Content uploaded by Richard Alan Fortey
Author content
All content in this area was uploaded by Richard Alan Fortey on Jan 08, 2019
Content may be subject to copyright.
Trilobites of the genus Dikelokephalina from Ordovician Gondwana and
Avalonia
RICHARD A. FORTEY*
Department of Palaeontology, The Natural History Museum, London, UK
The trilobite Dikelokephalina is widespread across the Tremadocian Gondwana, Avalonia and Baltica palaeocontinents. The hitherto poorly
known Anglo-Welsh species D. furca (Salter, 1866) is revised on the basis of new and well-preserved material. A new species, D. brenchleyi
sp. nov., is described from the Anti-Atlas, Morocco. The two species together provide a fuller understanding of the genus, and of the family,
Dikelokephalinidae (Kobayashi, 1934), to which it has been assigned. Suggestions that this family may be a junior synonym of Hungaiidae
(Raymond, 1924) are considered premature. D. brenchleyi sp. nov. apparently shows variation in mature segment number, a most unusual
feature in post-Cambrian trilobites. It is found in monospecific assemblages of large individuals which lack ventral median sutures, and a case
is made that these individuals represent a mass gathering of individuals that have ceased to grow, for the purpose of fertilizing eggs,as happens
in living Limulus. Copyright #2010 John Wiley & Sons, Ltd.
Received 29 January 2010; accepted 10 August 2010
KEY WORDS Ordovician; trilobites; mating; taphonomy; segmentation; classification
1. INTRODUCTION
The early Ordovician trilobite Dikelokephalina was wide-
spread across the former Gondwana supercontinent stretch-
ing from South China to Morocco, although the type species
was originally described from Ordovician Baltica. The first
species to be named was Dikelokephalina furca (Salter,
1866) from southern Britain. England and Wales are thought
to have been in the process of rifting off from the Gondwana
supercontinent by the Tremadocian as part of the Avalonia
microcontinent (Cocks and Fortey, 2009). D. furca has
hitherto been known from somewhat inadequate material,
but several better-preserved fossils allow the species to be
considerably clarified herein. In recent years spectacular
large, articulated specimens of another Dikelokephalina
species from Morocco have been appearing on the open
market. This is named as a new species in this paper. The
author has recently named a large asaphoid, Asaphellus
stubbsi (Fortey, 2009), from the same locality as the
Moroccan Dikelokephalina that shows similarly interesting
features in its field occurrence. These are briefly considered
in this paper. Furthermore, Dikelokephalinidae has been
considered a separate trilobite family (e.g. Moore, 1959),
including five genera according to Lu (1975), and a modern
appraisal of Dikelokephalina is evidently relevant to
suggestions (e.g. Ludvigsen et al., 1989) that Dikelokepha-
linidae should be synonymized with Hungaiidae. One of the
relatives of Dikelokephalina,Asaphopsis, has been used to
label most of what would now be recognized as the
Ordovician Gondwana supercontinent (Asaphopsis Pro-
vince), so this family as a whole is entrenched in notions of
Ordovician palaeobiogeography. Finally, Pat Brenchley
has contributed greatly to our understanding of Ordovician
clastic sedimentation over this former continent, and the
new Moroccan species of Dikelokephalina is named
appropriately to recognize this.
2. OCCURRENCE OF LARGE
DIKELOKEPHALINIDS IN MOROCCO
The large Tremadocian trilobites found in the Fezouata
Formation of the Anti-Atlas in Morocco are frequently found
in monospecific assemblages of similarly sized individuals.
I discussed this recently in relation to Asaphellus stubbsi
(Fortey, 2009) and the same is true of Dikelokephalina
brenchleyi described below. Many of these trilobites are
preserved on slabs that are for sale on the open market, and
unfortunately relatively few of these are available for
GEOLOGICAL JOURNAL
Geol. J. (2010)
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/gj.1275
* Correspondence to: R. A. Fortey, Department of Palaeontology, The
Natural History Museum, London, UK. E-mails: r.fortey@nhm.ac.uk;
r.fortey@dsl.pipex.com
Copyright #2010 John Wiley & Sons, Ltd.
scientific use. In the case of Asaphellus stubbsi it was evident
that some specimens had been put together from more than
one original source to make composite ‘plates’. The slabs
including Dikelokephalina brenchleyi sp. nov. are perhaps
less worrying in this regard, as they frequently show
individuals overlapping one another; this is an original
feature. It has been observed in slabs in the NHM, London,
the Royal Ontario Museum and the Oxford Museum of
Natural History. This arrangement would make it very
difficult for anyone wishing to insert a ‘replacement’ thorax,
for example. Individuals are always of large size, 15 cm
length or somewhat more, and they always seem to be dead
whole specimens rather than moults. They are flattened on
top of one another, but they also tend to only partially
overlap one another. I have never seen an example of nearly
complete overlap.
This collective orientation was probably the result of
a catastrophic ‘kill’ of a living assemblage, as previously
discussed for Asaphellus stubbsi (Fortey, 2009) and for a
slightly younger assemblage of giant trilobites from
Portugal (Gutie
´rrez-Marco et al., 2009). As with those
species, the assemblages of Dikelokephalina are remarkable
for the absence of smaller growth stages of the same
trilobite. This was a natural assembly of large individuals
gathered closely together which then suddenly perished
together. However, in the case of Dikelokephalina, the
marginal overlap of individuals is further suggestive of
mating behaviour. This is by comparison with the mass
mating behaviour of the living horseshoe crab Limulus in
North America (Shuster et al., 2003). During the time at
which females congregate to lay their eggs, males climb on
board to fertilize the freshly laid batches. Limulus males
outnumber the females in this process and it is common to
see three males partially mounting a laying female. Only
one male uses its specially adapted claspers to attach from
behind, and that individual does not fully cover the female;
hence other males approach obliquely from the side to
opportunistically gain access to the female’s opisthothorax.
In Limulus only large individuals that have ceased to grow
and moult perform this ritual. If such a breeding mass
were suddenly buried it would present as a series of partially
overlapping individuals not unlike the case with Dikeloke-
phalina. No smaller individuals would be present.
An interesting observation in this regard is that those
trilobites we have been able to examine from Morocco that
also show the cephalic doublure have lost the median suture
(see Figure 4aand b), and the cheeks are united as a single
piece. The doublure is extremely broad in this species, and it
is likely that moulting would have needed facilitation with
appropriate sutures. The fact that they appear to be lost in
these large, congregated populations is consistent with the
idea that the communal mating behaviour happened only in
individuals that no longer needed to moult and grow, as in
the Limulus example. It would be rather difficult to account
for the physical overlapping of several trilobites without
some explanation of this kind. A similar explanation may
apply to some of the trilobite-bearing slabs from Portugal
illustrated by Gutie
´rrez-Marco et al. (2009).
If this explanation were correct, another implication
would be that there was no sexual dimorphism in
Dikelokephalina. Dimorphism is claimed for some trilobites,
for example, those with putative brood pouches (Fortey and
Hughes, 1998) but is notoriously difficult to prove. The
present case would argue that dimorphism is not general.
3. DIKELOKEPHALINIDAE AND HUNGAIIDAE
Ludvigsen et al. (1989) made the suggestion that the Family
Dikelokephalinidae Kobayashi, 1934 might be a junior
synonym of Hungaiidae Raymond, 1924, following earlier
remarks by Henningsmoen (1959) and Jell and Stait (1985).
Ludvigsen et al. (1989) did not have available recent
descriptions of Dikelokephalina species; with the revision of
the type species by Ebbestad (1999) and the description of two
species herein, it may be a better time to evaluate this proposal
now. The greatest number of species belonging to genera
attributed to Dikelokephalinidae is found in the eastern part of
Ordovician Gondwana, including the Middle East, China and
Australia (Zhou and Zhen, 2008; Jell and Stait, 1985).
Dikelokephalina itself is reported from Tasmania (Jell and Stait,
1985), Korea (Kobayashi, 1934; Choi et al., 2001), China (Lu,
1975), Scandinavia (Ebbestad, 1999), Spain (Rabano
´, 1984,
p. 270) and Morocco and Avalonia (herein). It is probable that
the genus also extended to that part of the Ordovician Baltic
Plate located in Pai Khoy (Bursky, 1970). An occurrence in
Gorny Altai (Petrunina, 1960) may be from a peri-Gondwana
terrane. A record from Kazakhstan (Dikelokephalina firma
Apollonov and Chugaeva, 1983) seems to have a strongly
tapering glabella and may be better referred elsewhere. The
occurrence on the Baltic Plate is the only one of the ‘family’
outside Gondwana/Laurentia and associated terranes,
although there is much in common between trilobite faunas
in Avalonia and Norway/Sweden in the later Tremadocian
(Fortey and Cocks, 2003, Urals (Ancigin, 2001)). By contrast,
Hungaia is an Upper Cambrian Laurentian genus most
recently described from western Newfoundland by Ludvigsen
et al. (1989); in general, such faunas do not have much in
common with their Gondwana contemporaries.
The general design of the ‘dikelokephalinid’ is one which
has evolved on a number of occasions, and, given the
biogeographical differences noted in the previous paragraph,
it is necessary to be cautious before assuming that generally
similar trilobites are related. For example, large flattened
Copyright #2010 John Wiley & Sons, Ltd. Geol. J. (2010)
r. a. fortey
pygidia with marginal spines are known in Asaphidae
(Asaphelina), Taihungshaniidae and Ceratopygidae. More
critical characters are needed to determine close relation-
ships. If Hungaia and Dikelokephalina are indeed closely
related, this would be rather unusual among non-agnostid
trilobites between Laurentia and Gondwana.
Cephalic features, and especially the structure of the
glabella, may be most significant in determining relation-
ships; the glabellar structures of the asaphids, taihungsha-
niids and ceratopygids mentioned previously helped to
resolve those relationships (Fortey and Chatterton, 1988).
The fact that Hungaia has up to four pairs of pygidial
marginal spines compared with up to two in dikelokepha-
linids is perhaps not so significant, although their flattened
form is more like that of marginal spines of remopleur-
idioids, and they look like simple pleural extensions rather
than coming off the border. Curiously, Ludvigsen et al.
(1989, p. 29) state that the pygidia of H.magnifica and
H. devinei cannot be distinguished, whereas it seems fairly
obvious that the former has three, and the latter four pairs of
marginal pygidial spines (ibid. pl. 17, figure 1, compare
figure 9). A well preserved cranidium of Dikelokephalina
furca is illustrated herein (Figure 1E) and may be compared
with that of the type species (Ebbestad, 1999, figure 62) and
Hungaia (Ludvigsen et al., 1989, plate 17). Hungaia
obviously has eyes much closer to the glabella, but this is
scarcely a family level difference. More important is the fact
that the eye ridges on Hungaia approach the glabella well
behind the anterolateral corners of the glabella, whereas
in Dikelokephalina they advance to the very front of the
glabella. To this may be added the fact that Hungaia has
prominent inflated bacculae to either side of the base of the
glabella, whereas the type species of Dikelokephalina (and
many other dikelokephalinids) rather shows depressed alae.
However, D. furca shows neither. Glabellar furrows are
quite similar on Hungaia and Dikelokephalina dicraeura in
being pit-like and tending towards isolation within the
glabella; it is possible that S1 is widest abaxially in
Hungaia but adaxially in Dikelocephalina.OnD. furca only
the innermost part of S1 is clearly defined as a circular pit.
Ludvigsen et al. (1989, pl. 17, figure 11) illustrated a
free cheek of Hungaia that shows a broad doublure like
Dikelokephalina, and the paradoublural line crossing the free
cheek is very like D. furca. However, it is not clear (nor from
their description) whether the doublure terminates at a median
suture, or is merely broken at about the same position. A
species illustrated and described by Ludvigsen et al.(1989)
as Dikelokephalina?broeggeri Clark, 1924, from Le
´vis,
Quebec, shows the eye ridge in the posterior position, no
paradoublural line, and S1 is forked abaxially. It seems to be
more similar to Hungaia than to Dikelokephalina.
In summary, there is certainly a general resemblance
between Hungaia and Dikelokephalina in the broad cephalic
shield, narrow, strap-like postocular cheeks, glabellar
structure, wide preglabellar area and incised paradoublural
line. However, the position of the eyeridges and the structures
abutting the base of the glabella are different in the two
genera. The pygidium of Hungaia is very like those of early
remopleuridids such as Eorobergia, and is not like that of
Dikelokephalina. I know of no remopleuridid pygidium with
a steep posterior slope illustrated for D.dicraeura
by Ebbestad (1999). Furthermore, the dikelokephalinids in
general have a Gondwanan distribution, the exception being
the occurrence of the type species of Dikelokephalina in
Baltica. Although the synonymy of Dikelokephalinidae with
Hungaiidae suggested by Ludvigsen et al. (1989) has been
followed by a number of subsequent authors (e.g. Zhou
and Zhen, 2008) this seems to me to be premature. It seems
possible that the resemblances between the two families may
yet prove to be homoplastic. In any case, it seems preferable
to retain their separate taxonomic status until this is proved
not to be the case.
4. SYSTEMATIC DESCRIPTIONS
Repositories Specimens described herein are curated in
the following institutions, recognized by the appropriate
abbreviation: Natural History Museum, London (NHM),
Sedgwick Museum, Cambridge (SM) Royal Ontario
Museum (ROM) and Oxford University Museum (OUM).
Family DIKELOKEPHALINIDAE Kobayashi, 1934
Genus DIKELOKEPHALINA Broegger, 1896
Type species.Dikelokephalina dicraeura Broegger,
1896 from the ‘Ceratopyge Limestone’ (Bjørka
˚sholmen
Formation) late Tremadocian, Norway, original designation.
Diagnosis. Large dikelokephalinids with transversely
wide cephalic shields; gently tapering glabella with truncate
front; with or without alae; pygidium with one pair of
marginal spines relatively close to mid-line compared with
those of Asaphopsis.
Discussion. Jell and Stait (1985) noted that Dikeloke-
phalina was close to Asaphopsis, The revision of the type
species of the former by Ebbestad (1999) and the detailed
description of two additional species herein helps to clarify
the genus. The Moroccan species, D. brenchleyi sp. nov. and
the British species D. furca (Salter) are generally similar, but
differ consistently in the position of the eyes, the form of the
facial sutures, and the width of the cephalic doublure.
Both differ from the type species in the absence of alae. The
short and wide cephalon of Dikelokephalina is characteristic,
as is the subrectangular glabella. Asaphopsis has marginal
pygidial spines that are further apart than those of
Copyright #2010 John Wiley & Sons, Ltd. Geol. J. (2010)
trilobites of the genus dikelokephalina
Figure 1. Dikelokephalina furca (Salter, 1866). (a), (b). Latex cast from pygidium and four thoracic segments, 2, and detail (4) to show sculpture. Dol-cyn-
Afon Formation, Penmorfa, North Wales SM A39954; (c), (d). Complete exoskeleton, 2.5, and retrodeformed version. Note anterior thoracic segment is
incomplete. Y Garth, Minfford, North Wales, NHM I 8077; (e). Well preserved incomplete cranidium, 2, Shineton Shale Formation, Shineton Brook,
Shropshire NHM It 28341 (probably associated thorax and pygidium Figure 2c).
Copyright #2010 John Wiley & Sons, Ltd. Geol. J. (2010)
r. a. fortey
Dikelokephalina and may have developed from a different
pygidial segment. Hungioides has two pairs of pygidial
marginal spines: one pair is in the Asaphopsis position,
the other in the Dikelokephalina position, which supports
the idea that the marginal spines in these two genera
may not be homologous, and supports their generic
recognition.
Dikelokephalina furca (Salter, 1866)
Figure 1, Figure 2A-D
1866. Dikelocephalus (Centropleura?) furca Salter;
p.303, pl. vi, fig. 4, pl. viii, figs 9–10; pl. vi, fig. 4.
1896. Dikelokephalina furca (Salter); Broegger, p. 174, p.
198, text figs 2,3.
1919. Dikelocephalina furca (Salter); Lake, p. 118–120,
pl. 14, figs 11–13.
1927. Dikelokephalina furca (Salter); Stubblefield in
Stubblefield & Bulman, p. 136.
Diagnosis. Dikelokephalina with 12 thoracic segments;
paradoublural line on cephalon crosses preglabellar field
forward from front margin of glabella; eyes placed at about
40% glabellar length, such that posterior sections of facial
sutures curve outwards and backward behind eyes; where
seen, surface sculpture of fine raised lines.
Lectotype. External mould of a pygidium, from Moel-y-
Gest, near Porthmadog, N. Wales. Selected Morris (1988, p.
78), original of Salter (1866, pl. 8 figure 10; SM A 897a), and
re-illustrated by Lake (1919, pl. 14, figure 12). Two
specimens from Moel-y-Gest are registered in the Sedgwick
Museum under A897, the other being the original of Lake
(1919, pl.14, figure 13), which is easily distinguished from
the lectotype in having one partial thoracic segment attached
(A897b). Salter only figured one pygidium, so it is clear to
which specimen the lectotype designation referred. Lake
(1919) discussed additional reasons for supposing that this
pygidium is Salter’s original.
Other material. From the Dol-cyn-Afon Formation, North
Wales: a complete specimen from Y Garth BM I8077, and
SM A14677, pygidium SM A897b; from Penmorfa:
incomplete pygidium A539, from Salter’s original paper,
and from Fearnsides’ Collection pygidia with or without
associated thoracic segments, A39954-7. From the Shineton
Shale Formation, Shineton Brook, Shropshire (Salop),
cranidium, and incomplete thorax and pygidium, BM It
28340-1, and Birmingham University Lapworth Museum
pygidium BU 2436, free cheek and partial thorax BU 2435.
Age. Upper Tremadocian, Migneintian substage,
salopiensis to sedgwickii biozones.
Description. Material from North Wales is distorted to
some extent, but that from the Shineton Shale Formation is
not. Hence the most complete exoskeleton from North Wales
has been photographically retrodeformed (e.g. Fortey and
Rushton, 2003) (Figure 1d) until the pygidium was similar
to that from Shropshire to get the best idea of the original
proportions. All material is flattened to some extent, and
although dikelocephalinids are not convex trilobites it is
likely that some changes in proportion result, for example,
it is probable that there was a downward slope at the
preglabellar field that resulted in a ‘pushed up’ ridge in front
of the glabella on the flattened examples. This trilobite is a
large species, and it is likely that fragments came from
specimens at least 20 cm long. Elliptical exoskeleton not
quite twice as long as wide, with maximum width across
cephalon at genal spines, this being more than twice sagittal
cephalic length. Cephalic shield takes up a quarter, or
slightly more total dorsal length (sag.), and is shorter (sag.)
than the pygidium, including its lateral spines. Glabella
barrel shaped, as wide as long, with gentle forward taper
from occipital ring to distinctly truncate front. Glabellar
furrows are best seen on the large Shropshire cranidium
(Figure 1e): Occipital ring not reaching axial furrows and
terminating in a distinct elliptical (tr.) muscle impression;
first pair (S1) of glabellar furrows circular, isolated within
glabella quite close to mid-line, with a median inflated
boss surrounded by a circular depression; what are taken
to be two pairs of glabellar furrows anterior to this are rather
ill-defined, and similarly isolated within glabella, and
extending close to anterior glabellar margin. Less well-
preserved material shows only lateral deepening of occipital
furrow and rather vague depressions anteriorly. No alae
seen. Strongly developed eye ridges indent glabella very
close to anterolateral corners and slope outwards and
obliquely backwards to run into strongly curved palpebral
lobes of length (exsag.) one-third that of glabella. The
palpebral lobes are opposite S1. Facial sutures diverge in
front of eyes at a low angle (15–258) before converging
rather evenly across border and meeting on mid-line. If they
were like other Dikelokephalina species it seems likely that
they were supramarginal, but I have not seen a specimen that
is completely convincing in this regard. The best-preserved
cranidium (Figure 1e) does show the anterior-most part of
the sutures meeting at the mid-line at an oblique point
which indicates a dorsal position. The posterior sections
curve strongly outwards and backwards, but are never
transverse, before turning through a right angle distally to
cut the posterior margin perpendicularly. The strap-like
postocular fixed cheek is as wide (tr.) as the glabella and for
much of its length bisected by a deep posterior border
furrow. The anterior border furrow is narrower; it is probably
coincident with the paradoublural line. On the well-
preserved cranidium it falls short of the glabella leaving a
preglabellar field less than half as wide (sag.) as the occipital
ring. This area is probably exaggerated by crushing as
in Figure 1c. The border continues on to the narrowly
triangular free cheek, and is somewhat concave poster-
Copyright #2010 John Wiley & Sons, Ltd. Geol. J. (2010)
trilobites of the genus dikelokephalina
olaterally where it continues into a sharp genal spine that
extends backwards to reach the third thoracic segment.
Thorax comprises 12 segments; note that the anterior-
most of these is imperfectly preserved on Figure 1cand d.A
presumed moult also shows 12 segments (Figure 2a), and
I have seen a good entire specimen from Y Garth, near
Portmadog, N Wales, in the collection of Mr T. Unite
which also shows 12 segments, so this number appears to be
consistent for the species. The axis tapers very gradually
posteriorly, and pleurae nearly twice as wide (tr.) as axis.
Pleural tips curve backwards progressively down the thorax,
such that the posterior pair extend back as far as the tip of the
pygidial axis. Deep and narrow pleural furrows shallow
rapidly laterally short of pleural tips.
Pygidium wider than long, with prominent pair of
sharp-tipped marginal spines separated by an embayment,
the distance between which is about the same as the anterior
width of the thoracic axis. Distance behind axis is 25–30%
pygidial length. Axis continues backward taper of thorax
to bluntly rounded terminal piece and shows three, and a
weak fourth axial ring, and corresponding pleural furrows,
the fourth pair sloping steeply backwards at a low angle to
the sagittal line, and all fading out on the apparently weakly
concave border. Doublure closely underlies much more than
half the pleural field and is covered with strong terrace
ridges running subparallel to pygidial margin but crowded
around tip of axis. Similar doublure underlies about half the
thoracic pleurae. Best preserved exoskeleton shows fine,
dense raised lines as surface sculpture on pygidium and
pleural tips.
Discussion. The type species, D. dicraeura (Angelin,
1854) from the late Tremadocian Bjørka
˚sholmen Formation
(Ceratopyge Limestone) of Norway was fully revised by
Ebbestad (1999), from relief, although not articulated
material. The posterior pygidial margin of this species is
steeply downsloping, something which cannot be ascer-
tained from the largely flattened British material of D. furca.
Where fragments of exoskeleton adhere to the type species,
they also show a sculpture of finely crowded raised lines.
The glabellar furrows are much deeper on the Norwegian
material, but it is conceivable that this is the result of better
preservation. The occipital furrow reaches the axial furrow
in the type species. However, the best-preserved Shropshire
cranidium shows no signs of alae, which are prominent on
the type species. An additional good specific difference is
shown on the paradoublural line on cranidia; on D. furca this
does not approach the glabella medially, running straight
across the mid-line, whereas in D. dicraeura it curves
backwards medially towards the front of the glabella, The
postocular section of the facial suture of the latter runs
transversely, rather than sloping backwards as it does in
D. furca, and the posterior border furrow more nearly bisects
the cheek laterally in D. dicraeura. The doublure underlies a
much greater proportion of the pygidial pleural field in
D. furca.D. asiatica Kobayashi, 1934 was described from
fragmentary material from South Korea, but Jell and Stait
(1985) attributed more and better material from Tasmania
to the same species. It has no dorsal expression of the
paradoublural line on the cranidium and larger eyes than
D furca, there is evidence of granulose sculpture on the
glabella, and its pygidial spines are much closer to the mid-
line. Differences from D. brenchleyi sp. nov. are discussed in
the following section.
Dikelokephalina brenchleyi sp. nov.
Figures 2E–F, 3, 4
1985. Dikelokephalina sp.; Destombes, Hollard and
Willefert: 189.
Diagnosis. Dikelokephalina species usually with 13
thoracic segments; cephalic doublure wider than that of
other species of the genus, and eyes further back, such that
the postocular fixed cheek is narrower proximally than
distally, the facial sutures initially curving forwards behind
the eyes; posterior thoracic pleural tips long; lateral margins
of pygidium only slightly diverge from sagittal line.
Name. For Pat Brenchley
Holotype. Entire exoskeleton, ROM 55070C (Figure 3)
Paratypes. Exoskeletons: NHM It 28339; OUM BX 9–11;
ROM 55070B
Occurrence. Fezouata Formation, at Ouled Slimane, SW
of Agdz, Draa valley, Morocco; N 30833058400 W86890
55600. This locality appears to be the source of many
specimens available on the open market in 2009.
Age. Associated Clonograptus graptolite species (Fortey,
2009) indicate an age that is early Ordovician, Tremadocian,
Cressagian Substage.
Description. All material flattened, so comments similar
to D. furca apply. Exoskeletons commonly about 20–30 cm
long, and 1.25–1.35 times longer than wide, and widest
across cephalon at genal spines. Cephalon and pygidium of
similar length (the latter including lateral spines) and thorax
up to twice length of cephalon. Cephalon highly transverse,
maximum width three times sagittal length. Length (sag.) of
preglabellar area about two-thirds glabellar length, although
on some specimens, including the holotype, the anterior of
the glabella is not well-defined; this may be an original
feature. Subrectangular glabella tapers slightly forwards and
is about 0.7 times as wide as long. Material available does
not show glabellar furrows, apart from a shallow occipital
furrow and a faint indication of S1, but it is possible that
furrows were developed in the same way as displayed by the
best preserved specimen of D. furca, and then obscured by
flattening. Eyes up to almost half glabellar length (exsag.),
and strongly curved, and far back, such that the transverse
line connecting their posterior ends are at the level of the
Copyright #2010 John Wiley & Sons, Ltd. Geol. J. (2010)
r. a. fortey
Figure 2. Dikelokephalina furca (Salter, 1866). (a) Latex cast from thorax and pygidium, 1.25. Dol-cyn Afon Formation, Penmorfa, North Wales. (b) Well-
preserved pygidium, showing doublure, 2, Shineton Shale Formation, Shineton Brook, Shropshire, BU 2436. (c) and (d) Incomplete thorax and pygidium,
0.5, and detail of pygidial sculpture, same locality as b. NHM It 28340. Dikelokephalina brenchleyi sp. nov. Tremadocian. Fezouata Formation, Agdz, Draa
Valley Morocco. (e) Complete exoskeleton with 14 thoracic segments, 0.3, NHM It 28339a. (f) Complete exoskeleton (segments 3–6 disturbed) with 13
thoracic segments, 0.3. ROM 55070B.
Copyright #2010 John Wiley & Sons, Ltd. Geol. J. (2010)
trilobites of the genus dikelokephalina
occipital ring. Eye ridges strong and oblique. Holotype
clearly shows that the facial sutures are entirely dorsal, such
that the free cheeks are connected by a thin strip of dorsal
exoskeleton. Posterior sections of facial sutures curve
somewhat forwards behind eyes before curving sharply
backwards distally, thereby outlining a narrow posterior
fixed cheek which is considerably wider (tr.) than the
glabella. Posterior border furrow fades out before reaching
tip of postocular cheek. Anterior sections curve outwards
before converging across mid line. Narrowly triangular free
Figure 3. Dikelokephalina brenchleyi sp nov. Holotype, slightly reduced from natural size. Tremadocian, Fezouata Formation, Agdz, Draa Valley Morocco.
ROM 55070C.
Copyright #2010 John Wiley & Sons, Ltd. Geol. J. (2010)
r. a. fortey
cheeks are featureless apart from a triangular short genal
spine, which extends back as far as the third or fourth
thoracic segment.
Cephalic doublure is extremely wide (Figure 4a) extend-
ing back as far as the eye ridges, and underlying virtually all
the free cheek. Several specimens show that the median
suture is lacking. It could have been present at an earlier
stage in ontogeny, and become sealed on large individuals,
possibly those that had ceased to moult and grow. Up to
Figure 4. Dikelokephalina brenchleyi sp. nov. Locality and horizon as Figure 3. (a) Cephalic shield, NHM It 28339b. Approximately natural size, showing
doublure lacking median suture. (b) Cephalic shield, OUM BX 10, approx. natural size, showing lack of median suture on doublure with well preserved terrace
ridges. (c) Typical slab one-sixth natural size, showing partially overlapping dorsal exoskeletons. NHM It 28339.
Copyright #2010 John Wiley & Sons, Ltd. Geol. J. (2010)
trilobites of the genus dikelokephalina
twelve widely spaced terrace ridges are present on the
doublure,
Thorax tapers backwards, while axis tapers to a much
lesser extent. Spinose pleural tips droop progressively
backwards and are longer posteriorly. Doublure underlies
the backwardly curved parts of the pleurae. The number of
thoracic segments appears to be variable. Most specimens
show thirteen thoracic segments. I have confirmed this on
specimens held in other public collections in the Royal
Ontario Museum (David Rudkin, personal communication,
2009) and Natural History Museum, Oxford (Derek Siveter,
personal communication, 2009). However, the specimen
shown on Figure 2eshows 14 thoracic segments. Since this
specimen is overlain by another one on the right this should
rule out any major tampering, such as I described for
Asaphellus stubbsi from the same horizon (Fortey, 2009).
One other specimen on a large slab in the Natural History
Museum, London, may also show fourteen segments. On the
Internet there are specimens for sale that apparently have
twelve thoracic segments, though these cannot be vouched
for from examination. Dr D. Siveter (personal communi-
cation, 2010) has one specimen (OUM BX 11) in the Oxford
University Museum that probably has 12 segments. It seems
likely that there is indeed variation in mature segment
number in D. brenchleyi. This is unusual in post-Cambrian
trilobites, but is not unknown; for example, in the Silurian
trilobite Aulacopleura konincki (see Hughes and Chapman,
1995).
Pygidium slightly wider than long (sag.), and with broad-
based, triangular marginal spines about one-third as long as
axis. Sides of pygidium diverge only slightly from sagittal
line where the posterior thoracic pleurae extend alongside,
posteriorly converging at a higher angle. Axis extends to half
or slightly more pygidial length, and shows five segments of
equal length (sag.) and a terminal piece. Three, and a weak
fourth, pleural ribs fade out on ill-defined border, the anterior
two having a distinct proximal transverse portion. Doublure
underlays all but this part of the pleural fields, with wide-
spaced terrace ridges like those on the cephalon.
Discussion. The cephalic doublure of D. brenchleyi is the
widest of any species in the genus and this feature readily
distinguishes it from others. For example, on D. furca
and D. dicraeura the paradoublural line crosses through the
middle of the free cheek, while on D. brenchleyi it is
confined to the most posterior part. The position of the eye is
also characteristic, being far back enough to result in an
initial anterior swing to the course of the facial sutures; on
D. furca the same part of the facial suture slopes posteriorly
and on D. dicraeura it is more or less transverse. The
steeply backward-sloping lateral flanks of the pygidium
of D. brenchleyi are also distinctive. Like D. furca,
D. brenchleyi shows no evidence of basal alae, which
might suggest it is more closely related to that species than to
D. asiatica Kobayashi and D. dicraeura which have alae.
Not enough is known of the dorsal surface of D. brenchleyi
to be sure whether or not it carried surface sculpture.
ACKNOWLEDGEMENTS
I thank the Curators at the Sedgwick Museum, Cambridge,
and the Lapworth Museum, Birmingham, for looking out
specimens in their care. I particularly thank Dr R. Kennedy
for donating a Shineton Shale specimen to the Natural
History Museum, London. Dr D. Siveter kindly photo-
graphed a specimen in the Oxford University Museum,
and Dr D. Rudkin specimens in the Royal Ontario Museum.
Phil Crabb took excellent photographs, and Derek Adams
helped with making up the plates.
REFERENCES
Ancigin N. Ya. 2001. Tremadoc trilobites of the Urals. 244pp. Ekaterin-
burg. (in Russian).
Angelin, N.P. 1854.Palaeontologia Scandinavica I Crustacea Formationis
Transitionis, Fasc. 2, Sansom and Wallin: Lund; 21–92.
Apollonov, M.K., Chugaeva, M.N. 1983. Certain trilobites from the
boundary deposits of the Cambrian and Ordovician of Batyrbai, Maly
Karatau. In: Lower Palaeozoic Stratigraphy and Palaeontology of
Kazakhstan, Apollonov, M.K., Bandaletov, S.M., Ivshin, N.K. (eds).
Nauka: Leningrad (St. Petersburg); 66–90 (in Russian).
Broegger, W.C. 1896.U
¨ber die Verbreitung der Euloma-Niobe Fauna (Der
Ceratopygenkalk Fauna) in Europa. Nyt Magazin fur Naturvidenskab 36,
164–240.
Bursky, A.Z. 1970. Trilobites. In: Early Ordovician of Pai Khoy, Vaigach
and Novaya Zemlya, Bondarev, V.I. (ed.). Institute of Arctic Geology:
Leningrad (St. Petersburg) 96–138 (in Russian).
Choi, D.K., Kim, D.H., Sohn, J.W. 2001. Ordovician trilobite faunas and
depositional history of the Taebaeksan Basin, Korea: implications for
palaeogeography. Alcheringa 25, 53–68.
Clark, T.H. 1924. The paleontology of the Beekmantown Series at Levis,
Quebec. Bulletins of American Paleontology 10, 1–134.
Cocks, L.R.M., Fortey, R.A. 2009. Avalonia —a long lived Gondwana
terrane? In: Gondwana and Avalonia, Bassett, M.G. (ed.). Geological
Society, London, Special Papers 325, 3–21.
Destombes, J., Hollard, H., Willefert, S. 1985. Lower Palaeozoic rocks of
Morocco. In: Lower Palaeozoic Rocks of North-western and West-central
Africa, Holland, C.H. (ed.). Wiley: New York; 91–336.
Ebbestad, J.O.R. 1999. Trilobites of the Tremadoc Bjørka
˚sholmen
Formation in the Oslo Region, Norway. Fossils and Strata 47,
1–118.
Fortey, R. 2009. A new giant asaphid trilobite from the Ordovician of
Morocco. Memoirs of the Australasian Association of Palaeontologists
37, 9–16.
Fortey, R.A., Chatterton, B.D.E. 1988. Classification of the trilobite
suborder Asaphina. Palaeontology 31, 165–222, pls 17–19.
Fortey, R.A., Cocks, L.R.M. 2003. Palaeontological evidence bearing on
global Ordovician–Silurian continental reconstructions. Earth Science
Reviews 61, 245–307.
Fortey, R.A., Hughes, N.C. 1998. Brood pouches in trilobites. Journal of
Paleontology 72, 638–649.
Fortey, R.A., Rushton, A.W.A. 2003. A new aglaspidid arthropod from the
Lower Ordovician of Wales. Palaeontology 46, 1031–1038.
Copyright #2010 John Wiley & Sons, Ltd. Geol. J. (2010)
r. a. fortey
Gutie
´rrez-Marco, J.C., Sa
´, A.A., Garcı
´a-Bellido, D.C., Ra
´bano, I.,
Vale
´rio, M. 2009. Giant trilobites and trilobite clusters from the Ordo-
vician of Portugal. Geology 37, 443–446.
Henningsmoen, G. 1959. Rare Tremadocian trilobites from Norway. Norsk
Geologisk Tidsskrift 39, 153–173.
Hughes, N.C., Chapman, R.E. 1995. Growth and variation in the Silurian
proetide trilobite Aulacopleura koninki and its implications for trilobite
palaeobiology. Lethaia 28, 333–353.
Jell, P.A., Stait, B. 1985. Tremadoc trilobites from the Florentine Valley
Formation, Tim Shea area, Tasmania. Memoirs of the Queensland
Museum 30, 455–485.
Kobayashi, T. 1934. The Cambro-Ordovician faunas of South Chosen.
Palaeontology. Part II. The Lower Ordovician faunas. Journal of
the Faculty of Science, Imperial University of Tokyo Series 3(9): 521–
585.
Lake, P. 1919. Monograph on the British Cambrian Trilobites. Part V.
Palaeontographical Society Monographs 71(343): 89–120.
Ludvigsen, R., Westrop, S.R., Kindle, C. 1989. Sunwaptan (Upper
Cambrian) trilobites of the Cow Head Group, western Newfoundland,
Canada. Palaeontographica Canadiana 6, 1–175.
Lu, Y.-H. 1975. Ordovician trilobite faunas of central and southwestern
China. Palaeontologia Sinica, New Series B, 11, 1–261 (in Chinese,
English summary)
Moore, R.C. (ed.). 1959.Treatise on Invertebrate Paleontology. Part O
Trilobita. University of Kansas Press and Geological Society of America:
Lawrence Kansas.
Morris, S.F. 1988. A review of British trilobites, including a synoptic
revision of Salter’s monograph. Palaeontographical Society Monograph
140(574): 1–136.
Petrunina, Z.E. 1960. Trilobites. In: Biostratigraphy of the Palaeozoic in
the Sayan Altai alpine region. I. Lower Palaeozoic, Khalfina, L.L. (ed.).
Trudy Sibirskogo nauchno-issledovatelskogo Instituta Geologii, Geofi-
ziki i Mineral’nogo syr’ra 19, 409–433.
Rabano
´,I.1984. Trilobitos Ordovicicos del Macizo Hesperico
espanil: Lina Vision biostratigrafica. Cuadernos Geologia Iberica 9,
267–287.
Raymond, P.E. 1924. New Upper Cambrian and Lower Ordovician trilo-
bites from Vermont. Proceedings of the Boston Society of Natural
History, 37; 389–466.
Salter, J.W. 1866. On the fossils of North Wales. Appendix In: The Geology
of North Wales. Ramsay, A.C. (ed.). Memoirs of the Geological Survey of
Great Britain Vol. 3; 239–363, 372–381, pls 1–26.
Shuster, C.N., Barlow, R.B., Brockmann, H.J. (eds). 2003.The
American horseshoe crab. Harvard University Press: Cambridge, Mass;
1–427.
Stubblefield, C.J., Bulman, O.M.B. 1927. The Shineton Shales of the
Wrekin District. Quarterly Journal of the Geological Society of London
83, 96–145.
Zhou, Z.-Y., Zhen, Y.-Y. (eds). 2008.Trilobite Record of China. Science
Press: Beijing; 1–402.
Copyright #2010 John Wiley & Sons, Ltd. Geol. J. (2010)
trilobites of the genus dikelokephalina
