polarizing characters relating to these regions in
euchelicerates, although Palaeoisopus does retain a
segmented abdomen and telson. Instead the sister
group to Chelicerata informs on these characteristics,
and in this analysis megacheirans resolve in that
position. It is unclear whether they form a clade or
a grade, however, as a clade consisting of Alalcom-
enaeus and Leanchoilia forms a polytomy with Yohoia
and Chelicerata. Olenoides,Emeraldella, and Sid-
neyia form the basalmost clade in the analysis with
an unresolved internal topology. While this could be
consistent with the arachnomorph hypothesis, follow-
ing the argumentation of Scholtz & Edgecombe (2005)
for trilobites as stem-mandibulates results in this
clade being interpreted as representing Antennulata.
If the tree is rooted on the pycnogonids, as it would
be if the Cormogonida hypothesis were true, then the
topology rotates with euchelicerates forming the
sister group to a clade with a topology consisting
of Fuxianhuia as the sister taxon to Antennulata
with megacheirans forming a basal polytomy. Irre-
spective, the internal topology of Euchelicerata does
not change. Offacolus is shown to be the most basal
known euchelicerate, resolving as the sister taxon
to a clade comprising euchelicerates that have lost
the exopods on prosomal appendages II–V. This clade
is here termed Prosomapoda nom. nov., and includes
xiphosurans (sensu lato), chasmataspidids, euryp-
terids, and arachnids.
Figure 8. Summary cladogram of the internal relationships of Prosomapoda. Arachnids, eurypterids, and chas-
mataspidids form a clade, here termed Dekatriata. Xiphosurans are paraphyletic with respect to Dekatriata, with
xiphosurids forming a monophyletic clade of their own. All taxa outside the two labels are synziphosurines, which would
here be polyphyletic. For the full consensus tree see the Supporting information.
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
Xiphosura, as presently defined, is shown to be
paraphyletic (Fig. 8). Xiphosurids do indeed form a
monophyletic clade, although synziphosurines repre-
sent polyphyletic aquatic chelicerates that resolve at
the base of a number of well-supported chelicerate
clades. Weinbergina,Venustulus Moore in Moore et al.
2005b, Camanchia Moore et al., 2011 and Legrandella
Eldredge, 1974, taxa that have always been consid-
ered to be closely related (either placed within the
now defunct Weinberginidae or compared directly
with the morphology of Weinbergina), form a grade
at the base of Prosomapoda nom. nov. There are a
number of characters that support the position of
these taxa and the fact they form a grade rather than
a clade.
Lamsdell (2011) proposed two pieces of evidence that
suggested synziphosurines formed a paraphyletic
grade to a group inclusive of xiphosurids, eurypterids,
chasmataspidids, and arachnids, rather than being
confined to the xiphosurid stem lineage. One of these
lines of reasoning is no longer valid but the other
strongly attests to the non-monophyly of Xiphosura.
The articulation between the thoracetron and the
carapace in xiphosurids is formed by an anterior
extension of the articulating facet along the axis into
a pseudo half-ring (Fig. 9A), and this type of articu-
lation is vestigial in the tergites of the fused tho-
racetron of Lunataspis aurora (Fig. 9B). Lamsdell
(2011) followed Eldredge (1974) and Eldredge & Plot-
nick (1974) in suggesting that this type of articulation
was common among the synziphosurines, and hypoth-
esized that Weinbergina opitzi Richter & Richter,
1929 and Kasibelinurus amicorum Pickett, 1993,
which possess simple articulating facets without the
pseudo half-ring extension, may in fact be ancestral to
all the more derived chelicerates as opposed to just
Figure 9. A, Limulus polyphemus Linnaeus, 1758 from
the Recent of North America. Specimen positioned to show
the prosoma/opisthosoma joint, with its axially extending
articulation. B, Lunataspis aurora Rudkin, Young &
Nowlan, 2008 from the Upper Ordovician (Katian) of
Manitoba, Canada. Rubber mould of specimen MM I-3990,
showing the axial extension of the articulating facet on
each segment. Image courtesy of Graham Young. C, Will-
werathia laticeps (Størmer, 1936) from the Lower Devo-
nian (Emsian) of Willwerath, Germany. Specimen PWL
2011/5690-LS, showing a similar anterior extension of the
articulating facets. Image courtesy of Markus Poschmann.
Scale bars =10 mm.
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
Xiphosurida. With the exception of Willwerathia lati-
ceps (Størmer, 1936) (Fig. 9C), however, no synzipho-
surine demonstrably possesses any form of pseudo
half-ring. Instead an articulating facet is present
as in Weinbergina opitzi (Fig. 10). Furthermore, pos-
sessing an articulating facet is shown to be the
plesiomorphic state, being recognizable in megachei-
rans such as Leanchoilia superlata Walcott, 1912,
artiopods such as Emeraldella brocki, and a number
of aglaspidids including Aglaspis spinifer,Aglaspis
barrandei Hall, 1862, and Chlupacaris dubia Van
Roy, 2006. Therefore, the retention of an articulating
facet in synziphosurines provides no information
regarding their position on the chelicerate tree.
The second line of enquiry focuses on the appendages
of somite VII. It has been suggested that this append-
age pair is the origin of the xiphosurid chilaria
(Dunlop & Webster, 1999), the eurypterid metastoma
(Størmer, 1934), and the scorpion sternum (Jeram,
1998), and if so then all of these structures must be
treated as transformational homologues. The chilaria
have been shown through work on development in
Limulus to comprise the appendages of somite VII
(Farley, 2010), whilst scorpions possess an embryonic
limb pair anterior to the remaining trunk limbs
(Brauer, 1895) that is not expressed in adults. Its
eventual fate is unknown, but it is conceivable that
the pair could proceed to form the sternum. Evidence
for the appendicular nature of the eurypterid metas-
toma stems from the presence of paired muscle scars
on many well-preserved specimens and a possible
fused median suture in others. If one accepts the
homology of these structures, the fully pediform
appendage VII of Weinbergina (as shown by Stürmer
& Bergström, 1981 and Moore et al., 2005a) needs
to be accounted for. There is the possibility that
the pediform endopod is an autapomorphy of Wein-
bergina, especially given the flap-like structures
in the same position on Offacolus kingi, although
the appendages of Offacolus more closely resemble
exopods (bearing some similarity to the exopods of
megacheirans) and may represent part of an adaption
to a more nektonic mode of life. More likely is that the
pediform limb is a plesiomorphic character, irrespec-
tive of uncertainty about the euchelicerate outgroup,
and this is the case in the new analysis. A fully
pediform appendage VII is found in pycnogonids, tri-
lobites, and megacheirans, along with other mooted
ancestral taxa such as Sidneyia inexpectans and
Emeraldella brocki. Therefore, for Weinbergina to
resolve at the base of a monophyletic Xiphosura,
the endopod of appendage VII would have to be inde-
pendently reduced in both the lineage leading to
xiphosurids and that leading to eurypterids, chas-
mataspidids, and arachnids. A more parsimonious
model of evolution is for the fully pediform append-
ages of Weinbergina to be reduced into the chilaria
through the synziphosurine lineage (some of which,
such as Venustulus waukeshaensis Moore in Moore
et al., 2005b, clearly only have five post-cheliceral
pediform appendages) that were retained in xiphosu-
rids but fused to form the metastoma in eurypterids
and chasmataspidids.
There are also a number of further characteristics,
not explored by Lamsdell (2011), that suggest xipho-
suran paraphyly. One of the more conclusive is elon-
gation of the chelicerae, a condition found in Offacolus
kingi (Sutton et al., 2002) and in Weinbergina opitzi
as shown by the manner in which the chelicerae
project beyond the anterior carapace margin (Moore
et al., 2005a: fig. 4). The chelicerae are reduced to
their usual, shorter length in the synziphosurines
Venustulus waukeshaensis and Camanchia grovensis
Figure 10. Weinbergina opitzi Richter & Richter, 1929
from the Lower Devonian (Emsian) of Bundenbach,
Germany. Holotype specimen SMF VIII 7a, showing a flat
articulating facet at the anterior of each tergite. Scale
bar =10 mm.
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
Moore et al., 2011 as well as the xiphosurids. Short
chelicerae are also known in the chasmataspidid
Loganamaraspis dunlopi Tetlie & Braddy, 2004 and
in all eurypterids where preserved except for the
pterygotids, the enlarged chelicerae of which repre-
sent an independently derived condition. Budd &
Legg (2011) have suggested that the elongate cheli-
cerae are derived from multi-annulate antenniform
appendages such as those of Sidneyia and Emeral-
della, but no chelicerae show any sign of possessing
more than four segments [with the possible exception
of the chelifores of Palaeoisopus which according
to Bergström, Stürmer & Winter (1980) comprise five
articles] or being formed from fused annulations.
Instead, the increase in length is derived from the
peduncle (the first cheliceral segment, which is bipar-
tite in pycnogonids and Offacolus) or the segment
bearing the fixed ramus of the cheliceral claw. This
state is also shared with the fossil pycnogonids Pal-
aeoisopus problematicus and Haliestes dasos, which
have large, robust chelifores similar in width to
the walking limbs and of equal length to the palps,
while a number of megacheiran taxa also possess
great appendages composed of few articles where
the peduncle and first armature-bearing segment
are elongated (Haug et al., 2012b). Irrespective of
whether elongate chelicerae have a great appendage
or antenniform origin, it is clear that the condition
is plesiomorphic for Euchelicerata and so a similar
situation arises as with the form of appendage VII.
For Weinbergina to be positioned at the base of a
monophyletic Xiphosura, reduction of the chelicerae
would be required to have occurred in both the xi-
phosurid stem and the lineage leading to arachnids,
eurypterids, and chasmataspidids. Therefore, it is
more likely that the precursors to Weinbergina
diverged from the main chelicerate lineage prior to
the divergence of the xiphosurid clade.
Weinbergina,Venustulus,Bunodes, and Camanchia
also all lack a carapace marginal rim, something that
is present in all other synziphosurines, xiphosurids,
and eurypterids, even if it is deflected ventrally as
in most xiphosurids. The lack of a carapace marginal
rim is likely to be the plesiomorphic condition, it
being absent from most other arthropods (including
Offacolus) with the notable exception of trilobites and
Of a similar nature is the degree of tergopleural
overlap. In Offacolus, along with Weinbergina,Venu-
stulus, and Camanchia, the tergopleurae overlap
one another, as is the condition for the majority of
artiopods and megacheirans (with the exception of
Yohoia) and trilobites (the tergopleurae of which abut
each other). Among the remaining synziphosurines,
however, the tergopleurae are separated laterally to
form a ‘gape’; it is this condition that is also present
in Yohoia. Eurypterids, with their reduced epimera,
also exhibit a lateral gape, as do xiphosurids that
retain the epimera on the lateral margins of the
thoracetron despite the fusion of the tergites. This
again would be consistent with xiphosurids sharing
closer common ancestry with eurypterids than with
some of the synziphosurines that were previously
meant to comprise part of the xiphosurid stem.
Kasibelinurus and Willwerathia resolve as a para-
phyletic grade in a clade with xiphosurids, and are
the only two synziphosurines in the analysis that fall
within the revised Xiphosura. Xiphosura sensu stricto
is defined by the possession of a cardiac lobe that
extends onto the anterior half of the carapace and
potentially the possession of opthalmic ridges that
curve towards the carapace centre anteriorly and
merge with a central cardiac ridge to form a double
arch (or ‘m’) shape; although Kasibelinurus amicorum
does not appear to possess this ophthalmic ridge
morphology the dorsal carapace of the holotype is
flattened and these structures may have not been
preserved, while other species currently assigned to
Kasibelinurus [such as Kasibelinurus (?) randalli
(Beecher, 1902)] clearly show the double arch configu-
ration. Willwerathia is placed as the sister taxon to
Xiphosurida despite its unusual morphology, united
with the order by its possession of an axial anterior
extension to the articulating facet on each tergite.
Xiphosura sensu stricto is itself the sister group to a
clade named Planaterga nom. nov. consisting of buno-
dids, pseudoniscids, chasmataspidids, eurypterids,
and arachnids, a group defined at its base by the loss
of the distinctive axial and subaxial nodes found on
other synziphosurines. Chasmataspidids, eurypterids
and arachnids are further defined in their ground
plan by the possession of a 13-segmented opisthosoma
(although this is then reduced in arachnids) and form
a monophyletic clade, which is here named Deka-
triata nom. nov. The basal node of Planaterga nom.
nov. consists of a polytomy of taxa currently assigned
to Pseudoniscidae and Bunodidae. Bunodes and
Limuloides resolve as sister taxa, as do Pseudoniscus
and Cyamocephalus, but neither family is retrieved in
its entirety as unequivocally monophyletic. Both are
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
united with Dekatriata nom. nov., however, in lacking
axial nodes (with the exception of Limuloides) and in
possessing an opisthosoma that has its widest point
at the third or fourth tergite. Furthermore, the pseu-
doniscids and Dekatriata nom. nov. both show a pre-
dilection for possessing a carapace that is at least
equal in length and width and an increasing lateral
gape in the tergopleurae posteriorly. Bunodes and
Bembicosoma, meanwhile, possess a distinct tubercu-
late ornament that is similar to that found in chas-
mataspidids. At present there is not enough data to
indicate whether pseudoniscids and bunodids form
a paraphyletic grade leading to Dekatriata nom. nov.
or if they are themselves a diverse and previously
unrecognized order of chelicerates. Either way, the
recognition that they have a closer relationship to
chasmataspidids, eurypterids, and arachnids than
xiphosurids is important and has ramifications for
the evolution of the group; the presence of dorsally
visible reduced pre-opercular tergites in chas-
mataspidids, pseudoniscids, bunodids, and Willwer-
athia, for example, means that the tergite of somite
VII was reduced independently in Xiphosurida and
Dekatriata nom. nov. Although it cannot be seen in
Weinbergina,Camanchia, or Venustulus,Legrandella
clearly shows that while the tergite of somite VII was
already partially reduced it still retained its identity
as a fully expressed opisthosomal tergite with tergo-
pleurae prior to its reduction in each lineage.
With phylogenetic analysis supporting the contention
that Xiphosura is paraphyletic it is considered suffi-
cient justification to alter the systematics of Eucheli-
cerata to reflect this revised system. Systematics
(including taxonomy) should always aim to accurately
represent phylogeny whenever possible, and as such
formal names should only be applied to monophyletic
groups. Xiphosura is therefore restricted to xiphosu-
rids and those synziphosurines that form the stem
lineage within the clade exclusive of Planaterga nom.
nov.; in this analysis that comprises Willwerathia and
Kasibelinurus, although further synziphosurine taxa
should be restudied to ascertain whether they fall
within or outside of Xiphosura. Rather than muddy the
taxonomic waters by introducing a swathe of mono-
typic families (or orders!), synziphosurines that fall
outside of Xiphosura are at present left unranked,
although a number of more inclusive clades are named
in order to aid discussion of their relationships. This
will probably change through time as polytomies
such as those of the bunodids and pseudoniscids are
resolved and as more taxa are described. Being off-
shoots of the main chelicerate lineage with a ghost
range of at least 45 Myr it is certain that taxa such as
Weinbergina and Legrandella are actually representa-
tives of at least marginally successful clades of marine
chelicerates that extend as far back as the Ordovician,
even if species diversity remained at a minimum
throughout their history. It is likely that further
studies of Ordovician localities, as well as Silurian and
Devonian strata, will reveal both xiphosurids and
Diagnosis: Arthropoda with the preoral appendages of
somite I modified into chelate grasping appendages.
Included taxa: EUCHELICERATA Weygoldt & Paulus,
1979; PYCNOGONIDA Latreille, 1810 [ =ARACHNOPODA
Dana, 1853].
DANA, 1853]
Diagnosis: Chelicerata with all body segments retain-
ing dorsal differentiation; external proboboscis devel-
oped around mouth; appendage III modified into
ovigers; abdomen generally reduced.
Diagnosis: Chelicerata with the body divided into a
prosoma, mesosoma, and metasoma; prosoma consist-
ing of six appendage pairs united dorsally by a cara-
pace; mesosomal appendages modified into flap-like
Included taxa: Offacolus Orr et al., 2000; PRO-
SOMAPODA nom. nov.
Etymology: From the term prosoma, referring to the
anterior chelicerate tagma, and the Greek podi (foot).
Diagnosis: Euchelicerata with prosomal appendages
II–V lacking exopods in the adult instar.
Included taxa: ?Andarella Moore, McKenzie &
Lieberman, 2007; ?Borchgrevinkium Novojilov, 1959;
Camanchia Moore, Briggs, Braddy & Shultz, 2011;
Legrandella Eldredge, 1974; Venustulus Moore in
Moore et al. 2005b; Weinbergina Richter & Richter,
1929; PLANATERGA nom. nov.;XIPHOSURA Latreille,
1802 [ =MEROSTOMATA Dana, 1852].
Remarks: Prosomapoda incorporates all euchelicer-
ates that have uniramous prosomal appendages in
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
the adult instar apart for the flabellum found on
appendage VI. This node-based definition allows
for taxa to be incorporated or excluded from Pro-
somapoda in the future without further altering the
Diagnosis: Prosomapoda with a partially reduced
tergite of somite VII; appendages of somite VII
reduced to chilaria; opisthosoma broadest anteriorly;
cardiac lobe extends onto anterior half of carapace;
opthalmic ridges merge anteriorly with median ridge
to form double arch.
Included taxa: Kasibelinurus Pickett, 1993; ?Maldy-
bulakia Tesakov & Alekseev, 1998 [ =Lophodesmus
Tesakov & Alekseev, 1992]; Willwerathia Størmer,
1969; XIPHOSURIDA Latreille, 1802.
Remarks: Merostomata was originally proposed by
Dana (1852) as consisting solely of Xiphosura but
later expanded to include Eurypterida (Woodward,
1866–1878). Eurypterida being closer related to
Arachnida than Xiphosura necessitates their removal
from Merostomata and results in Merostomata being
a junior synonym of Xiphosura.
Elleria Raymond, 1944 and Kiaeria Størmer, 1934
probably have a fused thoracetron and are therefore
classified as xiphosurids. Maldybulakia has been con-
sidered to show similarity to Willwerathia and so is
tentatively included within Xiphosura.
Diagnosis: Xiphosura with the tergites of somites
VIII–XIV dorsally fused into a thoracetron.
Etymology: From the Latin plana (level) and terga
(back), in reference to the lack of axial nodes that
characterizes the clade’s ground plan.
Diagnosis: Prosomapoda lacking enlarged axial nodes
on opisthosomal tergites; genal spines vestigial;
opisthosoma broadest at third or fourth tergite;
appendages of somite VII reduced; tergite of somite
VII beginning to form microtergite.
Included taxa: Bembicosoma Laurie, 1899; ?Bunaia
Clarke, 1919; Bunodes d’Eichwald, 1854 [ =Exapinu-
rus Nieszkowski, 1859]; Cyamocephalus Currie,
1927; Limuloides Woodward, 1865 [ =Hemiaspis
Woodward, 1864]; Pasternakevia Selden & Drygant,
1987; Pseudoniscus Nieszkowski, 1859 [ =Neolimulus
Woodward, 1868]; DEKATRIATA nom. nov.
Remarks: It is currently equivocal as to whether pseu-
doniscids and bunodids form a clade of their own or
are paraphyletic in regard to Dekatriata.
This clade is defined as incorporating all taxa that
resolve along the arachnid stem, exclusive of Xipho-
surida and those taxa that diverge from the chelicer-
ate lineage prior to the divergence of Xiphosurida.
Etymology: From the Greek dekatria (thirteen), refer-
ring to the 13-segmented opisthosoma that defines
the clade’s ground plan.
Diagnosis: Planaterga with a total of 19 somites;
opisthosoma plesiomorphically consisting of 13 seg-
ments; appendages of somite VII fused into plate.
Included taxa: CHASMATASPIDIDA Caster & Brooks,
1956 [ =DIPLOASPIDA Simonetta & Delle Cave, 1978];
SCLEROPHORATA Kamenz, Staude & Dunlop, 2011.
Remarks: The character used to define this clade, the
presence of 13 segments in the opisthosoma, is con-
sidered to be present in the arachnid groundplan
(Dunlop & Webster, 1999). Although the number of
segments in other arachnid groups is often fewer, this
could be achieved either through suppression of the
VII somite or a paedomorphic reduction of segments
(see Dunlop, 1998 for a similar concept to this but
based on arachnid polyphyly).
Diagnosis: Dekatriata with the tergite of somite VII
retained as a dorsally visible microtergite; tergites of
somites VII–X dorsally fused into a buckler; buckler
underlain by ventral plate.
Diagnosis: Dekatriata possessing sclerophores as part
of a spermatophore-mediated sperm transfer system
Included taxa: ARACHNIDA Lamarck, 1801; EURYPTE-
RIDA Burmeister, 1843 [ =GIGANTOSTRACA Haeckel,
1866; =CYRTOCTENIDA Størmer & Waterston, 1968].
Remarks: It is possible that Sclerophorata and Deka-
triata are synonyms, but until more detail of the
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
genital structures of chasmataspidids is known this
clade is considered to comprise solely Arachnida and
Diagnosis: Sclerophorata with the appendages of
somites VIII and IX fused into a genital operculum.
Diagnosis: Sclerophorata with reduced head shield
lacking cardiac lobe; anteroventrally directed mouth;
proventricular crop lost; adult instars without
appendages of the first opisthosomal segment.
Remarks: A monophyletic Arachnida is one of the
most strongly supported clades within the analysis,
even without characters traditionally considered
to unite the group that have been suggested to rep-
resent convergences towards a terrestrial mode of
life (Dunlop & Webster, 1999). Dunlop et al. (2012)
recently reviewed the cephalic tagmosis in solifuges,
acariformes, schizomids, and palpigrades, which have
the prosoma separated anteriorly into a propeltidium
consisting of the first four appendage pairs, that they
suggested may be homologous to the ‘euarthropod
head’ (see Chen, 2009). Dunlop et al. suggest that a
divided prosomal region consisting of a propeltidium
and two separate segments bearing walking limbs
may be plesiomorphic for arachnids. This would,
however, necessitate a separate acquisition of the
prosoma in each of the aquatic euchelicerate groups.
The solifuge Galeodes armeniacus Birula, 1929 was
included in the current analysis with its head com-
position coded polymorphically as both 1 +3 and
1+5, representing the fact that it possesses a pro-
peltidium as a subdivision of the prosoma. The
results, however, show that the plesiomorphic condi-
tion for arachnids is a fully consolidated prosoma, and
the division into propeltidium is a derived state (or
possibly a reversal). Alternatively coding G. armenia-
cus as only possessing a 1 +3 head did not change the
tree topology, or the arachnid ground plan, in any
The redefinition of Xiphosura requires that we
rethink the patterns and timing of early chelicerate
evolution. With synziphosurines scattered across the
chelicerate tree, as either part of the stem lineage to
all other euchelicerates except Offacolus, the xipho-
surid stem lineage, or even the stem lineage of a clade
comprising chasmataspidids, eurypterids, and arach-
nids, it is clear that the phylogenetic diversity of
Palaeozoic aquatic chelicerates as previously defined
has been underestimated. Furthermore, the winnow-
ing of Xiphosura sensu stricto also indicates that,
except for the proliferation of Bellinurina in the Car-
boniferous, the clade has always had a relatively low
diversity. The new proposed topology of the chelicer-
ate tree also results in large ghost ranges for all of the
synziphosurine taxa. While further studies of Ordo-
vician strata in Europe and North America will prob-
ably reveal more aquatic chelicerate taxa that may be
recognizable as the ancestors of the Silurian and
Devonian synziphosurines, attention should also turn
to the Gondwanan provinces. Anderson (1996) sug-
gested that the appearance of seemingly primitive
synziphosurines during the Devonian was the result
of their radiation from Gondwanan refugia, while
Legrandella is itself from Bolivia. Lamsdell, Hos¸gör &
Selden (2012) suggested a Gondwanan origination for
eurypterids, the constituent clades of which also all
have ghost ranges extending into at least the Katian.
Both xiphosurids and synziphosurines have been dis-
covered from the Ordovician of Morocco (Van Roy
et al., 2010) and so it is possible that the earliest
euchelicerates may be found outside of Laurentia.
It is worth noting that megacheirans have a near
global distribution, having been recorded from North
America (e.g. Briggs & Collins, 1999; Haug et al.,
2012b), China (e.g. Chen et al., 2004; Liu, Hou &
Bergström, 2007), and Australia (Edgecombe, García-
Bellido & Paterson, 2011), and if they are in fact the
sister group to Chelicerata then the potential remains
for chelicerates to have originated anywhere within
this range.
While the balance of evidence presented herein
indicates that Xiphosura as has previously been
defined is paraphyletic, further discoveries are
needed to help elucidate the associations of some of
the more problematic taxa (such as Andarella and
Borchgrevinkium) and resolve the nature of the
relationship between bunodids and pseudoniscids. A
restudy of these problematic Palaeozoic ‘synzipho-
surines’ may provide further information on their
relationships, while similar cladistic work should be
carried out on the xiphosurids, which have never
received a proper phylogenetic treatment. The rear-
rangement of xiphosurans in a paraphyletic grade
also opens up the possibility of the existence of
synziphosurines that retain further plesiomor-
phic characteristics or share derived characters
with Dekatriata. While a Weinbergina-like form with
biramous prosomal limbs would previously have been
difficult to reconcile with the previous model of
chelicerate evolution, and a pseudoniscid or bunodid
with a genital appendage would have been deeply
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
unsettling, the discovery of either would now be
almost expected given the revised relationships set
out in this study.
Soon after the acceptance of this paper, Briggs
et al. (2012) described a new synziphosurine from the
Silurian of Herefordshire. This remarkable creature,
named Dibasterium durgae, closely resembles the
hypothesized ‘Weinbergina-like form with biramous
prosomal appendages’ postulated in the conclusion of
the present paper. Briggs et al. also include Dibaste-
rium in a phylogenetic analysis that retrieves the
traditional Xiphosura (synziphosurines and Xiphosu-
rida) as monophyletic along with Offacolus. Four
apparent synapomorphies are cited as supporting this
clade, although one of them (character 22 – raised
axial region) is simply an expression of the arthropod
axis as noted here. The status of two of the remaining
synapomorphies, character 77 (chelate walking limbs)
and character 88 (seventh appendage pair incorpo-
rated into the cephalic tagma), is also uncertain as
chelate prosomal appendages are known from Chas-
mataspis (which was not included in the analysis)
while the authors did not account for the probability
of the eurypterid metastoma being homologous to the
seventh appendage pair. This leaves a single synapo-
morphy, character 38 (an elongate telson), a subjec-
tive character that is sensitive to relative changes
in both telson and opisthosomal length. Although all
of the potential xiphosuran taxa sampled possess
elongate telsons, a large number of unsampled syn-
ziphosurines do not, and so its suitability as a synapo-
morphy for the clade is dubious. Synziphosurines
and Xiphosurida are united by characters 85 (append-
age VI modified into a pusher), 89 (appendage VII
reduced and spinose along margin) and 90 (append-
age VII as chileria); however, characters 89 and 90
are actually re-expressions of one another, and as
such one should be removed. This results in two
potential synapomorphies for the clade, both of which
were taken into account in the analysis presented in
herein. Therefore, while the excellent description and
analysis of Dibasterium by Briggs et al. does provide
new information about character evolution among
basal chelicerates it does not provide any new support
for xiphosuran monophyly.
Ewa Krzemin´ ska (Museum of the Institute of System-
atics and Evolution of Animals), Dave Marshall
(Ichron/Palaeocast), Markus Poschmann (Generaldi-
rektion Kulturelles Erbe Rheinland-Pfalz), and
Graham Young (Manitoba Museum) are thanked for
providing images of specimens. Paul Selden (Univer-
sity of Kansas) provided support and equipment that
allowed for the execution of this study. Martin Stein
(University of Copenhagen) is thanked for many dis-
cussions on arthropod morphology. Bruce Lieberman
(University of Kansas) read and commented on an
earlier version of the manuscript. Thanks are due
to Peter Van Roy (Ghent University), Jason Dunlop
(Museum für Naturkunde), and an anonymous re-
viewer for providing comments that greatly improved
the quality of the manuscript.
Anderson LI. 1996. Taphonomy and taxonomy of the Palaeo-
zoic Xiphosura. D. Phil. Thesis, University of Manchester.
Anderson LI. 1999. A new specimen of the Silurian synzi-
phosurine arthropod Cyamocephalus.Proceedings of the
Geologists’ Association 110: 211–216.
Anderson LI, Poschmann M, Brauckmann C. 1998. On
the Emsian (Lower Devonian) arthropods of the Rhenish
Slate Mountians: 2. The synziphosurine Willwerathia.Palä-
ontologische Zeitschrift 72: 325–336.
Anderson LI, Selden PA. 1997. Opisthosomal fusion and
phylogeny of Palaeozoic Xiphosura. Lethaia 30: 19–31.
Beecher CE. 1901. Discovery of eurypterid remains in the
Cambrian of Missouri. American Journal of Science 12: 364.
Beecher CE. 1902. Note on a new xiphosuran from the Upper
Devonian of Pennsylvania. American Geologist 29: 143–146.
Bergström J. 1971. Paleomerus – merostome or merosto-
moid. Lethaia 4: 393–401.
Bergström J. 1975. Functional morphology and evolution of
xiphosurids. Fossils and Strata 4: 291–305.
Bergström J, Stürmer W, Winter G. 1980. Palaeoisopus,
Palaeopantopus and Palaeothea, pycnogonid arthropods
from the Lower Devonian Hunsrück Slate, West Germany.
Paläontologische Zeitschrift 54: 7–54.
Berlese A. 1904. Acari nuovi III. Redia 2: 10–32.
Birula AA. 1929. Über Galeodes armeniacus n. sp. Zoolo-
gischer Anzeiger 84: 273–282.
Boxshall GA. 2004. The evolution of arthropod limbs.
Biological Reviews 79: 253–300.
Brauer A. 1895. Beiträge zur Kenntnis der Entwicklungsge-
schicte des Skorpiones, II. Zeitschrifte Für Wissenschaftliche
Zoologie 59: 351–433.
Bremer K. 1994. Branch support and tree stability. Cladis-
tics 10: 295–304.
Briggs DEG, Bruton DI, Whittington HB. 1979. Append-
ages of the arthropod Aglaspis spinifer (Upper Cambrian,
Wisconsin) and their significance. Palaeontology 22: 167–
Briggs DEG, Collins D. 1988. A middle Cambrian chelicer-
ate from Mount Stephen, British Columbia. Palaeontology
31: 779–798.
Briggs DEG, Collins D. 1999. The arthropod Alalcomenaeus
cambricus Simonetta, from the Middle Cambrian Burgess
Shale of British Columbia. Palaeontology 42: 953–977.
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
Briggs DEG, Fortey RA. 1989. The early radiation and
relationships of the major arthropod groups. Science 246:
Briggs DEG, Fortey RA, Wills MA. 1992. Morphological
disparity in the Cambrian. Science 256: 1670–1673.
Briggs DEG, Siveter DJ, Siveter DJ, Sutton MD,
Garwood RJ, Legg DA. 2012. Silurian horseshoe crab
illuminates the evolution of arthropod limbs. Proceedings
of the National Academy of Sciences of the United States of
America 109: 15702–15705.
Broili F. 1928. Crustaceenfunde aus dem rheinischen
Unterdevon. Sitzungsberichte der Bayerischen Akademie
der Wissenschaften, Mathematisch-Naturwissenschaftliche
Abteilung 1928: 197–201.
Budd GE. 2002. A palaeontological solution to the arthropod
head problem. Nature 417: 271–275.
Budd GE, Legg DA. 2011. Up the spout? Climbing up the
chelicerate stem-group. The Palaeontological Association
55th Annual Meeting, Programme and Abstracts, 17.
Burmeister H. 1843. Die Organisation der Trilobiten, aus
ihren lebenden Verwandten entwickelt; nebst systematischen
Uebersicht aller zeither beschrieben Arten. Berlin: G.
Reimer, 1–148.
Caster KE, Brooks HK. 1956. New fossils from the
Canadian-Chazan (Ordovician) hiatus in Tennessee. Bulle-
tins of American Palaeontology 36: 157–199.
Chen J. 2009. The sudden appearance of diverse animal body
plans during the Cambrian explosion. The International
Journal of Developmental Biology 53: 733–751.
Chen J, Edgecombe GD, Ramsköld L, Zhou G. 1995.
Head segmentation in early Cambrian Fuxianhuia: impli-
cations for arthropod evolution. Science 268: 1339–1343.
Chen J, Waloszek D, Maas A. 2004. A new ‘great-
appendage’ arthropod from the Lower Cambrian of China
and homology of chelicerate chelicerae and raptorial antero-
ventral appendages. Lethaia 37: 3–20.
Clarke JM. 1919. Bunaia woodwardi, a new merostome from
the Silurian waterlimes of New York. Geological Magazine
6: 531–532.
Clarke JM, Ruedemann R. 1912. The eurypterida of New
York. New York State Museum Memoir 14: 1–439.
Cotton TJ, Braddy SJ. 2004. The phylogeny of arach-
nomorph arthropods and the origin of the Chelicerata.
Transactions of the Royal Society of Edinburgh: Earth Sci-
ences 94: 169–193.
Currie LD. 1927. On Cyamocephalus, a new synxiphosuran
from the Upper Silurian of Lesmahagow, Lanarkshire. Geo-
logical Magazine 64: 153–157.
Daley AC, Budd GE, Caron J-B, Edgecombe GD, Collins
D. 2009. The Burgess Shale anomalocaridid Hurdia and its
significance for early euarthropod evolution. Science 323:
Dana JD. 1852. Crustacea, pt. I. United States exploring
expedition during the years 1838, 1839, 1840, 1841, 1842.
Under the command of Charles Wilkes, U.S.N.C. 13. Phila-
delphia: Sherman, 1–685.
Dana JD. 1853. Crustacea, pt. II, Arachnopoda or Pycnogo-
nida. United States exploring expedition during the years
1838, 1839, 1840, 1841, 1842. Under the command of
Charles Wilkes, U.S.N.C. 14. Philadelphia: Sherman, 1382–
Diedrich CJ. 2011. Middle Triassic horseshoe crab reproduc-
tion areas on intertidal flats of Europe with evidence of
predation by archosaurs. Biological Journal of the Linnean
Society 103: 76–105.
Dunlop JA. 1998. The origins of tetrapulmonate book
lungs and their significance for chelicerate phylogeny. In:
Selden PA, ed. Proceedings of the 17th European Colloquium
of Arachnology, Edinburgh, 1997. Burnham Beeches: The
British Arachnological Society, 9–16.
Dunlop JA. 2002. Arthropods from the Lower Devonian
Severnaya Zemlya Formation of October Revolution Island
(Russia). Geodiversitas 24: 349–379.
Dunlop JA. 2006. New ideas about the euchelicerate stem-
lineage. In: Deltshev C, Stoev P, eds. European Arachnology
2005. Acta zoologica bulgarica, Supplement 1, 9–23.
Dunlop JA. 2010. Geological history and phylogeny of
Chelicerata. Arthropod Structure and Development 39: 124–
Dunlop JA, Anderson LI, Braddy SJ. 1999. A new
chasmataspid (Chelicerata: Chasmataspida) from the Lower
Devonian of the Midland Valley of Scotland. Transactions
of the Royal Society of Edinburgh: Earth Sciences 89: 161–
Dunlop JA, Anderson LI, Braddy SJ. 2004. A redescription
of Chasmataspis laurencii Caster & Brooks, 1956 (Cheli-
cerata: Chasmataspidida) from the Middle Ordovician of
Tennessee, USA, with remarks on chasmataspid phylogeny.
Transactions of the Royal Society of Edinburgh: Earth
Sciences 94: 207–225.
Dunlop JA, Arango CP. 2005. Pycnogonid affinities: a
review. Journal of Zoological Systematics and Evolutionary
Research 43: 8–21.
Dunlop JA, Braddy SJ. 2001. Scorpions and their sister-
group relationships. In: Fet V, Selden PA, eds. Scorpions
2001: in memoriam Gary A. Polis. London: British Arach-
nological Society, 1–24.
Dunlop JA, Krüger J, Alberti G. 2012. The sejugal furrow
in camel spiders and acariform mites. Arachnologische
Mitteilungen 43: 8–15.
Dunlop JA, Selden PA. 1997. The early history and phyl-
ogeny of the chelicerates. In: Fortey RA, Thomas RH, eds.
Arthropod relationships. Systematics Association Special
Volumes Series, No. 55. London: Chapman & Hall, 221–235.
Dunlop JA, Webster M. 1999. Fossil evidence, terrestriali-
zation and arachnid phylogeny. The Journal of Arachnology
27: 86–93.
Edgecombe GD, García-Bellido DC, Paterson JR. 2011. A
new leanchoiliid megacheiran arthropod from the lower
Cambrian Emu Bay Shale, South Australia. Acta Palaeon-
tologica Polonica 56: 385–400.
Edgecombe GD, Ramsköld L. 1996. Classification of the
arthropod Fuxianhuia: response. Science 272: 747–748.
d’Eichwald CE. 1854. Die Grauwackenschichten von Liev-
und Esthland. Bulletin de la Société Imperiale des Natural-
istes de Moscou 27: 1–211.
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
Eldredge N. 1974. Revision of the suborder Synziphosurina
(Chelicerata: Merostomata), with remarks on merostome
phylogeny. American Museum Novitates 2543: 1–41.
Eldredge N, Plotnick RE. 1974. Revision of the pseudonis-
cine merostome genus Cyamocephalus Currie. American
Museum Novitates 2557: 1–10.
Farley RD. 2010. Book gill development in embryos and
first and second instars of the horseshoe crab Limulus
polyphemus L. (Chelicerata, Xiphosura). Arthropod Struc-
ture and Development 39: 369–381.
Farris JS, Albert VA, Källersjö M, Lipscomb D,
Kluge AG. 1996. Parsimony jackknifing outperforms
neighbour-joining. Cladistics 12: 99–124.
Felsenstein J. 1985. Confidence limits on phylogenies: an
approach using the bootstrap. Evolution 39: 783–791.
Fisher DC. 1984. The Xiphosurida: archetypes of bradytely?
In: Eldredge N, Stanley SM, eds. Living fossils. Berlin:
Springer, 196–212.
Flessa KW, Powers KV, Cisne JL. 1975. Specialization and
evolutionary longevity in the Arthropoda. Paleobiology 1:
Flower R. 1969. Merostomes from a Cotter horizon of the
El Paso Group. New Mexico Bureau of Mines and Mineral
Resources Memoir 22: 35–44.
Goloboff PA, Farris JS, Nixon KC. 2008. TNT, a free
program for phylogenetic analysis. Cladistics 24: 774–786.
Haeckel E. 1866. Generale Morphologie der Organismen.
Band 2. Berlin: 1–574.
Hall J. 1862. A new crustacean from the Potsdam Sandstone.
Canadian Naturalist 7: 443–445.
van der Hammen L. 1963. The addition of segments
during the postembryonic ontogenesis of the Actinotrichida
(Acarida) and its importance for the recognition of the
primary subdivision of the body and the original segmenta-
tion. Acarologia 5: 443–457.
van der Hammen L. 1980. Glossary of acarological termi-
nology, Vol. 1 General terminology. The Hague: Dr. W. Junk,
van der Hammen L. 1986a. Comparative studies in
Chelicerata IV. Apatellata, Arachnida, Scorpionida, Xipho-
sura. Zoologische Verhandelingen 226: 3–52.
van der Hammen L. 1986b. On some aspects of parallel
evolution in Chelicerata. Acta Biotheoretica 35: 15–37.
Haug C, Van Roy P, Leipner A, Funch P, Rudkin DM,
Schöllmann L, Haug JT. 2012a. A holomorph approach to
xiphosuran evolution – a case study on the ontogeny
of Euproops.Development Genes and Evolution 222: 253–
Haug JT, Waloszek D, Maas A, Liu Y, Haug C. 2012b.
Functional morphology, ontogeny and evolution of mantis
shrimp-like predators in the Cambrian. Palaeontology 55:
Hendricks JR, Lieberman BS. 2008. New phylogenetic
insights into the Cambrian radiation of arachnomorph
arthropods. Journal of Paleontology 82: 585–594.
Hesselbo SP. 1992. Aglaspidida (Arthropoda) from the Upper
Cambrian of Wisconsin. Journal of Paleontology 66: 885–
Heymons R. 1901. Die Entwicklungsgeschichte der Scolo-
pender. Zoologica (Stuttgart) 13: 1–244.
Hou X-G. 1987. Three new large arthropods from Lower
Cambrian Chengjiang, eastern Yunnan. Acta Palaeonto-
logica Sinica 26: 272–285.
Hughes NC. 2003a. Trilobite tagmosis and body patterning
from morphological and developmental perspectives. Inte-
grative and Comparative Biology 43: 185–206.
Hughes NC. 2003b. Trilobite body patterning and the evolu-
tion of arthropod tagmosis. Bioessays 25: 386–395.
Jeram AJ. 1998. Phylogeny, classification and evolution of
Silurian and Devonian scorpions. In: Selden PA, ed. Pro-
ceedings of the 17th European Colloquium of Arachnology,
Edinburgh, 1997. Burnham Beeches: The British Arachno-
logical Society, 17–31.
Kamenz C, Staude A, Dunlop JA. 2011. Sperm carriers in
Silurian sea scorpions. Die Naturwissenschaften 98: 889–
Krzemin´ ski W, Krzemin´ ska E, Wojciechowski D.
2010. Silurian synziphosurine horseshoe crab Pasterna-
kevia revisited. Acta Palaeontologica Polonica 55: 133–
Kühl G, Briggs DEG, Rust J. 2009. A great-appendage
arthropod with a radial mouth from the Lower Devonian
Hunsrück Slate, Germany. Science 323: 771–773.
Lamarck JBPA. 1801. Systême des animaux sans vertèbres.
Paris: Lamarck and Deterville, 1–432.
Lamsdell JC. 2011. The eurypterid Stoermeropterus conicus
from the lower Silurian of the Pentland Hills, Scotland.
Monograph of the Palaeontological Society, London 165:
1–84, pls 1–15.
Lamsdell JC, Hos¸ gör I
˙, Selden PA. 2012. A new Ordovician
eurypterid (Arthropoda: Chelicerata) from Southeast Turkey:
evidence for a cryptic Ordovician record of Eurypterida.
Gondwana Research. doi: 10.1016/
Lankester ER. 1881. Limulus an arachnid. Quarterly
Journal of Microscopical Science 21: 504–548.
Lankester ER. 1904. The structure and classification of the
Arthropoda. Quarterly Journal of Microscopical Science 47:
Latreille PA. 1802. Histoire naturelle, générale et particulière,
des Crustacés et des Insectes, vol. 3. Paris: Dufart, 1–467.
Latreille PA. 1810. Considérations générales sur l’Ordre
Naturel des Animaux composant les Classes des Crustacés,
des Arachnides et des Insectes. Paris: 1–446.
Laurie M. 1893. The anatomy and relations of the
Eurypteridæ. Transactions of the Royal Society of Edinburgh
37: 509–528.
Laurie M. 1899. On a Silurian scorpion and some additional
eurypterid remains from the Pentland Hills. Transactions of
the Royal Society of Edinburgh 39: 575–590.
Lauterbach K-E. 1980. Schlüsselereignisse in der Evolution
des Grundplans der Arachnata (Arthropoda). Abhandlungen
des Naturwissenschaftlichen Vereins in Hamburg 23: 163–
Lauterbach K-E. 1983. Synapomorphien zwischen Trilobiten-
und Cheliceratenzweig der Arachnata. Zoologischer Anzeiger
210: 213–238.
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
Lauterbach K-E. 1989. Trilobites and phylogenetic system-
atics: a reply to G. Hahn. Abhandlungen des Naturwissen-
schaftlichen Vereins in Hamburg 28: 201–211.
Leach WE. 1819. Entomostracés. In: Levrault FG, ed.
Dictionnaires des Sciences Naturelles, volume 14. Paris:
Linnaeus C. 1758. Systema Naturae 1: 1–824.
Liu Y, Hou X-G, Bergström J. 2007. Chengjiang arthropod
Leanchoilia illecebrosa (Hou, 1987) reconsidered. GFF 129:
Lucas H. 1835. Essai sur une monographie du genre Thely-
phone. Magasin de Zoologie 8: 17–24.
Maddison WP, Maddison DR. 2010. Mesquite: a modular
system for evolutionary analysis, Version 2.73. Available at:
Minelli A, Fusco G, Hughes NC. 2003. Tagmata and
segment specification in trilobites. Special Papers in Palae-
ontology 70: 31–43.
Moore RA, Braddy SJ. 2005. A glyptocystitid cystoid affinity
for the putative stem group chelicerate (Arthropoda: Aglas-
pidida or Xiphosura) Lemoneites from the Ordovician of
Texas, USA. Lethaia 38: 293–296.
Moore RA, Briggs DEG, Bartels C. 2005a. A new specimen
of Weinbergina opitzi (Chelicerata: Xiphosura) from the
Lower Devonian Hunsrück Slate, Germany. Paläontolo-
gische Zeitschrift 79: 399–408.
Moore RA, Briggs DEG, Braddy SJ, Anderson LI,
Mikulic DG, Kluessendorf J. 2005b. A new synzipho-
surine (Chelicerata: Xiphosura) from the late Llandovery
(Silurian) Waukesha lagerstätte, Wisconsin, USA. Journal
of Paleontology 79: 242–250.
Moore RA, Briggs DEG, Braddy SJ, Shultz JW. 2011.
Synziphosurines (Xiphosura: Chelicerata) from the Silurian
of Iowa. Journal of Paleontology 85: 81–91.
Moore RA, McKenzie SC, Lieberman BS. 2007. A Carbon-
iferous synziphosurine (Xiphosura) from the Bear Gulch
Limestone, Montana, USA. Palaeontology 50: 1013–1019.
Müller OF. 1785. Entomastraca, seu, Insecta testacea quae
in aquis Daniae et Norvegiae reperit, descripsit et iconibus
illustravit. Hauniae: Thiele, 1–134.
Nieszkowski J. 1859. Zusätze zur Monographie der Trilo-
biten der Osteeprovinzen, nebst der Beschreibung einiger
neuen obersilurischen Crustaceen. Archive für Naturkune,
Liv-Estonia und Kurlands 1: 345–384.
Novojilov NJ. 1959. Mérostomes du Dévonian inférieur et
moyen de Sibérie. Annales de la Société Géologique du Nord
78: 241–258.
O’Leary MA, Kaufman SG. 2012. MorphoBank 3.0: web
application for morphological phylogenetics and taxonomy.
Available at:
Orr PJ, Siveter DJ, Briggs DEG, Siveter DJ, Sutton MD.
2000. A new arthropod from the Silurian Konservat-
Lagerstätte of Herefordshire, UK. Proceedings of the Royal
Society of London 267: 1497–1504.
Ortega-Hernández J, Legg DA, Braddy SJ. 2012. The
phylogeny of aglaspidid arthropods and the internal relation-
ships within Artiopoda. Cladistics. doi: 10.1111/j.1096-0031.
Packard AS. 1886. On the Carboniferous xiphosurous fauna
of North America. Memoires of the National Academy of
Sciences 3: 145–157.
Pickett JW. 1993. A Late Devonian xiphosuran from near
Parkes, New South Wales. Memoirs of the Association of
Australian Palaeontologists 15: 279–287.
Poschmann M, Anderson LI, Dunlop JA. 2005. Chelicer-
ate arthropods, including the oldest phalangiotarbid arach-
nid, from the early Devonian (Siegenian) of the Rhenish
Massif, Germany. Journal of Paleontology 79: 110–124.
Raasch GO. 1939. Cambrian Merostomata. Special Papers of
the Geological Society of America 19: 1–146.
Ramsköld L, Edgecombe GD. 1991. Trilobite monophyly
revisited. Historical Biology 4: 267–283.
Raymond PE. 1944. Late Paleozoic xiphosurans. Bulletin of
the Museum of Comparative Zoology 94: 475–508.
Regier JC, Shultz JW, Zwick A, Hussey A, Ball B,
Wetzer R, Martin JW, Cunningham CW. 2010.
Arthropod relationships revealed by phylogenomic analysis
of nuclear protein-coding sequences. Nature 463: 1079–
Richter R, Richter E. 1929. Weinbergina opitzi n. g., n. sp.,
ein Schwertträger (Merost. Xiphos.) aus dem Devon (Rhein-
land). Senckenbergiana 11: 21–39.
Rominger C. 1887. Description of primordial fossils from
Mount Stephens, N. W. Territory of Canada. Proceedings
of the Academy of Natural Sciences of Philadelphia 39:
Rota-Stabelli O, Campbell L, Brinkmann H, Edgecombe
GD, Longhorn SJ, Peterson KJ, Pisani D, Philippe H,
Telford MJ. 2011. A congruent solution to arthropod phy-
logeny: phylogenomics, microRNAs and morphology support
monophyletic Mandibulata. Proceedings of the Royal Society
B278: 298–306.
Rudkin DM, Young GA, Nowlan GS. 2008. The oldest
horseshoe crab: a new xiphosurid from late Ordovician
konservat-lagerstätten deposits, Manitoba, Canada. Palae-
ontology 51: 1–9.
Scholl G. 1977. Beiträge zur Embryonalen entwicklung von
Limulus polyphemus L. (Chelicerata, Xiphosura). Zoomor-
phologie 86: 99–154.
Scholtz G. 1995. Head segmentation in Crustacea– an immu-
nocytochemical study. Zoology 98: 104–114.
Scholtz G, Edgecombe GD. 2005. Heads, Hox and the
phylogenetic position of trilobites. In: Koenemann S, Jenner
R, eds. Crustacea and arthropod relationships. Boca Raton:
CRC, 139–165.
Scholtz G, Edgecombe GD. 2006. The evolution of arthro-
pod heads: reconciling morphological, developmental and
palaeontological evidence. Development Genes and Evolu-
tion 216: 395–415.
Schopf TJM. 1984. Rates of evolution and the notion of
‘living fossils’. Annual Review of Earth and Planetary Sci-
ences 12: 245–292.
Sekiguchi K, Yamamichi Y, Costlow JD. 1982. Horseshoe
crab developmental studies I. Normal embryonic develop-
ment of Limulus polyphemus compared with Tachypleus
tridentatus. In: Bonaventura J, Bonaventura C, Tesh S, eds.
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
Physiology and biology of horseshoe crabs: studies on normal
and environmentally stressed animals. New York: Liss,
Selden PA. 1981. Functional morphology of the prosoma of
Baltoeurypterus tetragonophthalmus (Fischer) (Chelicerata:
Eurypterida). Transactions of the Royal Society of Edin-
burgh: Earth Sciences 72: 9–48.
Selden PA, Drygant DM. 1987. A new Silurian xiphosuran
from Podolia, Ukraine, USSR. Palaeontology 30: 537–542.
Selden PA, Siveter DJ. 1987. The origin of the limuloids.
Lethaia 20: 383–392.
Shultz JW. 1993. Muscular anatomy of the giant whipscor-
pion Mastigoproctus giganteus (Lucas) (Arachnida: Uropygi)
and its evolutionary significance. Zoological Journal of the
Linnean Society 108: 335–365.
Shultz JW. 2001. Gross muscular anatomy of Limulus
polyphemus (Xiphosura, Chelicerata) and its bearing on
evolution in the Arachnida. Journal of Arachnology 29:
Shultz JW. 2007. A phylogenetic analysis of the arachnid
orders based on morphological characters. Zoological
Journal of the Linnean Society 150: 221–265.
Simonetta AM. 1964. Osservazioni sugli artropodi non trilo-
biti della ‘ Burgess Shale’ (Cambriano medio). III Contributo.
I generi Molaria,Habelia,Emeraldella,Parahabelia. (Nov.),
Emeraldoides (Nov.). Monitore Zoológico Italia 72: 215–231.
Simonetta AM. 1970. Studies on non-trilobite arthropods of
the Burgess Shale (Middle Cambrian). Palaeontographia
Italica 66: 34–45.
Simonetta AM, Delle Cave L. 1978. Una possibile interpre-
tazione filogenetica degli artropodi paleozoici. Bollettino di
Zoologia 45: 87–90.
Siveter DJ, Selden PA. 1987. A new, giant xiphosurid from
the lower Namurian of Weardale, County Durham. Proceed-
ings of the Yorkshire Geological Society 46: 153–168.
Siveter DJ, Sutton MD, Briggs DEG, Siveter DJ. 2004. A
Silurian sea spider. Nature 431: 978–980.
Snodgrass RE. 1952. Textbook of arthropod anatomy. New
York: Cornell University Press, 1–363.
Stein M, Selden PA. 2012. A restudy of the Burgess Shale
(Cambrian) arthropod Emeraldella brocki and reassessment
of its affinities. Journal of Systematic Palaeontology 10:
Størmer L. 1934. Merostomata from the Downtonian
Sandstone of Ringerike, Norway. Skrifter utgitt av Det
Norske Videnskaps-Akademi i Oslo I. Matematisk-
Naturvidenskapelig Klasse 10: 1–125, pls 1–12.
Størmer L. 1936. Eurypteriden aus dem Rheinischen Unter-
devon. Abhandlungen der Preußischen Geologischen Lande-
sanstalt, Neue Folge 175: 1–74.
Størmer L. 1944. On the relationships and phylogeny
of fossil and recent Arachnomorpha. Skrifter utgitt av
Det Norske Videnskaps- Akademi i Oslo. I. Matematisk-
Naturvidenskapelig Klasse 5: 1–158.
Størmer L. 1955. Merostomata. In: Moore RC, ed. Treatise on
invertebrate paleontology part P. Arthropoda 2: chelicerata,
pycnogonida and palaeoisopus. Lawrence, KS: University of
Kansas Press, 4–41.
Størmer L. 1956. A Lower Cambrian merostome from
Sweden. Arkiv för Zoologi, Serie 2 9: 507–514.
Størmer L. 1969. Eurypterids from the Lower Devonian of
Willwerath, Eifel. Senckenbergiana Lethaea 50: 21–35.
Størmer L. 1972. Arthropods from the Lower Devonian
(Lower Emsian) of Alken an der Mosel, Germany. Sencken-
bergiana Lethaea 53: 1–29.
Størmer L, Waterston CD. 1968. Cyrtoctenus gen. nov., a
large late Palaeozoic arthropod with pectinate appendages.
Transactions of the Royal Society of Edinburgh 68: 63–101.
Ström H. 1762. Physik og Oeconomisk Beskrevelse over
Fogderiet Søndmør, beliggende i Bergens. Stift i Norge 1:
Stürmer W, Bergström J. 1981. Weinbergina, a xiphosuran
arthropod from the Devonian Hunsrück Slate. Paläontolo-
gische Zeitschrift 55: 237–255.
Sutton MD, Briggs DEG, Siveter DJ, Siveter DJ, Orr PJ.
2002. The arthropod Offacolus kingi (Chelicerata) from the
Silurian of Herefordshire, England: computer based mor-
phological reconstructions and phylogenetic affinities. Pro-
ceedings of the Royal Society of London B 269: 1195–1203.
Tesakov AS, Alekseev AS. 1992. Myriapod-like arthropods
from the Lower Devonian of central Kasakhstan. Paleonto-
logical Journal 26: 18–23.
Tesakov AS, Alekseev AS. 1998. Maldybulakia – new name
for Lophodesmus Tesakov and Alekseev, 1992 (Arthropoda).
Paleontological Journal 32: 29.
Tetlie OE, Braddy SJ. 2004. The first Silurian chas-
mataspid, Loganamaraspis dunlopi gen. et sp. nov. (Cheli-
cerata: Chasmataspidida) from Lesmahagow, Scotland, and
its implications for eurypterid phylogeny. Transactions
of the Royal Society of Edinburgh: Earth Sciences 94: 227–
Tetlie OE, Moore RA. 2004. A new specimen of Paleomerus
hamiltoni (Arthropoda; Arachnomorpha). Transactions of
the Royal Society of Edinburgh: Earth Sciences 94: 195–198.
Van Roy P. 2006. An aglaspidid arthropod from the Upper
Ordovician of Morocco with remarks on the affinities and
limitations of Aglaspidida. Transactions of the Royal Society
of Edinburgh: Earth Sciences 96: 327–350.
Van Roy P, Orr PJ, Botting JP, Muir LA, Vinther J,
Lefebvre B, el Hariri K, Briggs DEG. 2010. Ordovician
faunas of Burgess Shale type. Nature 465: 215–218.
Walcott CD. 1911. Middle Cambrian Merostomata. Cambrian
geology and paleontology II. Smithsonian Miscellaneous
Collections 57: 17–40.
Walcott CD. 1912. Middle Cambrian Branchiopoda, Malacos-
traca, Trilobita and Merostomata. Cambrian geology and
paleontology II. Smithsonian Miscellaneous Collections 57:
Waloszek D, Chen J, Maas A, Wang X. 2005. Early Cam-
brian arthropods – new insights into arthropod head and
structural evolution. Arthropod Structure and Development
34: 189–205.
Weygoldt P, Paulus HF. 1979. Untersuchungen zur
Morphologie, Taxonomie und Phylogenie der Chelicerata.
Zeitschrift für Zoologische Systematik und Evolutionsforsc-
hung 17: 85–116, 177–120.
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
Whittington HB, Almond JE. 1987. Appendages and habits
of the Upper Ordovician trilobite Triarthrus eatoni.Philo-
sophical Transactions of the Royal Society B 317: 1–46.
Wills MA. 1996. Classification of the arthropod Fuxianhuia.
Science 272: 746–747.
Wills MA, Briggs DEG, Fortey RA. 1994. Disparity as an
evolutionary index: a comparison of Cambrian and recent
arthropods. Paleobiology 20: 93–130.
Wills MA, Briggs DEG, Fortey RA, Wilkinson M,
Sneath PHA. 1998. An arthropod phylogeny based on fossil
and recent taxa. In: Edgecombe GD, ed. Arthropod fossils
and phylogeny. New York: Columbia University Press,
Woodward H. 1864. Descriptions of some new Palaeozoic
Crustacea. Geological Magazine 1: 196–200.
Woodward H. 1865. On a new genus of Eurypterida from the
Lower Ludlow rocks of Leintwardine, Shropshire. Quarterly
Journal of the Geological Society 21: 490–492.
Woodward H. 1866–1878. A monograph of the British fossil
Crustacea belonging to the order Merostomata. Monograph
of the Palaeontographical Society, London 19, 22, 26, 32:
Woodward H. 1867. On some points in the structure of the
Xiphosura, having reference to their relationship with the
Eurypteridæ. Quarterly Journal of the Geological Society
23: 28–40.
Woodward H. 1868. On a new limuloid crustacean (Neolimu-
lus falcatus) from the Upper Silurian of Lesmahagow, Lan-
arkshire. Geological Magazine 5: 1–3.
Woodward H. 1872. Notes on some British Palaeozoic
Crustacea belonging to the order Merostomata. Geological
Magazine 9: 433–441.
Woodward H. 1913. The position of the Merostomata.
Geological Magazine 10: 293–300.
von Zittel KA. 1885. Handbuch der Palaeontologie, Part 1.
Palaeozoologie Vol. 2. Munich and Leipzig: 1–893.
Additional Supporting Information may be found in the online version of this article:
Appendix S1. Morphological character list and character matrix used in the phylogenetic analysis and the full
strict consensus tree with branch support.
© 2012 The Linnean Society of London, Zoological Journal of the Linnean Society, 2013, 167, 1–27
  • ... 480 million years from the Early Ordovician to the Recent ( Babcock et al., 2000;Racheboeuf et al., 2002;Heethoff and Norton, 2009;Rudkin and Young, 2009;Van Roy et al., 2010Briggs et al., 2012). Furthermore, extant Xiphosura are employed as modern analogues to understand the functional morphology of extinct euarthropod taxa, such as the 508 million-year-old (mid-Cambrian) artiopodan Sidneyia inexpectans Walcott, 1911a from the Burgess Shale of British Columbia, Canada ( Zacaï et al., 2016), the early Cambrian radiodontan Amplectobelua symbrachiata Hou et al., 1995 from the Chengjiang biota of China ( Cong et al., 2017), and a variety of large, supposedly predatorial, potentially durophagous eurypterid taxa ( Selden, 1981;Babcock et al., 2000;Lamsdell, 2013;Poschmann et al., 2016). Of the extant xiphosurans, Limulus polyphemus (Linnaeus, 1758) is considered here. ...
    ... We present new information on the microstructure of gnathobasic spines in Limulus polyphemus to determine whether the gnathobases of the Silurian eurypterid Eurypterus tetragonophthalmus Fischer, 1839 from Estonia, and the Cambrian artiopodan Sidneyia inexpectans from Canada have similar structure and function. Eurypterus tetragonophthalmus was chosen because of the threedimensional preservation of appendage specimens that have been isolated from the carbonate host rock, allowing the spines to be sectioned ( Holm, 1898;Selden, 1981), and the relatively close phylogenetic relationship between eurypterids and xiphosurans (e.g., Legg et al., 2013;Lamsdell, 2013Lamsdell, , 2016Legg, 2014;Garwood and Dunlop, 2014;Selden et al., 2015;, in press). While some studies (see Dalingwater, 1975Dalingwater, , 1987Selden, 1981) have detailed certain cuticular microstructures of the gnathobasic spines of E. tetragonophthalmus, no study has sectioned the spines and imaged the internal features using scanning electron microscopy. ...
    Full-text available
    Gnathobasic spines are located on the protopodal segments of the appendages of various euarthropod taxa, notably chelicerates. Although they are used to crush shells and masticate soft food items, the microstructure of these spines are relatively poorly known in both extant and extinct forms. Here we compare the gnathobasic spine microstructures of the Silurian eurypterid Eurypterus tetragonophthalmus from Estonia and the Cambrian artiopodan Sidneyiainexpectans from Canada with those of the Recent xiphosuran chelicerate Limulus polyphemus to infer potential variations in functional morphology through time. The thickened fibrous exocuticle in L. polyphemus spine tips enables effective prey mastication and shell crushing, while also reducing pressure on nerve endings that fill the spine cavities. The spine cuticle of E. tetragonophthalmus has a laminate structure and lacks the fibrous layers seen in L. polyphemus spines, suggesting that E. tetragonophthalmus may not have been capable of crushing thick shells, but a durophagous habit cannot be precluded. Conversely, the cuticle of S. inexpectans spines has a similar fibrous microstructure to L. polyphemus, suggesting that S. inexpectans was a competent shell crusher. This conclusion is consistent with specimens showing preserved gut contents containing various shelly fragments. The shape and arrangement of the gnathobasic spines is similar for both L. polyphemus and S. inexpectans, with stouter spines in the posterior cephalothoracic or trunk appendages, respectively. This differentiation indicates that crushing occurs posteriorly, while the gnathobases on anterior appendages continue mastication and push food towards and into the mouth. The results of recent phylogenetic analyses that considered both modern and fossil euarthropod clades show that xiphosurans and eurypterids are united as crown-group euchelicerates, with S. inexpectans placed within more basal artiopodan clades. These relationships suggest that gnathobases with thickened fibrous exocuticle, if not homoplasious, may be plesiomorphic for chelicerates and deeper relatives within Arachnomorpha. This study shows that the gnathobasic spine microstructure best adapted for durophagy has remained remarkably constant since the Cambrian.
  • ... The Atlantic horseshoe crab, Limulus polyphemus (Linnaeus, 1758), is the best documented extant xiphosurid, and has been the subject of detailed anatomical (Owen, 1872;Lankester, 1881;Shultz, 2001;Battelle, 2006;Bicknell et al., 2018a), biochemical ( Kaplan et al., 1977), physiological (Sokoloff, 1978) and population dynamic (e.g., Botton, 1984;Brockmann, 1990;Schaller et al., 2005;Gerhart, 2007) investigations since the 1800s (van der Hoeven, 1838; Walls et al., 2002). Palaeontologists have studied xiphosu- rids for multiple reasons, including a fossil record that extends as far back as the lower Ordovician, 480 million years ago ( ; Van Roy et al., 2010;Briggs et al., 2012;Lamsdell, 2013;Bicknell et al., 2018a, b). Furthermore, pale- ontologists have been intrigued by the morphologi- cal similarities of L. polyphemus and fossil xiphosurids like Yunnanolimulus luopingensis Zhang et al., 2009 (Guanling Formation, China, Tri- assic; Hu et al., 2017), Mesolimulus walchi (Des- marest, 1822) (Solnhofen Limestone, Germany, Jurassic; Sekiguchi and Sugita, 1980;Smith and Berkson, 2005) and Limulus darwini Kin and Błażejowski, 2014 (Sławno Limestone, Kcynia Formation, Poland, Late Jurassic;Błażejowski, 2015). ...
    Full-text available
    Xiphosurida comprise an archetypal arthropod group of considerable interest to both biological and palaeontological researchers. This appeal is generated by a combination of unique anatomical features, utility as modern analogues for extinct arthropod groups, and an impressive fossil record. Although xiphosurids have been extensively studied, there are few published examples of abnormal specimens. Abnormalities in xiphosurids have mostly been attributed to injuries (either self-inflicted, from mating, or predation) or teratologies (developmental and genetic malfunctions). Here we summarise all previously recorded extant xiphosurid abnormalities and describe new examples of injuries and teratologies to Limulus polyphemus and Tachypleus tridentatus. Furthermore, we present the first evidence of injured fossil xiphosurids: Euproops danae and Mesolimulus walchi. We identify two main groups of telson teratologies and document new ‘U’ shaped cephalothoracic injuries to the anterior cephalothoracic margins of L. polyphemus and T. tridentatus. We show ‘V’ and ‘W’ shaped injuries to E. danae and M. walchi cephalothoracic sections. A further specimen of E. danae is described, which likely represents plastic deformation of a recently moulted exoskeleton, rather than an abnormality sensu stricto. We compare injuries on extant xiphosurids to extinct Cambrian trilobite injuries to suggest that rare cephalic injuries to trilobites were incurred during soft-shelled exoskeletal stages. Reviewing xiphosurid injuries through time is a pivotal step towards understanding how Recent and extinct arthropods responded to injuries.
  • ... We follow in this contribution the systematic taxonomy of lamsdell (2016). The anatomical terminology is that of tasHman (2014), but incorporates some prior sources as well (siveteR & selden 1987;andeRson & selden 1997;and lamsdell 2013). The measurements used in this paper for Psammolimulus lange, 1922 are from a reconstruction in meiscHneR (1962). ...
    The fossil record of horseshoe crabs (Xiphosurida) from the Mesozoic of North America consists of only three name-bearing specimens from the Cretaceous. We add to this depauperate record the first report of a horseshoe crab body fossil from the Triassic of North America. It comes from a locality in the Olenekian (Spathian) Thaynes Group, near Paris, Idaho, USA. This mostly complete and moderately well preserved specimen is assigned to the family Austrolimulidae RIEK, 1955 as Vaderlimulus tricki, n. gen., n. sp. Vaderlimulus is the second austrolimulid taxon to be reported from the Mesozoic of North America. Its discovery adds a fourth austrolimulid genus to the global Triassic fossil record. Vaderlimulus had large genal spines that are most comparable to the Early to late Middle Triassic austrolimulid genera Psammolimulus (Spathian) and Austrolimulus (Ladinian). Heightened enlargement and proportional reduction of body elements, sometimes resulting in bizarre forms, is seen throughout the biostratigraphic range (Serpukhovian-Maastrichtian) of the Austrolimulidae. The discovery of Vaderlimulus provides additional fossil evidence of this evolutionary process. Vaderlimulus likely inhabited a shallow, possibly transitional freshwater coastal setting in the Moenkopi depositional basin along the western Pangean coastal margin. © 2017 E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, Germany.
  • Article
    Full-text available
    The North American vinegaroon, Mastigoproctus giganteus (Lucas, 1835), is demonstrated to comprise a complex of range-restricted species rather than a single widespread polymorphic species. Seven species are recognized based on morphological characters of the adult males, including the arrangement of spines on the prodorsal margin of the pedipalp trochanter, the position of the epistoma on the carapace, the presence of a stridulatory organ on opposing surfaces of the chelicerae and the pedipalp coxa, the presence of a patch of setae on sternite V, and the shape and macrosculpture of the retrolateral surface of the pedipalp femur. The two currently recognized subspecies are elevated to species: Mastigoproctus mexicanus (Butler, 1872), stat. nov., and Mastigoproctus scabrosus (Pocock, 1902), stat. nov. Mastigoproctus floridanus (Lönnberg, 1897) is revalidated from synonymy with M. giganteus. Redescriptions of M. giganteus and the other three species, based on both sexes, are provided, and three new species described: Mastigoproctus cinteotl, sp. nov., from Tamaulipas, Mexico; Mastigoproctus tohono, sp. nov., from Arizona and Sonora, Mexico; Mastigoproctus vandevenderi, sp. nov., from Sonora, Mexico. The present contribution raises the diversity of the Order Thelyphonida Latreille, 1804, in North America from one species to seven. Three species occur in the United States (one each in Arizona, Texas, and Florida), six species occur in Mexico, and two species occur in both countries.
  • Article
    In recent years, methods for investigating the exo-morphology of zoological specimens have seen large improvements. Among new approaches, auto-fluorescence imaging offers possibilities to document specimens under high resolution without introducing additional artifacts as, for example, seen in scanning electron microscopy (SEM) imaging. Additionally, while SEM imaging is restricted to the outer morphology of the current instar, auto-fluorescence imaging can be used to document changes of the outer morphology of the next instar underneath the cuticle of the current instar. Thus, reinvestigating seemingly well known species with these methods may lead to interesting new insights. Here we reinvestigate the late embryonic development of the xiphosuran (“sword tail”) Limulus polyphemus, which is often treated as a proxy for early eucheliceratan evolution. In addition to entire specimens, the appendages of the embryos were dissected off and documented separately with composite-autofluorescence microscopy. Based on these data, we can distinguish six developmental stages. These stages do not match exactly the formerly described stages, as these were largely based on SEM investigation. Our stages appear to represent earlier or later phases within what has in other studies been identified as one stage. This finer subdivision is visible as we can see the developing cuticle under the outer cuticle. In comparison to data from fossil xiphosurans, our results and those of other studies on the ontogeny of L. polyphemus point to a derived mode of development in this species, which argues against the idea of L. polyphemus as a “living fossil.”
  • Article
    Xiphosurans have often been considered as archaic appearing cheliceratan arthropods, with a rich fossil record. We describe here parts of the post-embryonic ontogeny of the 300 million year old xiphosuran Euproops danae (Xiphosura sensu stricto, Euchelicerata), from the Mazon Creek Lagerstätte (Upper Carboniferous), USA. Recently, the ontogeny of a closely related species, Euproops sp. from the Upper Carboniferous Piesberg quarry, Osnabrück, Germany (informally called ‘Piesproops’), has been reconstructed. This analysis has drawn characters into question that were used to differentiate E. danae from another species occurring at the same time, Euproops rotundatus from the British Middle Coal Measures. More precisely, early post-embryonic stages of Piesproops resemble E. danae; later stages resemble E. rotundatus. Based on this earlier study, the here-described reinvestigation of E. danae has been performed as the ontogenetic sequence itself may yield more reliable characters for differentiating species of Euproops. We could identify eight different growth stages for E. danae. This ontogenetic sequence shows a comparable growth to that of Piesproops, but differs markedly in the development of the opisthosomal flange. This character may serve as a basis for reliably differentiating these species. Additionally, analysing the ontogeny of further species may offer the basis for identifying heterochronic shifts in the evolution of xiphosurans.
  • Article
    Full-text available
    Background: Chelicerata represents a vast clade of mostly predatory arthropods united by a distinctive body plan throughout the Phanerozoic. Their origins, however, with respect to both their ancestral morphological features and their related ecologies, are still poorly understood. In particular, it remains unclear whether their major diagnostic characters were acquired early on, and their anatomical organization rapidly constrained, or if they emerged from a stem lineage encompassing an array of structural variations, based on a more labile "panchelicerate" body plan. Results: In this study, we reinvestigated the problematic middle Cambrian arthropod Habelia optata Walcott from the Burgess Shale, and found that it was a close relative of Sanctacaris uncata Briggs and Collins (in Habeliida, ord. nov.), both retrieved in our Bayesian phylogeny as stem chelicerates. Habelia possesses an exoskeleton covered in numerous spines and a bipartite telson as long as the rest of the body. Segments are arranged into three tagmata. The prosoma includes a reduced appendage possibly precursor to the chelicera, raptorial endopods connected to five pairs of outstandingly large and overlapping gnathobasic basipods, antennule-like exopods seemingly dissociated from the main limb axis, and, posteriorly, a pair of appendages morphologically similar to thoracic ones. While the head configuration of habeliidans anchors a seven-segmented prosoma as the chelicerate ground pattern, the peculiar size and arrangement of gnathobases and the presence of sensory/tactile appendages also point to an early convergence with the masticatory head of mandibulates. Conclusions: Although habeliidans illustrate the early appearance of some diagnostic chelicerate features in the evolution of euarthropods, the unique convergence of their cephalons with mandibulate anatomies suggests that these traits retained an unusual variability in these taxa. The common involvement of strong gnathal appendages across non-megacheirans Cambrian taxa also illustrates that the specialization of the head as the dedicated food-processing tagma was critical to the emergence of both lineages of extant euarthropods-Chelicerata and Mandibulata-and implies that this diversification was facilitated by the expansion of durophagous niches.
  • Article
    Full-text available
    In addition to true tagmata, various pseudotagmata are present in chelicerates. Greatly miniaturized and morphologically simplified phytoparasitic acariform mites of the superfamily Eriophyoidea demonstrate a distinct ability to form pseudotagmata. The prodorsum and opisthosoma are the primary divisions of the eriophyoid body. In more evolutionary derived lineages, there is a trend towards the formation of additional opisthosomal subdivisions (pseudotagmata). These subdivisions are termed here “cervix”, “postprodorsum”, “pretelosoma”, “telosoma” and “thanosoma”. Among phytoptids, only the telosomal pseudotagma is present in several sierraphytoptine genera. In diptilomiopids, pseudotagmata have not been recorded. The most diverse examples of pseudotagmatization concern vagrant mites from the family Eriophyidae. Remarkably, well developed and unusually shaped pseudotagmata are peculiar to phyllocoptines from palms, especially in the new vagrant mite Pseudotagmus africanus n. g. & n. sp., found on leaves of Hyphaene coriacea (Arecaceae) in South Africa. Pseudotagmosis is one form of body consolidation in Eriophyoidea, reducing flexibility and therefore decreasing the ability for worm-like locomotion. Consequently, the legs become more important for locomotion. The other form of body consolidation is strengthening of the exoskeleton via armoring with microtubercles, and topographical changes (e.g. formation of opisthosomal ridges and furrows). The data at hand suggest that ancestrally, eriophyoids had an elongate body comprising many annuli, which can be regarded as pseudosegments. Later, they convergently evolved various pseudotagmata via the apparent fusion of these pseudosegments. Two morphotypes of vagrant mites (“armadillo” and “pangolin”) are proposed based on the difference in the modification of dorsal opisthosomal annuli. The minimal number of dorsal annuli (six) is equal to the number of dorso-longitudinal peripheral body muscles; however, this number is unlikely to reflect the true number of segments situated behind the prodorsum in Eriophyoidea. Although legs III and IV are absent in Eriophyoidea, the cervical pseudotagmata might be reminiscent of metapodosomal segments. Future comparative myo- and neuroanatomy studies of groups of genes involved in segmentation development are necessary to reach the final conclusion on the pattern of body segmentation in Eriophyoidea.
  • Article
    Full-text available
    Horseshoe crabs are classic “living fossils”, supposedly slowly evolving, conservative taxa, with a long fossil record back to the Ordovician. The evolution of their exoskeleton is well documented by fossils, but appendage and soft-tissue preservation is extremely rare. Here we analyse details of appendage and soft-tissue preservation in Yunnanolimulus luopingensis, a Middle Triassic (ca. 244 million years old) horseshoe crab from Yunnan Province, SW China. The remarkable preservation of anatomical details including the chelicerae, five pairs of walking appendages, opisthosomal appendages with book gills, muscles, and fine setae permits comparison with extant horseshoe crabs. The close anatomical similarity between the Middle Triassic horseshoe crabs and their recent analogues documents anatomical conservatism for over 240 million years, suggesting persistence of lifestyle. The occurrence of Carcinoscorpius-type claspers on the first and second walking legs in male individuals of Y. luopingensis indicates that simple chelate claspers in males are plesiomorphic for horseshoe crabs, and the bulbous claspers in Tachypleus and Limulus are derived.