Segmentation in early Xiphosura and the evolution of the thoracetron

Abstract

Xiphosuran chelicerates, also known as horseshoe crabs, are a long-lived clade characterized by a highly distinctive morphology and are a classic example of supposed evolutionary stasis. One key feature of horseshoe crabs is the fusion of the opisthosomal segments into a single sclerite referred to as a thoracetron. There has been historical uncertainty as to whether the thoracetron originated once or multiple times within the clade. Here we review criteria for determining whether segments are fused and apply them to a broad census of taxa for which their ontogeny is known or the adult status of specimens can be reasonably asserted to explore the evolution of the thoracetron within a developmental framework. Our findings indicate that the thoracetron evolved once in the common ancestor to Xiphosura. However, subsequent independent loss of the thoracetron segment boundaries is identified and shown to be the result of heterochronic processes acting on a shared developmental pathway. The multiple cases of effacement of the thoracetron within Limuloidea are cases of peramorphically driven parallelism, while the effacement of the thoracetron in the pedomorphic Belinurina is a case of convergence. Xiphosurids therefore represent an interesting case study for recognizing parallelism and convergence on the same structure within closely related lineages. We also demonstrate that somite VII has been incorporated into the prosoma multiple times within the chelicerate lineage, which has implications for interpreting the ground pattern of the group.

Full text

Segmentation in early Xiphosura and the evolution of the thoracetron
James C. Lamsdell*and Samantha B. Ocon
Department of Geology and Geography, West Virginia University, 98 Beechurst Avenue, Brooks Hall, Morgantown, West Virginia 26506,
USA <james.lamsdell@mail.wvu.edu><sbo00001@mix.wvu.edu>
Non-technical Summary.—Horseshoe crabs are characterized by the fusion of their body segments into a thoracetron;
however, it has been debated whether this fusion happened once or multiple times during their evolutionary history. By
looking at many species of fossil horseshoe crab for which juvenile and adult specimens are known, this study demon-
strates that the thoracetron evolved once in the shared horseshoe crab ancestor. The loss of visible segment boundaries,
however, has occurred multiple times within horseshoe crab evolution. This research shows that the loss of segment
boundaries in three lineages of horseshoe crabs represents cases of parallel evolution, where these lineages independently
lost their segment boundaries by progressing along the same shared developmental pathway. In one group, however, the
segment boundaries are not expressed due to the retention of juvenile characteristics into adulthood, which indicates a
case of convergent evolution.
Abstract.—Xiphosuran chelicerates, also known as horseshoe crabs, are a long-lived clade characterized by a highly
distinctive morphology and are a classic example of supposed evolutionary stasis. One key feature of horseshoe crabs
is the fusion of the opisthosomal segments into a single sclerite referred to as a thoracetron. There has been historical
uncertainty as to whether the thoracetron originated once or multiple times within the clade. Here we review criteria
for determining whether segments are fused and apply them to a broad census of taxa for which their ontogeny is
known or the adult status of specimens can be reasonably asserted to explore the evolution of the thoracetron within a
developmental framework. Our findings indicate that the thoracetron evolved once in the common ancestor to Xiphosura.
However, subsequent independent loss of the thoracetron segment boundaries is identified and shown to be the result of
heterochronic processes acting on a shared developmental pathway. The multiple cases of effacement of the thoracetron
within Limuloidea are cases of peramorphically driven parallelism, while the effacement of the thoracetron in the ped-
omorphic Belinurina is a case of convergence. Xiphosurids therefore represent an interesting case study for recognizing
parallelism and convergence on the same structure within closely related lineages. We also demonstrate that somite VII
has been incorporated into the prosoma multiple times within the chelicerate lineage, which has implications for inter-
preting the ground pattern of the group.
Introduction
With a known evolutionary history spanning approximately 480
million years (Van Roy et al., 2010), horseshoe crabs (Xipho-
sura Latreille, 1802) are often referred to as “living fossils”
due to apparent superficial similarities between modern and fos-
sil representatives. Further examination of their fossil record,
however, reveals a hidden morphological and ecological diver-
sity (Lamsdell, 2016,2021a,b). One of the most easily recog-
nizable sources of variation across the evolutionary history of
Xiphosura is the expression of visible tergite boundaries on
the thoracetron, or lack thereof. The thoracetron (Fig. 1), formed
by the external fusion of the opisthosoma (Størmer, 1955;
Anderson and Selden, 1997), is a morphological character
unique to Xiphosura (Lamsdell, 2013,2020).
Characterizing the evolution and segmental composition of
the thoracetron has been integral to our understanding of xipho-
suran taxonomy and phylogeny (Selden and Siveter, 1987;
Anderson and Selden, 1997;Lamsdell,2013,2020). Notoriously
segmented, the arthropod body is composed of serially homolo-
gous units known as somites or metameres (Lankester, 1904).
Lankester (1904) further divided somites into eight meromes,
although four are of primary focus: the tergite (dorsal sclerite),
sternite (ventral sclerite), appendages, and musculature. The
modification of meromes, and therefore the corresponding somite,
to form specialized regions of the body that are distinctive from
those preceding and succeeding is referred to as tagmosis (Lanke-
ster, 1904). The differentiation of somites is the underlying mech-
anism behind the evolution of morphological complexity and
specialization in arthropods; therefore, characterizing the evolu-
tionary history behind patterns of tagmosis in a clade is a funda-
mental underpinning of arthropod paleobiology.
Xiphosura are generally recognized as ancestrally compris-
ing 18 somites (Lankester, 1881; Anderson and Selden, 1997;
*Corresponding author.
Guest Editor: Carrie Schweitzer
Journal of Paleontology, page 1 of 20
Copyright © The Author(s), 2025. Published by Cambridge University Press on behalf of
Paleontological Society. This is an Open Access article, distributed under the terms of the
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permits unrestricted re-use, distribution and reproduction, provided the original article is
properly cited.
0022-3360/25/1937-2337
doi: 10.1017/jpa.2024.31
1
https://doi.org/10.1017/jpa.2024.31 Published online by Cambridge University Press

Lamsdell, 2013), although recent reports of lunataspids with an
opisthosoma consisting of 12 tergites (Lamsdell et al., 2023)
indicates that the ancestral condition for xiphosurans may in
fact be 19 somites. Anatomically modern limulines (as exempli-
fied by Limulus) exhibit 16 clear somites, the last somite consist-
ing of an unspecified number of suppressed or completely
merged somites (Shultz, 2001; Dunlop and Lamsdell, 2017).
The preocular somite is numbered 0, followed by somites
I–XVII (Anderson and Selden, 1997). In terms of tagmata,
somites I–VI comprise the prosoma, VII–XIII comprise the
mesosoma, and XIV–XVII form the metasoma, following
Lamsdell (2013). The opisthosoma is a pseudotagma composed
of all post-cephalic somites. Previously, the fusion of somites
IX–XVII (Anderson and Selden, 1997) or VIII–XIV (Lamsdell,
2013) have been variably considered to constitute the formation
of the thoracetron. In actuality, either scheme results in the
exclusion of taxa with a thoracetron from being considered as
possessing one, with, for example, Rolfeia exhibiting a freely
articulating 8 (Waterston, 1985) and Lunataspis retaining a
freely articulating postabdomen of somites XV–XVII (Rudkin
et al., 2008). The thoracetron therefore consists of somites
IX–XIV and later expands to comprise somites VIII–XVII.
The tagmatic affinity of somite VII, which bears the chi-
laria, is of some controversy. During development, somite VII
and part of somite VIII, the opercular tergite, are integrated
into the prosoma (Scholl, 1977). Stürmer and Bergström
(1981) and Haug et al. (2012) therefore argued that somite VII
should be considered prosomal in nature, contra the conven-
tional view that it is inherently opisthosomal in nature (Snod-
grass, 1952). The definition of tagmata used herein explicitly
addresses that tagmata are defined at the earliest ontogenetic
stage (Lamsdell, 2013); therefore, we consider somite VII to
be originally opisthosomal in nature.
In past iterations of xiphosuran taxonomy, the presence of
unfused opisthosomal tergites was used to differentiate the
superfamily Belinuroidea Packard, 1886—comprising Beli-
nurus and Bellinuroopsis—from other Paleozoic xiphosurids
with a thoracetron, namely the superfamilies Limuloidea
Leach, 1819 and Euproopoidea Eller, 1938 (see Størmer,
1955; Selden and Siveter, 1987; Anderson and Selden, 1997).
Belinuroids were suspected to have a variable number of freely
articulating tergites located anteriorly to a partially fused
opisthosoma (Størmer, 1955; see Anderson and Selden, 1997
for a review); however, a reexamination of Belinurus Bronn,
1839 material determined that the anterior opisthosomal tergites
were indeed fused in belinuroid taxa and established the thorace-
tron as a synapomorphy for Xiphosurida (Anderson and Selden,
1997). Anderson and Selden (1997) also included Synziphosur-
ina Packard, 1886 within Xiphosura as a paraphyletic grade of
stem xiphosurids lacking a thoracetron; further work by Lams-
dell (2013) determined that synziphosurines were a polyphyletic
grouping of basal aquatic chelicerates and defined a monophy-
letic Xiphosura united by the possession of a thoracetron (Lams-
dell, 2013,2016,2020), thereby rendering Xiphosura and
Xiphosurida broadly synonymous.
The timing of opisthosomal fusion in Xiphosura is still
undetermined. Anderson and Selden (1997) speculated that it
had to have occurred before the late Devonian. The surprising
discovery of Lunataspis aurora Rudkin, Young, and Nowlan,
2008 from the Ordovician of Canada prompted a reevaluation
of this timeline and demonstrated that a thoracetron was present
in the earliest xiphosurans (Rudkin et al., 2008). Further reports
of Ordovician xiphosurids have confirmed that a thoracetron was
ubiquitous among these taxa (Van Roy et al., 2010; Lamsdell
et al., 2023). This narrative is challenged, however, by conflict-
ing interpretations of a variety of Devonian taxa that resolve
Figure 1. Anatomy and terminology of an anatomically modern thoracetron.
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phylogenetically crownward of Lunataspis according to proso-
mal characteristics (Lamsdell, 2020,2021a) but have been inter-
preted as possessing an unfused opisthosoma (Pickett, 1993;
Bicknell et al., 2019; Bicknell and Smith, 2021), indicating
either: (1) that the thoracetron developed independently multiple
times within Xiphosura (a scenario previously suggested by
Raymond [1944] and Fisher [1981]), (2) that the Devonian
taxa diverged from the main xiphosuran lineage before Lunatas-
pis, or (3) that the interpretations of an unfused opisthosoma in
these taxa are erroneous. Complicating matters further, xipho-
surids appear to exhibit conflicting patterns of segment bound-
ary effacement within the thoracetron, with tergite expression
occurring variably within Belinurina, Paleolimulidae, and Aus-
trolimulidae, potentially suggesting a high degree of plasticity
within the thoracetron or further support for its convergent
development.
Here we review the opisthosomal morphology of Xiphosura
across their evolutionary history. Patterns of opisthosomal
fusion and segment effacement are documented in each of the
major horseshoe crab clades, and a revised scenario for the evo-
lution of the thoracetron is suggested. We focus particularly on
Ordovician and Devonian taxa that represent the earliest records
of thoracetron evolution and for which interpretations of the
opisthosoma have proven contentious, as well as the Belinurina,
a group for which interpreting the condition of the prosomal/
opisthosomal articulation has been historically problematic.
We also consider the development of the thoracetron during
ontogeny where possible. While we have attempted to be as
accurate and comprehensive as possible, we stress that this is a
preliminary review and that a number of our interpretations are
tentative; our goal here is to challenge prevailing assumptions
about opisthosomal fusion and thoracetron evolution and to
stress the need for further detailed descriptive work on key
taxa, especially those for which a large number of individuals
are known.
Materials and methods
We use the distinction between segments and somites employed
by Selden and Siveter (1987) and Lamsdell (2013): segments are
demarcated by the external expression of a tergite or sternite and
are referred to using Arabic numerals, while somites (numbered
using roman numerals) may not always be externally differen-
tiated, as exemplified by the fusion of the prosoma in chelicerates.
In addition, we follow the distinction between pseudotagma and
tagma as originally proposed by van der Hammen (1963,
1986a,b) and elaborated on by Lamsdell (2013), from which
we draw additional associated morphological terminology. Mor-
phological terminology for xiphosurids follows Selden and Siv-
eter (1987) and Lamsdell (2013,2016,2020,2021a).
One major challenge in interpreting trends in xiphosuran
morphology is determining whether the individuals being con-
sidered represent adults or juveniles, an important concern
when species undergo marked morphological changes during
their development (as is the case in modern horseshoe crabs;
Sekiguchi et al., 1982,1988a,b; Sekiguchi, 1988; Shuster and
Sekiguchi, 2003) and when heterochronic processes underlie
periods of evolution within a group (a scenario also supported
in Xiphosura; Lamsdell, 2021a,b). Adult forms may be more
readily distinguished in the fossil record among taxa that
undergo metamorphosis as part of their development (Haug
and Haug, 2012; Zippel et al., 2022;Prokopetal.,2023) but
can be more difficult to determine in arthropod groups that
undergo anamorphic or more gradualistic development, in part
because arthropods generally lack a reliable size-independent
indication of age (Higgins and Rankin, 1996) without detailed
descriptive and comparativework. Adults have been determined
in fossil taxa known from a large number of individuals where
ontogenetic sequences are known, determined either by their ter-
minal position on a size regression (e.g., Andrews et al., 1974;
Kues and Kietzke, 1981; Wagner et al., 2017) or by a stabiliza-
tion in their expressed morphology (utilized predominantly in
trilobites as determined by their segmental expression; see,
e.g., Hughes et al., 2006; Dai and Zhang, 2013a,b;Holmes
et al., 2021). A number of fossil horseshoe crab species are
known from a large number of specimens for which size regres-
sion has been plotted (Haug et al., 2012; Haug and Rötzer,
2018a; Tashman et al., 2019; Bicknell et al., 2022; Naugolnykh
and Bicknell, 2022). In combination with developmental data
from extant species, these studies support previous observations
that the majority of Paleozoic and Mesozoic horseshoe crabs
were smaller than their modern counterparts (Siveter and Sel-
den, 1987) and, critically for our purposes, that any ontogenetic
changes in opisthosomal fusion and segmental expression occur
in the earliest post-hatching instars (see also Haug and Haug,
2020; Lamsdell, 2021a), suggesting that even if individuals
are subadults, the condition of their thoracetron is likely repre-
sentative of the adult morphology.
We take a relatively conservative approach to inferring the
adult condition of specimens and focus only on taxa for which
we consider the available material to likely represent adult indi-
viduals. Therefore, we focus mainly on species for which a large
number of specimens—with associated ontogenetic data—are
known (adulthood determined on the basis of primary criteria).
We also consider species known from a limited number of indi-
viduals where those individuals are an equivalent size or larger
than the adults of closely related species within their own or a
closely related clade (adulthood determined on the basis of sec-
ondary criteria). In total, we directly consider 23 species from
across all xiphosuran clades. Of these, eleven (Lunataspis aur-
ora, the undescribed Fezouata xiphosurid, Belinurus trilobi-
toides [Buckland, 1837], Euproops danae Meek and Worthen,
1865,Liomesaspis laevis Raymond, 1944,Alanops magnificus
Racheboeuf, Vannier, and Anderson, 2002,Paleolimulus signa-
tus [Beecher, 1904], Boeotiaspis longispinus [Schram, 1979],
Mesolimulus walchi Desmarest, 1822,Tachypleus syriacus
[Woodward, 1879], and Limulus polyphemus [Linnaeus,
1758]) include adult representatives as determined from primary
criteria, while the remaining twelve (Lunataspis borealis Lams-
dell et al., 2023,Patesia randalli [Beecher, 1902], Kasibeli-
nurus amicorum Pickett, 1993,Pickettia carterae [Eller,
1940], Bellinuroopsis rossicus Chernyshev, 1933,Rolfeia foul-
denensis Waterston, 1985,Xaniopyramis linseyi,Norilimulus
woodae [Lerner, Lucas, and Mansky, 2016], Tasmaniolimulus
patersoni Bicknell, 2019,Vaderlimulus tricki Lerner, Lucas,
and Lockley, 2017,Austrolimulus fletcheri Riek, 1955, and Vic-
talimulus mcqueeni Riek and Gill, 1971) include adults as
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determined through secondary criteria—although Lunataspis
borealis is also known from juvenile material, and Patesia ran-
dalli is represented by several specimens.
Of equal importance for exploring the evolution of the thor-
acetron is discriminating between freely articulating and fused
tergites. Anderson and Selden (1997) set out four criteria for
determining when opisthosomal tergites were unfused: (1) dis-
tinct anterior and posterior boundaries to the opisthosomal ter-
gites should be visible in the fossils; (2) dorsoventral
compression of the tergites should result in the asymmetrical
disposition of the lateral spines in the fossils; (3) on enrollment,
the axial portions of the opisthosomal tergites should show some
degree of flexure in the vertical plane; (4) fully or partially dis-
articulated free tergites should be found in the rock matrix. We
largely follow this scheme but recognize some limitations and
biases in the original criteria and present updated guidelines
for determining tergite fusion here. First, we note that Anderson
and Selden (1997) were considering fusion of the entire opistho-
soma and not a smaller subsection of tergites within it; their ori-
ginal criteria are therefore likely to not be clearly fulfilled if only
one or two tergites are free within the opisthosoma, as isthe case
in Rolfeia,Euproops, and Belinurus, which were considered to
have a freely articulating somite VIII (in Rolfeia) and microter-
gite of somite VII (in Euproops and Belinurus) but met none of
the stated criteria. Size may also bias the likelihood of several of
the criteria occurring or being detected. Asymmetrical dispos-
ition of the lateral spines due to compression (criterion 2) is
caused by the original three-dimensionality of the fossil; this
can be reduced in smaller individuals with less convexity–the
cited examples of aglaspidids are all significantly larger than
most fossil Xiphosura (Hesselbo, 1992)—and similar displace-
ment is not seen in eurypterids with even large epimeral spines,
likely due to their shallower cross section in life (e.g., Wills,
1965; Lamsdell et al., 2020a). Similarly, the occurrence of dis-
articulated tergites (criterion 4) will be less common if only one
or a couple of the tergites are freely articulating and may be
extremely difficult to detect in smaller individuals where their
length may be measured in millimeters; for example, chasmatas-
pidids (which are generally small and of a similar size to Paleo-
zoic xiphosurans) are not found in association with
disarticulated tergites even when specimens occur in large con-
centrations (e.g., Størmer, 1972; Marshall et al., 2014), likely
due to their difficulty to detect and fragile nature rather than a
lack of occurrence.
We recognize six possible criteria for determining whether
opisthosomal tergites exhibit fusion. While ideally free tergites
would be indicated by the co-occurrence of several criteria, it is
important to accept that free tergites may in some cases be indi-
cated by only a single criterion (or none; tergites are, after all,
freely articulating—or not—irrespective of our ability to iden-
tify them). Our revised criteria are, in order of decreasing unam-
biguity: (1) free tergites occur disarticulated, as indicated by
their occurrence as isolated sclerites, their absence from a
detached but otherwise complete thoracetron, or their attach-
ment to a prosomal carapace from which the thoracetron has
become detached; (2) free tergites exhibit flexure at the axis dur-
ing enrollment, with the degree of enrollment indicative of the
number of free segments located anterior to the thoracetron;
(3) the pleural spines of free tergites overlap with those of
other segments; (4) free tergites show clear anterior and poster-
ior boundaries across their entire width as determined by the
degree of incision between segments; (5) free tergites are sepa-
rated from the thoracetron doublure; (6) free tergites exhibit
some form of physical differentiation, in the form of a change
in tergite shape, a change in pleural angle, or a change in the dis-
tribution of thoracic nodes.
Phylogenetic analysis of Xiphosura was conducted on the
basis of a modified matrix derived from Lamsdell (2021a),
which is itself an evolution of the matrices presented in Lamsdell
(2013,2020), Lamsdell and McKenzie (2015), Selden et al.
(2015), and Lamsdell et al. (2015). Five taxa (Belinurus lunatus
[Baldwin, 1905], Belinurus arcuatus [Baily, 1859], Belinurus
reginae Baily, 1863,Belinurus truemanii [Dix and Pringle,
1929], and Belinurus bellulus König, 1851) were removed
from the analysis as they have been shown to represent syno-
nyms of Belinurus trilobitoides (Lamsdell and Clapham,
2021; Lamsdell, 2022). The outgroup taxa included in the ana-
lysis were also revised on the basis of recent reinterpretations of
a variety of Cambrian taxa as putative stem chelicerates (Aria
and Caron, 2017,2019); as such, Fuxianhuia protensa Hou,
1987,Leanchoilia illecebrosa Hou, 1987,Alalcomenaeus cam-
bricus Simonetta, 1970,Emeraldella brocki Walcott, 1912,Syd-
neyia inexpectans Walcott, 1911, and Olenoides serratus
(Rominger, 1887) were removed from the matrix, with Yohoia
tenuis Walcott, 1912 retained as the new outgroup taxon and
Sanctacaris uncata Briggs and Collins, 1988,Habelia optata
Walcott, 1912, and Mollisonia plenovenatrix Aria and Caron,
2019 included to aid in resolving character polarity at the base
of Xiphosura. The treatment of the thoracetron in the matrix is
also modified, with the thoracetron considered to be present
when the fusion of somites XI to XIV occurs, rather than somites
VIII to XIV as in a previous version of the matrix. The fusion of
somites VIII, IX, and X into the thoracetron are therefore coded
as separate, distinct characters. The revised matrix compromises
259 characters coded for 156 taxa. Tree inference was performed
through maximum parsimony analysis performed using TNT
(Goloboff et al., 2008). The search strategy employed 100,000
random addition sequences with all characters unordered and
of equal weight (Congreve and Lamsdell, 2016), each followed
by tree bisection–reconnection (TBR) branch swapping. Most
parsimonious trees were summarized through a strict consensus.
Jackknife (Farris et al., 1996), Bootstrap (Felsenstein, 1985),
and Bremer (Bremer, 1994) support values were also calculated
in TNT. Bootstrapping was performed with 50% resampling for
1,000 repetitions, while jackknifing was performed using simple
addition sequence and TBR branch swapping for 1,000 repeti-
tions with 33% character deletion.
Repositories and institutional abbreviations.—AM, Australian
Museum, Sydney, New South Wales, Australia; BMS, Buffalo
Museum of Science, Buffalo, New York, USA; CCMGE,
Chernyshev Central Museum of Geological Exploration, Saint
Petersburg, Russia; FMNH, Field Museum of Natural History,
Chicago, Illinois, USA; GIUS, Silesian University Faculty of
Earth Sciences, Sosnowiec, Poland; KUMIP, University of
Kansas Biodiversity Institute, Lawrence, Kansas, USA; MM,
Manitoba Museum, Winnipeg, Manitoba, Canada; MNHN,
Museum National d’Histoire Naturelle, Paris, France;
Journal of Paleontology:1–204
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NHMUK, Natural History, London, England, UK; NMMNH,
New Mexico Museum of Natural History, Albuquerque, New
Mexico, USA; NMS, National Museums of Scotland,
Edinburgh, Scotland, UK; NMV, Museums Victoria, Carlton,
Victoria, Australia; NMW, National Museum of Wales,
Cardiff, Wales, UK; NSM, Nova Scotia Museum, Halifax,
Nova Scotia, Canada; OUM, Oxford University Museum of
Natural History, Oxford, England, UK; ROM, Royal Ontario
Museum, Toronto, Canada; USNM, National Museum of
Natural History, Washington, DC, USA; UTGD, Rock Library
and Geological Museum of the University of Tasmania,
Hobart, Tasmania, Australia; YPM, Yale Peabody Museum,
New Haven, Connecticut, USA.
Results
Analysis of the character matrix retrieved one most parsimoni-
ous tree resulting in a phylogenetic framework congruent with
those of previous analyses (Lamsdell, 2020,2021a,b), differing
solely in the position of bunodids, which resolve here as stem
chelicerates diverging before the origin of Xiphosura, and of
Patesia, which here resolves as sister taxon to Pickettia—united
by the shared possession of an inflated axis of the anterior ter-
gites—within a monophyletic Kasibelinuridae. Kasibelinurids
as a whole maintain their position as the sister group to the Beli-
nurina/Limulina clade. Taxa are considered within the context of
their clades as defined here.
Lunataspidae and Fezouata xiphosurid.—Within the
Lunataspidae, adults of both Lunataspis aurora and
Lunataspis borealis have a thoracetron with six tergites
expressed axially but not laterally on the dorsal surface
(Fig. 2.1,2.2). Posterior to the thoracetron is a freely
articulating postabdomen comprising four segments, the last
of which is an elongated pretelson. Anterior to the thoracetron
are two freely articulating tergites as demarcated by clear,
deeply incised segmental boundaries across their entire
breadth, the thoracic doublure terminating before the two
anterior tergites, and the tergites being differentiated by an
anterolateral flexure (fulfilling criteria 4–6 for determining the
state of tergite articulation). An isolated juvenile thoracetron of
Lunataspis aurora (Fig. 2.3) also shows that these two tergites
can become disarticulated from the thoracetron (thereby
fulfilling the first criterion also). Articulated juvenile
specimens for Lunataspis aurora (see Rudkin et al., 2008) and
Lunataspis borealis (Fig. 2.4,2.5) demonstrate that these free
tergites are present in earlier ontogenetic stages. These
juveniles also exhibit lateral expression of the tergite
boundaries in the thoracetron, indicating that the effacement of
the lateral segment boundaries in the thoracetron occurs
during ontogeny in these species.
Study of a limited number of specimens (Fig. 2.6–2.11)of
the undescribed xiphosurid from the lower Ordovician of
Fezouata, Morocco, indicates marked ontogenetic change within
the thoracetron during the species’development. Adults exhibit
faint axial segmentation in the thoracetron (indicating the pres-
ence of maybe six segments; Fig. 2.9) with no evidence of lat-
erally expressed tergite boundaries. Anterior to these six
segments, the thoracetron possesses a dorsally inflected lip
that articulates directly with a flange on the posterior of the pro-
somal carapace (Fig. 2.7), although this articulation is generally
obscured in dorsal view by the overlapping of the carapace
posterior over the anterior regions of the thoracetron (Fig. 2.6).
Posterior to the thoracetron are freely articulating postabdominal
segments. In juveniles the thoracetron clearly expresses segment
boundaries both axially and laterally (Fig. 2.8) with the postab-
domen shown to comprise four segments. The anterior of the
opisthosoma is again obscured dorsally by the prosomal cara-
pace posterior; however, one specimen shows two deeply
incised anterior tergites posterior to the prosomal carapace
(Fig. 2.10), and another preserved in an oblique dorsolateral
view shows the presence of two freely articulating tergites anter-
ior to the thoracetron (Fig. 2.11). The free nature of these tergites
is demonstrated by flexure of the tergites and clear, deeply
incised anterior and posterior boundaries (criteria two and
four). These tergites appear absent in adults, and so it seems
that the species undergoes a loss of the anterior free tergites
and the lateral expression of segment boundaries in the thorace-
tron alongside the development of the prosomal flange and
anterior thoracetron lip over the course of its ontogeny.
Kasibelinuridae.—Kasibelinurus amicorum,Pickettia
carterae, and Patesia randalli all exhibit a similar thoracetron
structure. The thoracetron of Kasibelinurus amicorum
comprises six segments expressed both axially and laterally
with an enlarged pretelsonic unit incorporated into the
thoracetron to their posterior (Fig. 3.1). These segments are all
considered fused because of the shallow nature of the
segmental boundaries and the lack of gaps between the distal
pleural regions. Anterior to the thoracetron are two freely
articulating tergites, determined as such by the deeper incision
of the segment boundaries and the presence of articulation
facets on the pleurae (criteria four and six, with the third
criterion filled indirectly). A small anterior projection of the
first free tergite is also present, mostly obscured by the
posterior of the prosomal carapace.
Pickettia carterae possesses a thoracetron comprising six
segments with boundaries expressed axially and laterally and
an elongated pretelsonic unit incorporated into its posterior
(Fig. 3.2). Fusion of the thoracetron is indicated by the presence
of a continuous ventral doublure and the shallow nature of the
segment boundary incisions. Two freely articulating tergites
are located in front of the thoracetron, their articulating nature
indicated by deeply incised anterior and posterior boundaries,
their separation from the thoracetron doublure, and differenti-
ation of the sclerites through enlarged pleurae bearing articulat-
ing facets and an inflated axial region (fulfilling criteria 4–6).
Once again, a small projection from the first tergite is observed
partially covered by the prosomal carapace. A similar opisthoso-
mal configuration is observed in Patesia randalli (Fig. 3.3–3.6).
The thoracetron—again comprising six segments and an
enlarged pretelson—is indicated to be fused through the shallow
divisions between segments and the lack of axial flexion within
the thoracetron in enrolled specimens. The boundaries of the
thoracetron segments are expressed axially and laterally. Two
freely articulating tergites are located anterior to the thoracetron.
These segments show clear axial flexure, facilitating the partial
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enrollment of individuals, the anterior and posterior boundaries
of the two anterior tergites are deeply incised, and the tergites are
differentiated from those of the thoracetron by inflation of the
axis and pleural articulating furrows (criteria 2, 4, and 6). Data
for earlier ontogenetic stages are currently unknown for any of
these species.
Belinurina.—Belinurines exhibit significant variation in the
thoracetron. The morphology in basal forms, as exemplified
by Belinurus trilobitoides (Fig. 4.1–4.3), comprises seven
segments with their boundaries expressed both axially and
laterally. Anterior to the thoracetron, a single free tergite may
be present (Fig. 4.3), indicated by a deeper expression of the
segment boundaries and a change in angle of the pleurae
(criteria 4 and 6). Juvenile individuals exhibit a greater
number of free tergites—up to three—in front of the
thoracetron. The freely articulating nature of these tergites is
demonstrated by their occurrence attached to the posterior of
the prosomal carapace in some specimens where the
thoracetron has become detached (Fig. 4.1), flexure of the
axial region and overlap of the pleurae in specimens
exhibiting enrollment (Fig. 4.2), and clear and deeply incised
anterior and posterior segment boundaries (criteria 1–4). The
thoracetron of juveniles with an increased number of free
tergites comprises six segments, indicating that the reduction
of free tergites in adults occurs via the incorporation of at least
one of these tergites into the thoracetron during ontogeny.
Adults also exhibit a posterior prosomal flange that appears
absent in juveniles.
Euproops (Fig. 4.4–4.9) possess a thoracetron consisting of
seven segments, the boundaries of which are expressed axially
and laterally. Anterior to the thoracetron is a microtergite or
anterior projection that appears to be fused onto the thoracetron
in larger adults (Figs. 4.8,5.4) but is freely articulating in smaller
adults or subadults (Figs. 4.7,4.9,5.2,5.3). Adults also possess
a posterior flange to the prosomal carapace that becomes more
heavily sclerotized laterally in a manner similar to the anterior-
most tergite of the thoracetron and may be free of the carapace at
its extremities (Fig. 5.2–5.4). Juveniles exhibit a broader free ter-
gite as opposed to a microtergite (Figs. 4.6,5.1) that is clearly
differentiated from the rest of the thoracetron by a deep furrow,
with the sclerite separated from the doublure and the pleurae
deflected anteriorly (fulfilling criteria 4–6). The articulation of
the microtergite in subadults is demonstrated by the degree of
enrollment observed in subadults (Fig. 6.1,6.3) compared
with the partial enrollment in adults (Fig. 6.2). While the
extreme folding in the smaller specimens is likely due in part
to compression, the cross-sectional view of the microtergite in
combination with the thoracetron oriented parallel to the proso-
mal carapace indicates that the sclerite must have an articulation
with both the prosoma and thoracetron.
Among the more-derived belinurines, Liomesaspis laevis
has a thoracetron comprising seven segments that are expressed
only axially (Fig. 4.10); no freely articulating tergites are appar-
ent in any specimens of the species. Alanops magnificus also
lacks freely articulating tergites, with the thoracetron articulating
with the prosoma via a microtergite thatis fully fused to the thor-
acetron anterior. The thoracetron itself does not express any seg-
ment boundaries and exhibits an undifferentiated axial region
(Fig. 4.11); this highly effaced condition is observed in the earli-
est known instars and maintained throughout the species’
ontogeny (Racheboeuf et al., 2002).
Rolfeiidae and Bellinuroopsis.—These basal Limulina exhibit a
similar overall thoracetron structure, although with some key
differences. Bellinuroopsis rossicus Chernyshev, 1933 is
interpreted as possessing a fused thoracetron consisting of at
least five and potentially six segments, the last of which is an
enlarged pretelson that may or may not be freely articulating.
The fusion of the thoracic segments is determined by the fact
that the boundaries between these segments in the lateral
regions of the thoracetron express as fused, raised ridges rather
than incisions. The pleural nodes on these segments also
occur across the boundaries, which also indicates that these
segments did not articulate (Fig. 7.1). Anterior to the
thoracetron are two tergites that are here considered to be
freely articulating, as indicated by the partial overlap of the
pleurae, the deeply incised anterior and posterior segment
boundaries, and the differentiation of the tergites in lacking
pleural nodes (criteria 3, 4, and 6).
Rolfeia fouldenensis has a thoracetron composed of seven
fused segments (Waterston, 1985), including an elongated pre-
telson, which express their segment boundaries both axially
and laterally. A single freely articulating tergite is located anter-
ior to the thoracetron (Fig. 7.2), marked as such by the deeply
incised boundary between the tergite and the thoracetron, the
separation of the tergite from the thoracetron doublure, and the
differentiation of the pleural region of the tergite—which
forms an elongated projection that may be homologous to the
free lobes of other taxa—from those within the thoracetron
Figure 2. Representatives of Lunataspidae and an undescribed Moroccan Ordovician taxon. (1, 3)Lunataspis aurora;(2, 4, 5)Lunataspis borealis;(6–11) Unde-
scribed xiphosurid species. (1)Lunataspis aurora (MM I-4583), Upper Ordovician (Katian), Churchill River Group, Canada, adult or subadult. (2)Lunataspis bor-
ealis (ROM IP 64616), Upper Ordovician (Sandbian), Gull River Formation, Canada, adult or subadult. (3)Lunataspis aurora (MM I-3990), Upper Ordovician
(Katian), Churchill River Group, Canada, juvenile thoracetron. (4)Lunataspis borealis (ROM IP 64617), Upper Ordovician (Sandbian), Gull River Formation, Can-
ada, juvenile. (5)Lunataspis borealis (ROMIP 64618), Upper Ordovician (Sandbian), Gull River Formation, Canada, juvenile. (6) Undescribed xiphosurid (YPM IP
526014), Lower Ordovician (Floian), Fezouata Formation, Morocco, adult. (7) Undescribed xiphosurid (YPM IP 532152), Lower Ordovician (Floian), Fezouata
Formation, Morocco, adult. (8) Undescribed xiphosurid (YPM IP 530781), Lower Ordovician (Floian), Fezouata Formation, Morocco, juvenile. (9) Undescribed
xiphosurid (YPM IP 526014), Lower Ordovician (Floian), Fezouata Formation, Morocco, detail of adult thoracetron. (10) Undescribed xiphosurid (YPM IP
531837), Lower Ordovician (Floian), Fezouata Formation, Morocco, large juvenile showing the two free tergites under the prosomal carapace posterior. (11) Unde-
scribed xiphosurid (YPM IP 531656), Lower Ordovician (Floian), Fezouata Formation, Morocco, juvenile lateral view. Lunataspids possesstwo free tergites anterior
to the thoracetron, with juveniles exhibiting both axial and lateral expression of the tergites within the thoracetron (3–5) while the adults expresstergites within the axis
only (1, 2). The undescribed Lower Ordovician xiphosurid exhibitstwo anterior free tergites and fully expressed tergites within the thoracetron (10, 11). In adults, the
first free tergite appears to be incorporated into the prosomal carapace as a prosomal flange while the second tergite fuses onto the thoracetron as an anterior lip (7).
The thoracetron itself greatly reduces the tergite expression, so they are only faintly visible in the axis (9). In both juveniles and adults, the degree of overlap between
the prosoma and thoracetron generally obscures the articulation (6, 8). Scale bars = 1 mm.
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(criteria 4–6). Data for earlier ontogenetic stages are currently
unknown for either of these species.
Paleolimulidae.—Despite being a relatively small group,
paleolimulids exhibit a degree of variation within the
thoracetron. The basalmost forms, exemplified here by
Norilimulus woodae (Lerner et al., 2016), express segmental
boundaries both axially and laterally in the thoracetron. The
thoracetron comprises at least five segments, although the
posterior margin is not preserved and so it is unclear whether
a pretelson is present. Two anterior segments show
differentiation in having deeply incised anterior and posterior
boundaries, especially in the axis, and in the angle of the
pleural margins, which are inflected anteriorly (Fig. 8.2). This
may indicate that these segments were freely articulating in
front of the thoracetron, although the first tergite is overlapped
Figure 3. Representatives of Kasibelinuridae. (1)Kasibelinurus amicorum (AM F68969), Upper Devonian (Famennian), Mandagery Sandstone, Australia, pre-
sumed adult or subadult. (2)Pickettia carterae (BMS E9644), Upper Devonian (Famennian), Cattaraugus Formation, Pennsylvania, USA, presumed adult or sub-
adult. (3–6)Patesia randalli:(3)Patesia randalli (FMNH PE56581), Upper Devonian (Famennian), Chadakoin Formation, Pennsylvania, USA, apparent adult or
subadult; (4)Patesia randalli (USNM PAL 4845), Upper Devonian (Famennian), Chadakoin Formation, Pennsylvania, USA, apparent adult or subadult; (5)Patesia
randalli (FMNH PE57077), Upper Devonian (Famennian), Chadakoin Formation, Pennsylvania, USA, apparent adult or subadult; (6)Patesia randalli (FMNH
PE56589), Upper Devonian (Famennian), Chadakoin Formation, Pennsylvania, USA, apparent adult or subadult. All taxa exhibit a small sclerite located partially
underneath the prosomal carapace posterior and two free articulatingtergites anterior to the thoracetron. The thoracetronitself shows lateral and axial dorsal expression
of the tergites. Scale bars = 10 mm. (1) Reproduced from Bicknell and Pates (2020) under a CC BY 4.0 license.
Journal of Paleontology:1–208
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laterally by free lobes, which indicates the presence of a tergite
anterior to these two that is largely suppressed or incorporated
into the prosoma axially. Free lobes that are fused with and
overlap the succeeding segment are otherwise observed only
at the anterior of a fused thoracetron, and so it is unclear
whether these anterior segments in Norilimulus are freely
articulating or simply differentiated but otherwise fully
incorporated into the thoracetron (in which case the
thoracetron would comprise eight segments, including that of
the free lobes laterally). Another possibility is that the known
specimens of Norilimulus are those of juveniles or early
subadults, and that these anterior segments are freely
articulating but fuse into the thoracetron over the course of
their later ontogeny.
Xaniopyramus linseyi Siveter and Selden, 1987, a member
of the group that resolves as the sister clade to Paleolimulus,
expresses segmental boundaries both axially and laterally within
the thoracetron. Longitudinal pleural ridges running the length
Figure 4. Representatives of Belinurina. (1–3)Belinurus trilobitoides:(1)Belinurus trilobitoides (NMW 70.17G.9), Carboniferous (Bashkirian), South Wales
Lower Coal Measures Formation, Wales, UK, apparent adult or subadult, juvenile; (2)Belinurus trilobitoides (NMW 29.197.G1), Carboniferous (Bashkirian),
South Wales Lower Coal Measures Formation, Wales, UK, juvenile; (3)Belinurus trilobitoides (GIUS 5-845/7), Carboniferous (Moscovian), Orzesze Beds, Poland,
adult. (4–9)Euproops danae:(4)Euproops danae (YPM IP 255613), Carboniferous (Moscovian), Carbondale Formation, Illinois, USA, juvenile; (5)Euproops
danae (YPM IP 168054), Carboniferous (Moscovian), Carbondale Formation, Illinois, USA, juvenile; (6)Euproops danae (YPM IP 168032), Carboniferous (Mos-
covian), Carbondale Formation, Illinois, USA, juvenile; (7)Euproops danae (YPM IP 50754), Carboniferous (Moscovian), Carbondale Formation, Illinois, USA,
subadult or adult; (8)Euproops danae (YPM IP 428963), Carboniferous (Moscovian), Carbondale Formation, Illinois, USA, presumed adult; (9)Euproops danae
(YPM IP 168011), Carboniferous (Moscovian), Carbondale Formation, Illinois, USA, subadult or adult. (10)Liomesaspis laevis (YPM IP 18050), Carboniferous
(Moscovian), Carbondale Formation, Illinois, USA, presumed adult. (11)Alanops magnificus (MNHN SOT 1951), Carboniferous (Kasimovian), Great Seams For-
mation, France, adult. Basal-most belinurines as exemplified by Belinurus exhibit three freely articulating tergites in front of the thoracetron (1–3), which display both
axial and lateral expression of the constituent tergites. The thoracetron of Euproops also expresses individual tergites axially and laterally, with juveniles exhibiting an
upwardly inflected anterior lip of the thoracetron (4, 5) and an anterior free tergite (6). More-mature Euproops develop a broad posterior prosomal flange that resem-
bles a tergite fused to the prosomal carapace, the lateral margins of which are more differentiated than the axial region (8), while the anterior free tergite reduces to a
microtergite (9) and eventually fuses into the thoracetron (7). More-derived belinurines show a progressive decrease in tergite expression on the thoracetron, with
Liomesaspis having tergites expressed only in the axis (10) and Alanops having a completely undifferentiated axial region devoid of tergite expression (11).
Scale bars = 5 mm. (3) Reproduced from Bicknell and Pates (2020) under a CC BY 4.0 license.
Lamsdell and Ocon—The evolution of the xiphosurid thoracetron 9
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of the thoracetron, punctuated by pleural nodes that occur across
the segment boundaries, indicate that all visible segments were
fully fused into the thoracetron (Fig. 8.1). The posterior of the
thoracetron is not preserved, so it is unclear whether a pretelso-
nic sclerite was present either fused to the thoracetron or articu-
lating. The anteriormost segment of the thoracetron is produced
laterally into free lobes derived from the lateral portions of an
anterior segment that is either incorporated in the prosoma or
largely suppressed axially.
Paleolimulus signatus exhibits change in the thoracetron
over the course of its ontogeny. Juveniles possess a thoracetron
composed of seven segments, including free lobes, which are
expressed both laterally and axially (Fig. 8.3). Posterior to the
thoracetron is an elongated pretelson that is freely articulating
as indicated by its occasional disarticulation from the thorace-
tron, the clear and deeply incised boundary between the two
sclerites, and the total separation of the pretelson from the thor-
acetron doublure (criteria 1, 4, and 5). Adults also possess a
thoracetron of seven segments with free lobes; however, the seg-
ment boundaries are greatly effaced and clearly expressed only
axially (Fig. 8.4). The pretelsonic sclerite is less clearly defined
in adults and may become fused to the thoracetron during devel-
opment, although the presently available material of the species
does not show this with clarity.
Austrolimulidae.—Austrolimulids also exhibit interspecific
variation in the thoracetron. Boeotiaspis longispinus,an
apparent basal form, has a thoracetron comprising eight
segments that are expressed only axially (Fig. 9.3), the
posteriormost of which is a pretelson that is somewhat more
differentiated than the preceding segments. No free lobes are
visible on the thoracetron, and the anteriormost segment has
more deeply incised anterior and posterior margins as well as
anteriorly inflected pleural margins, indicating that it may
potentially be freely articulating (criteria 4 and 6). These traits
are unusual and potentially unique among Austrolimulidae,
however, and Boeotiaspis may have affinities outside of the
clade (see Discussion). Tasmaniolimulus pattersoni Bicknell,
2019 is more representative of the general morphology among
basal species, with a thoracetron comprising at least six
segments (the posterior margin is not preserved). The
segmental boundaries are expressed axially but not laterally on
the thoracetron, and free lobes are present at the anterolateral
margins (Fig. 9.1).
Among more-derived austrolimulids, as exemplified here
by Vaderlimulus tricki, the thoracetron lacks clear dorsal seg-
ment demarcations in either the axial or pleural regions,
although the presence of small moveable spines indicates
that the thoracetron is composed of six segments and a pretel-
son that is fully incorporated into the thoracetron doublure
(Fig. 9.2). Free lobes are also present on the thoracetron, indi-
cating the presence of an additional segment that is suppressed
or partially incorporated into the prosoma axially. Another
derived austrolimulid, Austrolimulus fletcheri,islesswell
preserved, so the details of the thoracetron are somewhat ten-
tative. The thoracetron is largely devoid of visible segmenta-
tion and does not preserve either fixed or movable spines, so
the number of segments within the thoracetron is unknown.
Free lobes are apparent at the thoracetron anterior, however,
as is an elongated pretelson with an apparent anterior demar-
cation where it connects to the thoracetron, although it is
unclear whether the pretelson was freely articulating or
fused onto the thoracetron.
Limulidae.—The overall morphology of the thoracetron is
consistent across Limulidae as demonstrated by exemplars
across both the major constituent clades. Mesolimulus walchi
possesses a thoracetron lacking tergite demarcations with free
lobes at its anterior (Fig. 10.1). This thoracetron morphology
is shared in the freshwater representative Victalimulus
mcqueeni (Fig. 10.2). The presence of apodemes and movable
spines indicates the thoracetron to comprise six segments in
addition to the free lobes and a fused pretelson composed of
an undetermined number of segments. Tachypleus syriacus,
representative of the other major limulid clade, exhibits a
fundamentally identical thoracetron composition (Fig. 10.3).
Limulus polyphemus (Linnaeus, 1758) demonstrates the
Figure 5. Detailed view of the prosomal/opisthosomal joint in Euproops
danae.(1) YPM IP 168032, juvenile exhibiting free tergite. (2) YPM IP
50754, subadult or adult exhibiting microtergite and prosomal flange. (3)
YPM IP 168011, subadult or adult exhibiting microtergite and prosomal flange.
(4) YPM IP 428963, presumed adult exhibiting microtergite and prosomal
flange. Scale bars = 5 mm.
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ontogeny of the group, with pre-hatchling embryos first
developing a topographic expression of segments without
clear boundaries and with no differentiated free lobes or
defined axial region (Fig. 10.4) before developing clearly
demarcated tergites in the axis (Fig. 10.5). Post-hatchling
instars first exhibit both lateral and axial tergites expression as
well as clear free lobes (Fig. 10.6) before progressively
effacing first the lateral tergite boundaries (Fig. 10.7) and later
the axial tergite boundaries as the thoracetron matures into the
typical adult morphology (Fig. 10.8).
Discussion
The thoracetron is recognized to have originated once early in the
evolution of Xiphosura; however, subsequent convergent
evolution of the thoracetron morphology is prevalent across the
clade (Fig. 11). When the Fezouata species is considered to
represent the sister taxon to all other xiphosurans (suggested by
the Early Ordovician age of the taxon and the lack of clear lateral
eyes or raised cardiac lobe on the prosomal carapace), lateral ter-
gite expression is considered to be suppressed in the xiphosuran
ancestor, as indicated by their absence in adults within Lunataspi-
dae and the Fezouata taxon. Under this scheme, xiphosurids are
considered to have undergone a reversal in expression of the thor-
acetron lateral segment boundaries that occurred before the diver-
gence of Kasibelinuridae from the main xiphosurid lineage.
Alternatively, if the Fezouata taxon resolves in a clade with the
lunataspid species, the ancestral condition of the thoracetron is
for the segments to be fully expressed, and their lateral suppression
in the Ordovician species would represent another case of
convergence.
Figure 6. Specimens of Euproops exhibiting folding. (1)Euproops danae (YPM IP 50813), Carboniferous (Moscovian), Carbondale Formation, Illinois, USA. (2)
Euproops danae (YPM IP 50951) part and counterpart, Carboniferous (Moscovian), Carbondale Formation, Illinois, USA. (3)Euproops danae (YPM IP 50863) part
and counterpart, Carboniferous (Moscovian), Carbondale Formation, Illinois, USA. Smaller juveniles or early subadults (1, 3) exhibit a greater degree of enrollment,
with the axis of the thoracetron compressed directly against the prosomal carapace indicating that the thoracetron is lying directly beneath the prosoma with the telson
projecting anteriorly. The microtergite and anterior margin of the thoracetron are visible end-on at the prosomal carapace posterior, however, suggesting that the thor-
acetron in juveniles may have had some anterior flexibility. Larger subadults or adults are preserved only partially enrolled (2) and preserve the main body of the
thoracetron in line with the microtergite. Scale bars = 5 mm.
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The xiphosurid thoracetron exhibits convergent patterns of
evolution between Belinurina and Limulina as well as within
Limulina across Paleolimulidae, Austrolimulidae, and Limulidae;
suppression of the tergite boundaries laterally, suppression of the
tergite boundaries axially, incorporation of somite VII into the
prosoma, and incorporation of somite VIII into the thoracetron
are all interpreted to have occurred at least three separate times.
Suppression of the lateral segment boundaries occurs independ-
ently in Belinurina and Paleolimulidae and at the base of Limuloi-
dea, while suppression of the axial segment boundaries occurs in
Belinurina, Austrolimulidae, and the base of Limulidae. Somite
VII is interpreted as becoming fully incorporated intothe prosoma
in the Fezouata xiphosurid, within Belinurina, and before the
divergence of Rolfeiidae within Limuloidea, and somite VIII is
interpreted as being fully incorporated into the thoracetron in
the Fezouata xiphosurid and within Limulidae before the diver-
gence of Paleolimulidae.
The convergent changes within the thoracetron generally
follow a consistent pathway, with the lateral expression of seg-
mentation becoming suppressed before that of the axis, and
the somite of segment VII generally being incorporated into
the prosoma before segment boundaries begin to become
effaced. This pattern is seen clearly in Belinurina. Both Beli-
nurus (Filipiak and Krawczyn
ski, 1996) and Euproops (Ander-
son, 1994) exhibit full axial and lateral expression of the
segments; the lateral segment margins are largely effaced in
more-derived taxa such as Liomesaspis (Malz and Poschmann,
1993; Anderson, 1997). Subsequently, the axial segment bound-
aries are suppressed in Alanops (Rachebeouf et al., 2002) and
Prolimulus (Lustri et al., 2021). The tergite of somite VII is
considered to be free in Belinurus but incorporated into the pro-
soma of Euproops—with the pleural margins clearly delineated
at the prosomal carapace posterior (Fig. 4.8,4.9)—and all other
belinurines. Somite VIII is interpreted to be a free tergite in Beli-
nurus and a reduced microtergite fused onto the thoracetron in
Euproops (Anderson, 1994; Anderson and Selden, 1997) and
Alanops (Rachebeouf et al., 2002).
The trend in Limulina begins with the incorporation of
somite VII into the prosoma in Rolfeia (Waterston, 1985) and
continues with the full incorporation of the tergite of somite
VIII into the thoracetron before the divergence of Paleolimulidae
and Limuloidea. Within paleolimulids, the full segment bound-
aries are expressed in the basal taxa Moravurus (Pr
ibyl, 1967),
Xaniopyramis (Selden and Siveter, 1987), and Norilimulus (Ler-
ner et al., 2016) but are suppressed laterally in Paleolimulus
(Babcock et al., 2000; Bicknell et al., 2022; Naugolnykh and
Bicknell, 2022). The lateral segment boundaries are independ-
ently suppressed in Limuloidea, the earliest offshoot of
which—Valloisella—exhibits axial but not lateral tergite expres-
sion (Anderson and Horrocks, 1995). Limulidae subsequently
undergoes suppression of the axial tergite boundaries before
the divergence of the group, as evidenced by the lack of anyseg-
mental expression within the thoracetron across the clade,
including the most basal offshoots such as Yunnanolimulus
(Hu et al., 2017). The condition is ubiquitous among both the
large clade of stem limulids that includes Mesolimulus—as fur-
ther demonstrated by Guangyanolimulus (Hu et al., 2022) and
Ostenolimulus (Lamsdell et al., 2021)—and the extant species
with their close extinct relatives such as Crenatolimulus (Feld-
mann et al., 2011; Kin and Błaz
ejowski, 2014).
Figure 7. Representatives of the Limulina stem lineage. (1)Bellinuroopsis rossicus (CCMGE 1/3694), Upper Devonian (Famennian), Lebedjan Formation, Russia,
adult or mature subadult. (2)Rolfeia fouldenensis (NMS G.1984.67.1) part and counterpart, Carboniferous (Tournaisian), Cementstones Group, Scotland, UK, adult
or mature subadult. Bellinuroopsis exhibits lateral and axial expression of the tergites within the thoracetron and two anterior free tergites. While Rolfeia also displays
lateral and axial tergite expression, only the first tergite is freely articulating. Scale bars = 10 mm. (1) Reproduced from Bicknell and Pates (2020) under a CC BY 4.0
license. (2) Made available under a CC BY-NC-SA 3.0 license courtesy of the GB3D type fossils database.
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The axial segmentation of the thoracetron is lost independ-
ently in Austrolimulidae, as several more-basal forms such as
Tasmaniolimulus and Panduralimulus (Allen and Feldmann,
2005) exhibit axial segmentation, while derived forms such as
Vaderlimulus and Austrolimulus do not. The condition of seg-
mentation in the thoracetron of Psammolimulus is unclear; ori-
ginally interpreted as exhibiting no segmentation, the
thoracetron seems to have segmentation within the axial region
(Meischner, 1962). The distribution of axial segmentation in
austrolimulids may indicate it was lost multiple times, as Limu-
litella—the sister taxon to Psammolimulus—is definitively
devoid of segmentation (Błaz
ejowski et al., 2017; Klompmaker
et al., 2023). The exact affinities of Limulitella are uncertain,
however, as some species are potentially diagnosed from speci-
mens representing multiple different species, and the genus may
in actuality resolve outside of Austrolimulidae. Boeotiaspis,a
supposed basal form, is also unusual for an austrolimulid as it
potentially has a freely articulating tergite of somite VIII. Previ-
ous authors have suggested the species comprising the genus
may show affinities to Rolfeia (Anderson and Selden, 1997);
the species needs redescription. Other derived austrolimulid
taxa lacking segmentation—Batracholimulus (Hauschke and
Wilde, 1987) and Dubbolimulus (Pickett, 1984)—also have a
poorly defined axial region and may represent juveniles. None-
theless, these uncertainties do not invalidate the overall trend in
axial effacement within austrolimulids, or the fact that it
Figure 8. Representatives of Paleolimulidae. (1)Xaniopyramis linseyi (OUM E.03994), Carboniferous (Mississippian), Upper Limestone Group, England, UK,
apparent adult. (2)Norilimulus woodae (NSM 005GF045.374), Carboniferous (Tournasian), Horton Bluff Formation, Canada, potential adult. (3, 4)Paleolimulus
signatus:(3)Paleolimulus signatus (KUMIP 399929), Carboniferous (Gzhelian), Wood Siding Formation, Kansas, USA, juvenile; (4)Paleolimulus signatus
(KUMIP 399962), Carboniferous (Gzhelian), Wood Siding Formation, Kansas, USA, subadult or adult. More-basal taxa, such as Xaniopyramis and Norilimulus,
exhibit lateral and axial expression of the tergites within the thoracetron. Norilimulus also shows differentiation of the two anterior-most tergites byan anterior angling
of their lateral expression as in lunataspids, although it is unclear whether these tergites are freely articulating or whether they are an adult trait. More-derived taxa
within the clade, as exemplified by Paleolimulus, display clear lateral and axial expression of the tergites within the thoracetron in juveniles while the lateral tergite
expression is largely effaced in adults. Scale bars = 5 mm. (1) Made available under a CC BY-NC-SA 3.0 license courtesy of the GB3D type fossils database.
Lamsdell and Ocon—The evolution of the xiphosurid thoracetron 13
https://doi.org/10.1017/jpa.2024.31 Published online by Cambridge University Press

occurred independently from the same trend observed in
Limulidae.
Developmental mechanisms of opisthosomal fusion.—The
evolution of the thoracetron within Limulina broadly
recapitulates the development of the tagma as seen in modern
species. Our understanding of developmental mechanisms
underlying thoracetron evolution in fossil limulines is limited
by the paucity of detailed ontogenetic descriptions in the
literature. Although several extinct species are known from
multiple ontogenetic stages or subadult material (Gall, 1971;
Kin and Błaz
ejowski, 2014; Lamsdell and McKenzie, 2015;
Lamsdell, 2021a), most of our understanding of development
in limulines is based on modern representatives. Further
documentation of ontogeny in extinct taxa, especially that
focusing on descriptions of morphological change rather than
quantitative measurement-based surveys, would be
enlightening. In Limulus polyphemus and Tachypleus
tridentatus Leach, 1819, the two species for which
development is best known (Scholl, 1977; Sekiguchi et al.,
Figure 9. Representatives of Austrolimulidae. (1)Tasmaniolimulus patersoni (UTGD 123979), Early Triassic (Induan), Jackey Shale, Tasmania, Australia. (2)
Vaderlimulus tricki (NMMNH P-81445), Early Triassic (Olenekian), Thaynes Group, Idaho, USA, adult or mature subadult. (3)Boeotiaspis longispinus (ROM
IP 49769), Carboniferous (Bashkirian), Bear Gulch Limestone, Montana, USA, adult or mature subadult. (4)Austrolimulus fletcheri (AM F38274), Middle Triassic
(Anisian), Beacon Hill Shale, Australia, adult or mature subadult. More-basal taxa within the clade, such as Tasmaniolimulus and potentially Boeotiaspis, retain the
axial expression of tergites within the thoracetron. This is lost in more-derived taxa such as Austrolimulus and Vaderlimulus, the thoracetrons of which lack any
expression of the tergites. Scale bars = 10 mm. (1) Reproduced from Bicknell (2019) under a CC BY-NC-SA 4.0 license.
Journal of Paleontology:1–2014
https://doi.org/10.1017/jpa.2024.31 Published online by Cambridge University Press

1982,1988a,b; Sekiguchi, 1988; Shuster and Sekiguchi, 2003;
Farley, 2010; Haug and Rötzer, 2018b; Lamsdell, 2021a), the
embryo first develops faint segmentation through topographic
expression of the segments without incisions at the anterior or
posterior margins before developing a differentiated axial
region with the segment boundaries within the thoracetron
being more deeply incised. After hatching, the larval
horseshoe crabs undergo an effacement of first the lateral and
later axial segment margins within the thoracetron. The
segment of somite VII and the axial region of somite VIII are
incorporated into the prosoma during the embryonic stage,
with the lateral portions of VIII forming the free lobe on the
thoracetron.
At the base of Limulina, both somites VII and VIII are con-
sidered to be freely articulating anterior to the thoracetron but
soon become integrated into the prosoma and thoracetron as in
the extant species. The lateral tergite expression is then sup-
pressed independently in Paleolimulidae and Limuloidea, with
the axial tergite expression subsequently suppressed independ-
ently in Austrolimulidae and Limulidae. This progression
along the developmental trajectory seen in modern taxa fits
with the general peramorphic heterochronic trend observed in
these clades, which is most strongly expressed in Austrolimuli-
dae (Lamsdell, 2021a). Paleolimulus, for which juvenile speci-
mens are known, exhibits a similar developmental trajectory to
the modern species, including a weakly expressed axis and
lack of deep incisions at the segment boundaries of larval
forms. The juveniles exhibit a freely articulating pretelson, how-
ever, and maintain the lateral segment boundaries well into sub-
adulthood (Naugolnykh and Bicknell, 2022). Juveniles of
Mesolimulus, meanwhile, have already effaced the tergite
boundaries in the thoracetron (Lamsdell et al., 2020b); these
species are also larger than those of Paleolimulus, suggesting
that these morphological changes may have been achieved by
either lengthening the time or increasing the rate of
development.
Figure 10. Representatives of Limulidae. (1)Mesolimulus walchi (MNHN F.A33516), Upper Jurassic (Tithonian), Altmühltal Formation, Germany, adult. (2)
Victalimulus mcqueeni (NMV P22410B), Early Cretaceous (Aptian), Korumburra Group, Australia, adult. (3)Tachypleus syriacus (NHMUK IA 188), Late Cret-
aceous (Cenomanian), Hjoûla Konservat-Lagerstätten, Lebanon, adult. (4–8)Limulus polyphemus:(4)Limulus polyphemus, recent, United States of America, pre-
hatchling larva imaged under SEM; (5)Limulus polyphemus, recent, United States of America, later pre-hatchling larva; (6)Limulus polyphemus, recent, United
States of America, fourth-molt hatchling; (7)Limulus polyphemus (YPM IZ 070174), recent, United States of America, twelfth-molt juvenile; (8)Limulus polyphe-
mus, recent, United States of America, adult. Adults across all taxa possess a thoracetron lacking tergite demarcations; however, the earlier developmental stages of
Limulus exhibit varying degrees of tergite expression within the thoracetron, with the pre-hatchling phase showing lateral expression of the tergites, the hatchling
showing an undifferentiated thoracetron without a clear axis, and the juvenile exhibiting axial tergite expression. (1–3, 8) Scales bars = 10 mm; (4) scale bar =
100 μm; (5–7) scale bars = 1 mm. (1) Made available as part of the RECOLNAT (ANR-11-INBS-0004) program.
Lamsdell and Ocon—The evolution of the xiphosurid thoracetron 15
https://doi.org/10.1017/jpa.2024.31 Published online by Cambridge University Press

The pattern of thoracetron consolidation through ontogeny
is maintained basally within Belinurina. Belinurus appears to
exhibit a more-extreme degree of anterior tergite fusion into
the thoracetron over the course of its development, with poten-
tially as many as three free tergites being progressively incorpo-
rated into the thoracetron beginning with the posteriormost. This
is interesting in that it provides a mechanism by which the pos-
terior “pygidium”of the Silurian basal prosomapod Offacolus
(Sutton et al., 2002) could develop into the xiphosurid thorace-
tron and is diametrically opposite to the ontogeny of trilobites,
which develop and release segments from the pygidium as
they grow (Hughes et al., 2006). The earliest available instars
of Euproops have two free tergites between the prosoma and
thoracetron; later instars have a free tergite and small sclerite
(microtergite) located posterior to the prosoma. By adulthood,
these two tergites are incorporated into the prosoma and opistho-
soma, although incompletely as indicated by the free lateral
margins of segment VII within the prosomal carapace. The ter-
gite of somite VIII decreases in size progressively to a reduced
tergite then microtergite before suturing onto the thoracetron in
adults. Euproops also exhibits a reduced degree of enrollment as
it matures (Fig. 6), likely due to the progressive fusion of free
segments into the prosoma and thoracetron through develop-
ment. This could potentially be the explanatory mechanism
behind the variable types of enrollment seen in Euproops (see
Anderson, 1994; Anderson and Selden, 1997; Haug et al.,
2012) and could indicate some degree of ontogenetic niche par-
titioning. It is unclear from the description of the highly derived
belinurine Alanops whether juveniles exhibit any free tergites as
the description focused on adult morphology (Rachebeouf et al.,
2002). The species is known from some 140 specimens, includ-
ing juveniles, however, and an evaluation of the earlier instars
would be illuminating. It is also unclear whether juvenile Ala-
nops exhibit any clear segmental boundaries within the
Figure 11. Phylogeny of Xiphosura derived from analysis of the character matrix as described in the Materials and methods section, strict consensus of two most
parsimonious trees with clades collapsed for ease of display. Character transitions for the thoracetron in adults are shown. Where multiple transitions are shown within
a collapsed clade, they are presented in the order in which they occur. Widespread convergence in thoracetron evolution is apparent, interpreted as being driven by
developmental parallelism. Incorporation of somite VII into the prosoma and somites VIII and IX into the thoracetron is interpreted as having occurred independently
in the Fezouata xiphosurid, Belinurina, and Limulina; suppression of the tergites laterally in the thoracetron occurs independently in Belinurina, Paleolimulidae, and
Limuloidea; and the suppression of tergite expression within the thoracetron axis occurs independently in Belinurina, Austrolimulidae, and Limulidae. Importantly, a
single reversal is inferred near the base of Xiphosura in the expression of tergites laterally within the thoracetron, which likely represents the retention of juvenile traits
into adulthood (pedomorphosis).
Journal of Paleontology:1–2016
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thoracetron. Belinurus and Euproops exhibit axial and lateral
segment boundaries within the thoracetron throughout their
development, and juvenile material has not been reported from
any derived belinurine aside from Alanops.Liomesaspis exhi-
bits axial but not lateral segmental expression in adults, although
some species do exhibit faint evidence of lateral segmentation in
the form of undulations or topographic expression (e.g., Tasch,
1961), while Prolimulus and Alanops have the axial as well as
the lateral segment margins effaced, although in Alanops faint
evidence of the axial segmentation may sometimes be seen in
the form of topographic expression (Rachebeouf et al., 2002),
in a manner similar to the lateral segmentation in Liomesaspis.
The independent development of similar forms or struc-
tures in organisms is known as convergent evolution (McGhee,
2011; Pearce, 2012). Convergent evolution may be the result of
distinct developmental or genetic pathways—sometimes result-
ing in the co-option of different aspects of anatomy to develop
similar structure—or the independent expression of the same
developmental or genetic pathways in (usually closely related)
lineages (McGhee, 2011; Pearce, 2012). When convergent
change has a shared developmental or genetic cause, it is
referred to as parallelism, or parallel evolution (Arendt and
Reznick, 2008; Scotland, 2011; Hall, 2012). Critically, while
convergence is generally considered to be an adaptational
response to similar environmental or mechanical pressures, par-
allel evolution may be the result of developmental (as opposed
to functional) constraints (Mahler et al., 2017). Developmental
constraints may also shape broad heterochronic trends within a
lineage, so it is possible for a suite of morphological characters
to exhibit nonselective parallel patterns of evolution due to
selection favoring a heterochronically derived condition within
a single trait. Differentiating between convergence and parallel-
ism in closely related taxa—such as is the case here—therefore
requires an assessment of not only whether changes are due to a
shared developmental pathway, but also whether these changes
progress along the same trajectory within those pathways (i.e.,
whether the observed changes are consistently pedomorphic or
peramorphic in nature). Within Paleolimulidae, Austrolimuli-
dae, and Limulidae, the effacement of the thoracetron segment
boundaries progresses along the general developmental trajec-
tory that remains relatively conserved within Xiphosura (Lams-
dell, 2021a; Bicknell et al., 2022; Lamsdell et al., 2023), fitting
with the prevalence of peramorphic changes within these
lineages (Lamsdell, 2021a,b) and indicating that the shared
changes observed within the thoracetron represent cases of par-
allel evolution within these groups. The evolutionary changes in
the thoracetron of Belinurina appear to follow the same trend in
reducing the expression of the lateral followed by the axial
boundaries and so at first may appear to represent another case
of parallelism within the group; however, belinurines are
known to exhibit a pedomorphic heterochronic trend (Lamsdell
2021a,b; Lustri et al., 2021) as opposed to the general pera-
morphic condition across Limuloidea. As noted in the preceding
discussion, belinurines with largely suppressed segmental
boundaries may still exhibit a topographic expression of the seg-
ments as seen in the earliest larvae of modern limulids. Belinur-
ina are therefore interpreted here as exhibiting a retrograde trend
along the common developmental trajectory, as extant larvae
develop incised margins along the axis before incisions develop
in the lateral segments. The pedomorphic nature of the thorace-
tron is further supported by the reduced number of opisthosomal
opercula in Alanops, which is a condition also seen in larval
extant limulids that increase the number of opercula in succes-
sive molts up to their full complement (Dunlop, 1998). The
extreme effacement of the prosomal carapace of Alanops,Proli-
mulus, and Stilpnocephalus is also a condition seen only in lar-
vae; that these represent adult individuals with larval traits rather
than larvae or juveniles themselves is confirmed by the large size
of Stilpnocephalus (Selden et al., 2019). The progressive efface-
ment of thoracetron segmentation in belinurines is therefore the
result of truly independent convergence rather than parallelism.
Conclusions
The xiphosuran thoracetron is present within the earliest known
representatives of the group (Anderson and Selden, 1997; Lams-
dell, 2013,2020) and therefore is a key synapomorphy of the
clade. Previous suggestions that the thoracetron originated inde-
pendently in disparate xiphosurid clades are here disproven;
however, convergent and parallel trends are present within the
subsequent development of this functional pseudotagma. Evolu-
tionary trends in xiphosurids are, however, obfuscated by a var-
iety of biotic and abiotic processes; these challenges reinforce
the critical necessity for a consideration and understanding of
both taphonomy and ontogeny when interpreting fossils. Further
documentation of ontogeny in fossil Xiphosura is integral to our
understanding of developmental macroevolutionary mechan-
isms. Presently, the identification of parallelism and conver-
gence within the development of the thoracetron by
progression and retrogression along a shared developmental tra-
jectory represents an important case study into the significance
of developmental macroevolutionary mechanisms in morpho-
logical evolution and provides further support for the role of het-
erochronic processes in xiphosuran evolution. This work also
further elucidates the highly conserved developmental pathway
within Xiphosura and the heterochronic push and pull toward
endmember morphologies along the same shared pathway of
development while still conserving the overall trajectory.
Finally, it is interesting to note that juveniles of the
Fezouata xiphosurid have two freely articulating tergites anterior
to a fused thoracetron, whereas adults possess a prosomal flange
and anterior lip on the thoracetron. These features are possibly
formed from the fusion of the two freely articulating segments
seen in juveniles onto the posterior edge of the prosoma and
anterior edge of the thoracetron, respectively, paralleling the
fusion of somites VII and VIII to the prosoma and thoracetron
during the development of modern Limulus embryos (Scholl,
1977). Somite VII is considered to be represented by a freely
articulating tergite in kasibelinurids and basal members of
both Belinurina and Limulina, which would indicate its consoli-
dation has occurred independently a number of times within
Xiphosura. Expanding to view Chelicerata more generally,
somite VII is considered to be consolidated into the prosoma
in arachnids and potentially eurypterids (Dunlop, 1998), while
it is retained as an opisthosomal microtergite in chasmataspidids
(Marshall et al., 2014); the incorporation of somite VII into the
prosoma is clearly a somewhat plastic trait among basal
Lamsdell and Ocon—The evolution of the xiphosurid thoracetron 17
https://doi.org/10.1017/jpa.2024.31 Published online by Cambridge University Press

chelicerates, and this realization should be enlightening for dis-
cussions regarding the ground pattern of the clade and whether
the consolidation of somite VII into the prosoma in the stem che-
licerate Mollisonia (Aria and Caron, 2019) represents the plesio-
morphic condition for the group.
Acknowledgments
We thank G. Young for photographs of MM I-4583 and MM
I-3990, R. Schmidt for the photograph of NMV P22410B,
M. McCurry for the photograph of AM F38274, A. Lerner for
photographs of NSM 005GF045.374 and NMMNH P-81445,
K. Page for the photograph of BMS E857, and D. Rudkin for
the photograph of ROM IP 45851. J. Dunlop and an anonymous
referee are thanked for reviewing the manuscript. The software
TNT was made available with the sponsorship of the Willi Hen-
nig Society. This research was funded by National Science
Foundation CAREER award EAR-1943082 “Explaining Envir-
onmental Drivers of Morphological Change through Phylogen-
etic Paleoecology”to J.C. Lamsdell.
Declaration of competing interests
The authors declare no competing interests.
Data availability statement
Data available from the Dryad Digital Repository: http://doi.org/
10.5061/dryad.2fqz612z2
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