Arthropods from the Lower Devonian
Severnaya Zemlya Formation of October
Revolution Island (Russia)
Jason A. DUNLOP
Institut für Systematische Zoologie,
Museum für Naturkunde der Humboldt-Universität zu Berlin,
Invalidenstraße 43,
D-10115 Berlin (Germany)
jason.dunlop@museum.hu-berlin.de
406
Dunlop J. A. 2002. — Arthropods from the Lower Devonian Severnaya Zemlya Formation of
October Revolution Island (Russia). Geodiversitas 24 (2) : 349-379.
ABSTRACT
KEY WORDS
Arthropoda,
Chasmataspida,
Cheloniellida,
Phyllocarida,
Eurypterida,
Devonian,
Severnaya Zemlya,
Russia,
phylogeny,
genital appendage,
metastoma,
new genera,
new species.
New arthropods from the Lower Devonian (lower Lochkovian) Severnaya
Zemlya Formation, October Revolution Island, Russia, are described. Three
groups of non-ostracode arthropods are present. Cheloniellids (Arachnata,
Cheloniellida) are represented by Paraduslia talimaae n. gen., n. sp.,
Chasmataspids (Chelicerata, Chasmataspida) by Octoberaspis ushakovi
n. gen., n. sp., and phyllocarids (Crustacea, Malacostraca, Phyllocarida) by
Elymocaris urvantsevi n. sp. This fauna is significant as both cheloniellids
and chasmataspids are extremely rare groups, while phyllocarids remain
uncommon. Furthermore, this new chasmataspid material reveals two
important morphological characters for the group: a genital appendage and
metastoma. Both were previously considered autapomorphic for
eurypterids; their presence in chasmataspids strongly suggests that
Chasmataspida and Eurypterida are sister groups.
GEODIVERSITAS • 2002 • 24 (2) © Publications Scientifiques du Muséum national d’Histoire naturelle, Paris.
www.mnhn.fr/publication/
349
Dunlop J. A.
RÉSUMÉ
MOTS CLÉS
Arthropoda,
Chasmataspida,
Cheloniellida,
Phyllocarida,
Eurypterida,
Dévonien,
Severnaya Zemlya,
Russie,
phylogénie,
appendice génital,
metastoma,
nouveaux genres,
nouvelles espèces.
Arthropodes du Dévonien inférieur de la Formation Severnaya Zemlya, Île
de la Révolution d’Octobre (Russie).
De nouveaux arthropodes du Dévonien inférieur (Lochkovian inférieur) de
la Formation Severnaya Zemlya, Île de la Révolution d’Octobre, Russie,
sont décrits. Trois groupes d’arthropodes non-ostracodes sont présentés.
Les cheloniellides (Arachnata, Cheloniellida) sont représentés par Paraduslia
talimaae n. gen., n. sp., les chasmataspides (Chelicerata, Chasmataspida) par
Octoberaspis ushakovi n. gen., n. sp. et les phyllocarides (Crustacea,
Malacostraca, Phyllocarida) par Elymocaris urvantsevi n. sp. Cette faune est
remarquable par la présence de cheloniellides et chasmataspides qui sont des
groupes extrêmement rares de même par celle des phyllocarides. En outre, ce
nouveau matériel de chasmataspides révèle deux caractères morphologiques
importants pour le groupe : un appendice génital et un metastoma, qui ont
été considérés précédemment comme autapomorphies des eurypterides.
Leur présence chez les chasmataspides suggère fortement que ces derniers et
les Eurypterida sont des groupes-frères.
INTRODUCTION
In 1978 an expedition led by Dr Vladimiz
Menner investigated the geology of the
Severnaya Zemlya Archipelago off the Tajmyr
peninsular, Russia (see Männik et al. 2002: fig. 1,
for locality maps). A large number of Silurian
and Devonian fossils were collected, mostly
comprising vertebrate material. However, from
one of the islands, October Revolution Island,
over 200 arthropod fossils were recovered from
the Lower Devonian Servernaya Zemlya
Formation (Karatajūtē-Talimaa et al. 1986).
Though previously figured in the literature
(Novilskaya et al. 1983), these fossils have not
been studied in detail. This paper describes the
three groups of arthropod (excluding ostracodes) recorded from this locality: cheloniellids,
chasmataspids and phyllocarid crustaceans.
MATERIALS AND METHODS
All specimens are held in the Geological
Institute of Lithuania (GIL), T. Sevcenkos 13,
LT 2600, Vilnius, collection number 35. The
majority of specimens are provided with an outcrop number (oc) and a bed number (bd) indi-
350
cating where in the sequence they were collected
(see geological setting). Material derived from
other localities is noted in the text. Unless noted
otherwise, descriptions are of the appearance of
the animal in life with respect to ridges, tubercles, etc. Specimens were studied under a binocular microscope and, where necessary, were
carefully prepared using a large needle.
Interpretative drawings were made from the
specimens and photographs. Specimens of fossil
eurypterids and extant xiphosurans were studied for comparative anatomy. All the measurements of the description parts are in mm.
GEOLOGICAL SETTING
AND PRESERVATION
The material described here comes from the
October Revolution Island which is part of the
Severnaya Zemlya Archipelago (see Männik
et al. 2002 for details). The material was collected from loose nodules exposed at the surface, not in situ from the rock. The arthropods
come primarily from three outcrops: outcrop 1
on the Matusevich River, bed 21 (Männik et al.
2002: figs 2, 7); outcrop 41 on the Spokojnaya
River, bed 12 (Männik et al. 2002: fig. 6); out-
GEODIVERSITAS • 2002 • 24 (2)
Severnaya Zemlya arthropods
crop 67 on the Pod’’emnaya River, bed 12
(Männik et al. 2002: fig. 1). The detailed stratigraphy and age ranges of the exposures at each
of these localities are given in Männik et al.
(2002) and the arthropods all come from the
Severnaya Zemlya Formation, of Lower
Devonian (lower Lochkovian) age. The fossils
of the Servernaya Zemlya Formation comprise
abundant vertebrates, mostly jawless heterostracans, osteostracans and anaspids, but also small
to large predatory acanthodians. The arthropods
comprise ostracodes as well as the phyllocarids,
cheloniellids and chasmataspids described here.
Articulated plant fossils are also recorded
(Novilskaya et al. 1983). The occurrence of the
plant remains and non-ostracode arthropods
only in the Severnaya Zemlya Formation indicates a unique depositional environment in the
early Lochkovian time in the Severnaya Zemlya
region.
The arthropod fossils are preserved in flattened
nodules originally from a finely laminated argyolitic limestone. The nodules range in colour
from dark grey to light brown, though this
brown colour is a weathering effect and the
natural matrix colour is grey. The nodules from
the Pod’’emnaya River tend to be darker in
colour than those from the Matusevitch and
Spokoinaya rivers. Counterparts were not
recovered with any of this material. The specimens themselves are generally darker than the
surrounding matrix, typically dark brown or
grey in colour. Most specimens retain some
three-dimensional relief and details of cuticle
ornamentation. Some specimens are better preserved than others, and most have been at least
partially deformed due to compression. The best
specimens are typically those preserved as external moulds and in general these show the most
morphological detail. A number of the chasmataspid and crustacean specimens are preserved as internal moulds, some through
replacement by a crystaline mineral, possibly
calcium phosphate. These retain their original
three-dimensional shape better than some noncrystaline specimens but lack details of
morphology.
GEODIVERSITAS • 2002 • 24 (2)
SYSTEMATICS
CHELONIELLIDA Broili, 1932
REMARKS
Seven specimens can be referred to the taxon
Cheloniellida Broili, 1932, recognised as a distinct arthropod clade by Dunlop & Selden
(1997) (see also below). The four most complete
specimens (GIL 35/700-703, Figs 1-4) are
described and figured here. A number of additional but incomplete specimens (35/704-706)
may also represent cheloniellids. The identity of
these fossils as cheloniellids, compared to previous records of this group (e.g., Stürmer &
Bergström 1978; Selden & White 1984; Chlupác
1988) is based on their broadly oval shape with a
series of tergites bearing a distinct median axis,
ending posteriorly in a terminal furca, clearly
visible in 35/700 and 35/702 (Figs 1; 2). In addition to this, 35/702 (Figs 1B; 2B) and 35/703
(Figs 3B; 4B) preserve a number of mostly dissociated, filamentous fragments throughout the
length of their bodies. These are similar to the
stout lamellar spines described in Cheloniellon
calmani Broili, 1932 by Stürmer & Bergström
(1978) where they were believed to be associated
with the outer branch of a series of biramous
appendages (see also below).
Paraduslia n. gen.
TYPE SPECIES. — Paraduslia talimaae n. gen., n. sp.,
by monotypy.
ETYMOLOGY. — From its similarity to the Ordovician
form Duslia insignis Jahn, 1893.
DIAGNOSIS. — Small, narrow cheloniellid, approximately twice as long as broad, with at least 12 trunk
segments. Terminal furca short and eyes apparently
absent. Marginal fringe of spines (seen in Duslia
insignis) absent.
REMARKS
In overall appearance these fossils resemble the
Upper Ordovician cheloniellid Duslia Jahn,
1893. Both have a narrow median axial region
which extends onto the prosomal dorsal shield
351
Dunlop J. A.
A
B
FIG. 1. — Paraduslia talimaae n. gen., n. sp.; A, No. 35/700 (holotype), 29 × 16 mm; B, No. 35/702, 33 × 15 mm.
(or carapace). Both appear to lack eyes and both
have a relatively small pair of terminal furcae,
as compared to the very long furcae of
Cheloniellon Broili, 1932. Because of this similarity to Duslia the new generic name Paraduslia
n. gen. is proposed for this material. The proportions of the body, the larger number of trunk
segments and the lack of marginal tergal spines
in the new fossils distinguish them from Duslia.
Paraduslia talimaae n. sp.
(Figs 1-5)
HOLOTYPE. — GIL No. 35/700 (oc 41/bd 12).
ETYMOLOGY. — In honour of Dr Valentina Talimaa
(GIL) for her permission to work on this material and
her hospitality in Vilnius.
OTHER MATERIAL EXAMINED. — GIL Nos. 35/701 (oc
1/bd 21), 35/702 (oc 41/bd 12), 35/703 (oc 41/bd 12),
35/704 (right bank of upper part of Spokoinaya river)
(not figured), 35/705 (oc 41/bd 12) (not figured),
35/706 (oc 1/bd 21) (not figured).
352
G EOGRAPHICAL AND STRATIGRAPHICAL DISTRIBU TION. — All material is from the Severnaya Zemlya
Formation (Lower Devonian [lower Lochkovian]),
October Revolution Island.
DIAGNOSIS. — As for the genus.
REMARKS
Though it is possible that this material represents more than one species there are no reliable
characters to separate the specimens other than
size. On these grounds all material is referred to
a single species.
DESCRIPTION
No. 35/700 (holotype)
Specimen in dorsal view and quite well preserved (Figs 1A; 2A). Maximum preserved
length 29, maximum width 16. Carapace shape
and carapace margins indistinct. Median axis of
opisthosoma narrow and distinct, width 2.
Tergite margins quite clear on left side where
GEODIVERSITAS • 2002 • 24 (2)
Severnaya Zemlya arthropods
an
A
B
sp
ma
tg
fu
fu
FIG. 2. — Paraduslia talimaae n. gen., n. sp., interpretative drawings of the specimens shown in Fig. 1; A, No. 35/700 (holotype);
B, No. 35/702. Abbreviations: an, bases of antennae; fu, furca; ma, median axis of opisthosoma; sp, spines, probably lamellar
spines; tg, tergite. Scale bars: 5 mm.
four tergites show shape and curvature very
clearly. Lateral margins of up to eight tergites
distinct with median axis suggesting at least 12
trunk segments were present in life. Furca distinct, each ramus composed of three pieces, a
very short proximal section, a longer median
section which widens distally, and a shorter,
tapering distal section.
No. 35/701
Specimen preserved in dorsal view though
details lacking and whole animal slightly
GEODIVERSITAS • 2002 • 24 (2)
skewed about its median axis (Figs 3A; 4A).
Maximum length 18, width 9. Carapace reasonably well preserved with a broadly rounded anterior margin. Posterior margin of
carapace curved. Median axis evident along
length of specimen and extends onto posterior
region of carapace to form a raised diamondshaped area with a slight anterior projection.
Additional furrows faintly visible more laterally on the carapace. Eye tubercles absent. Axial
region clearly preserved on trunk with up to
13 tergites present. Pleural region of tergites
353
Dunlop J. A.
groups and slightly paler than the surrounding
fossil. Some filaments orientated along the long
axis of the body (as in 702). Other filaments
longer and more regular in their distribution,
approximately perpendicular to the long axis of
the body. Furca and appendages not preserved.
MORPHOLOGICAL RECONSTRUCTION
A
B
FIG. 3. — Paraduslia talimaae n. gen., n. sp.; A, No. 35/701,
18 × 9 mm; B, No. 35/703, 20 × 16 mm.
poorly preserved and tergite boundaries indistinct here. Furca not preserved.
No. 35/702
Specimen probably in ventral view. Outline and
segmentation of body very indistinct but medial
axis visible along part of its length (Figs 1B; 2B).
Maximum length 33, width 15. Pair of tiny
(length 0.5) white structures orientated parallel
with the long axis of the body clearly preserved.
Interpreted here as antennae. Much of body with
covering of short (length typically 2), filamentous structures, slightly paler than the surrounding fossil. Some filaments orientated along long
axis of body, especially close to median axis, but
most with no particular pattern of orientation.
Most filaments grouped into associations of
about five to ten elements. Filaments interpreted
here as part of appendages, though appendages
themselves absent. Furca well preserved.
No. 35/703
Poor specimen, antero-posterior and dorsoventral orientation difficult to distinguish.
Segmentation very indistinct, though part of
median axis preserved (Figs 3B; 4B). Maximum
length 20, width 12. Specimen with numerous
filamentous structures, often associated into
354
The new material ranges in length from two to
four cm. Compared to other cheloniellids these
examples are relatively elongate, being approximately twice as long as broad. Eyes appear to be
absent. One specimen (35/701, Figs 3A; 4A)
preserves an area of the median axis on the prosomal dorsal shield (or carapace), but shows no
evidence for raised lateral eye tubercles on this
carapace comparable with those of, for example,
Cheloniellon. None of the specimens clearly
show the total number of trunk segments
though counting segments in the axial regions of
both 35/700 and 35/701 (Figs 1A; 2A; 3A; 4A)
suggests there were at least 12 in total. By comparison Cheloniellon has nine, Duslia has 10 and
the other cheloniellids are incomplete making
their segment count uncertain. Specimen 35/700
clearly shows that the tergites bend posteriorly
at their lateral margins (Figs 1A; 2A) and have a
shallow groove running close to the posterior
margin of the tergite. This specimen also suggests that the tergites have a shallow depression
across the median axis (Figs 1A; 2A).
Unfortunately appendages are mostly absent in
this new material. A small pair of white structures at the front of 35/702 (Figs 1B; 2B) are
interpreted as the bases of the antennae as just in
front of them are two small white circles suggesting a narrow pair of antennae continuing
deeper into the matrix. Long antennae are present in Cheloniellon. The rest of the appendages
are absent in the present material although by
comparison with Cheloniellon a series of
gnathobasic head appendages and biramous
trunk appendages would be expected. The
lamellar spines of the outer ramus of the
opisthosomal appendages are seen in 35/702 and
35/703 as described above. Stürmer &
Bergström (1978) noted that these spines were
GEODIVERSITAS • 2002 • 24 (2)
Severnaya Zemlya arthropods
A
B
cp
sp
ma
FIG. 4. — Paraduslia talimaae n. gen., n. sp., interpretative drawings of the specimens shown in Fig. 3; A, No. 35/701; B, No. 35/703.
Abbreviations: cp, carapace (note raised, diamond-shaped central area); ma, median axis of opisthosoma; sp, spines, probably
lamellar spines from appendages. Scale bar: 5 mm.
so stiff in Cheloniellon that at times the cuticle
of the rest of the body would be bent around
them during compression. Likewise, these
spines are unusually well preserved in the
Severnaya Zemlya material, especially 35/702,
suggesting that their cuticle was very resiliant.
Their distribution is somewhat random in
35/702 where the spines are mostly short and
fragmentary. However in 35/703 there is more
of a pattern with groups of slightly longer spines
projecting laterally from the midline, as though
associated with an appendage series. A reconstruction of the dorsal surface of this new cheloniellid is presented in Fig. 5.
PHYLOGENY
The affinities of the cheloniellids as a group
remain problematic and this new material adds
little to the debate, except to confirm the presence of the stout lamellar spines in an additional
taxon to Cheloniellon. Originally Cheloniellon
was believed to be similar to trilobites (Broili
1932), though subsequent authors have sug-
GEODIVERSITAS • 2002 • 24 (2)
gested that it is related to trilobites and
Cambrian “merostomoids” (Størmer 1944), that
it is a late representative of a trilobitomorph
group giving rise to Chelicerata (Stürmer &
Bergström 1978), that it is related to “loricate”
crustaceans (Delle Cave & Simonetta 1991) or
that it is the sister group to Chelicerata (Wills et
al. 1995; Dunlop & Selden 1997).
The clade Arachnata (alternatively Arachnomorpha) has been erected for chelicerates, trilobites, and a number of related arthropods
comprising many of the problematic Burgess
Shale-type animals (e.g., Wills et al. 1995), although clear apomorphies defining Arachnata
and relationships within the clade remain unresolved. Dunlop & Selden (1997) proposed that a
number of Palaeozoic arachnate genera, Duslia,
Cheloniellon, Triopus Barrande, 1872,
Neostrabops Caster & Macke, 1952 and
Pseudarthron Selden & White, 1984 could be
grouped together into a clade for which Broili’s
(1932) name Cheloniellida was available. The
new taxon, Paraduslia n. gen., can be added to
355
Dunlop J. A.
MODE OF LIFE
Stürmer & Bergström (1978) believed Cheloniellon had the overall body plan of a benthic animal (though not a burrower) and that it was a
predator, since they identified xiphosuran-like
gnathobases for masticating food. Chlupác (1988)
regarded the eyeless Duslia as a probable shallow
burrower in a low energy environment.
Paraduslia n. gen. also appears to lack eyes which
might favour a burrowing mode of life. The function of the lamellate spines in both Paraduslia n.
gen. and Cheloniellon remains unresolved.
Broili’s (1932) original suggestion that they aided
swimming seems unlikely, though Stürmer &
Bergström (1978) regarded the spines as very stiff
compared to known respiratory lamellae, e.g.,
xiphosuran book gills. In the absence of any
other known respiratory organs in cheloniellids a
respiratory function for the lamellar spines seems
the most plausible. Perhaps if these animals were
living in close association with the substrate the
stiffness of the spines prevented mechanical damage by the substrate. Overall, Paraduslia n. gen.
may have had a mode of life analogous to living
xiphosurans, acting as a predator and/or scavenger either on, or just within, the substrate.
FIG. 5. — Reconstruction of the dorsal surface of Paraduslia talimaae n. gen., n. sp. Scale bar: 5 mm.
CHELICERATA Heymons, 1901
CHASMATASPIDA Caster & Brooks, 1956
this list and as such represents only the sixth
genus of this group. Cheloniellids range from the
Ordovician to the Lower Devonian (Dunlop &
Selden 1997) making Paraduslia n. gen. one of
the youngest records of this group. Paraduslia n.
gen. could be interpreted as a sister taxon to
Duslia based on the synapomorphies of a reduced furca and loss of eyes. However this interpretation must be treated with caution as the
number of trunk segments is greater in
Paraduslia n. gen., while the terminal end of the
body and the morphology of the furca, and hence
the plesiomorphic furcal condition, is not known
for all cheloniellid taxa. Likewise, the absence of
eyes is based only on a single Paraduslia n. gen.
specimen in which the preservation is not ideal.
356
REMARKS
In an early report on the Severnaya Zemlya
Formation, Novilskaya et al. (1983) figured
three specimens which they provisionally interpreted as eurypterids. In fact these fossils do not
show the typical eurypterid pattern of opisthosomal tagmosis, i.e. a preabdomen of seven segments and a postabdomen of five segments.
Instead, they have a broad preabdomen of three
large segments and a postabdomen of nine segments (Figs 6-10). This pattern of tagmosis is
consistant with another, rare chelicerate taxon,
Chasmataspida. Originally thought to be xiphosurans, only four chasmataspid genera have been
described (Caster & Brooks 1956; Størmer 1972;
GEODIVERSITAS • 2002 • 24 (2)
Severnaya Zemlya arthropods
Dunlop et al. 1999) with Chasmataspida being
recognised as a separate clade within Chelicerata
by Dunlop & Selden (1997) and Dunlop et al.
(1999). In total there are about 70 chasmataspid
specimens from Severnaya Zemlya, ranging in
size from about 1 to 4 cm long, with the better
preserved examples (GIL 35/324, 35/336-337,
35/339, 35/707, 35/711-714, 35/719, 35/735)
described and figured here (Figs 6-9).
Family DIPLOASPIDIDAE Størmer, 1972
DIAGNOSIS. — Small chasmataspids with semicircular
or subrectangular carapace and with a distinctly tapering postabdomen and a short telson; postabdomen in
Chasmataspididae elongate and not strongly tapering.
No division of preabdominal tergites into medial and
lateral plates (after Anderson et al. 2000).
REMARKS
Heteroaspis is now regarded as a junior synonym
of Diploaspis following the revision of Dunlop et
al. 2001: 253-269.
There are clear differences between Chasmataspis
Caster & Brooks, 1956, in its monotypic family
Chasmataspididae, and the known Devonian
examples, Diploaspis Størmer, 1972, Heteroaspis
Størmer, 1972 and Forfarella Dunlop, Anderson
& Braddy, 1999, which can probably all
be referred to a single family Diploaspididae (see
Dunlop et al. 1999 for a discussion). Additionally,
the one known appendage in Chasmataspis is
chelate (Caster & Brooks 1956) while the Devonian fossils, as far as we know, have pediform
appendages (Størmer 1972). The Devonian examples may all have had paddles, but so far these
have only been found in Diploaspis Størmer, 1972
and now in this Severnaya Zemlya material (see
below). These Russian fossils, with their undivided preabdomen, tapering postabdomen and
short telson, can be referred to Diploaspididae
with some confidence.
Octoberaspis n. gen.
TYPE SPECIES. — Octoberaspis ushakovi n. gen., n. sp.,
by monotypy.
GEODIVERSITAS • 2002 • 24 (2)
ETYMOLOGY. — From October Revolution Island,
where this material was discovered, and the typical
chasmataspid suffix, aspis.
DIAGNOSIS. — Diploaspid with a pair of short, styliform epimera on the terminal opisthosomal segment
which lie adjacent to the telson. Preabdomen rounded
and sculptured with tubercles.
REMARKS
The single pair of epimera on the pretelson, adjacent to the telson of the Siberian specimens is not
seen in Diploaspis and Heteroaspis although
broader postabdominal epimera are present in the
new taxon from Scotland (Anderson et al. 2000).
Tuberculation is absent in Diploaspis and
Heteroapsis, although new, undescribed chasmataspid material from Germany is also tuberculate (Markus Poschmann pers. comm.). Forfarella
is poorly preserved, but has a trapezoidal, posteriorly tapering preabdomen which differs from the
more rounded preabdomen seen in the Severnaya
Zemlya material. Based on these differences, a
new genus, Octoberaspis n. gen., is proposed.
Octoberaspis ushakovi n. sp.
(Figs 6-10)
Eurypteridae – Novilskaya et al. 1983: 94, fig. 5.
HOLOTYPE. — GIL No. 35/336 (oc 67/bd 12).
E TYMOLOGY . — In honour of G. A. Ushakov, an
Arctic explorer and the leader of the first expedition
to Severnaya Zemlya in 1930-1932.
ADDITIONAL MATERIAL. — GIL Nos. 35/324, 35/337,
35/338 (the latter two on the same block as the holotype), 35/339, 35/379, 35/707 (all oc 67/bd 12),
35/711, 35/712 (both oc 1/bd 21), 35/713, 35/714
(both oc 67/bd 12), 35/719 (oc 1/bd 21), 35/735 (oc
67/bd 12).
ADDITIONAL MATERIAL (NOT FIGURED). — GIL Nos.
35/708-710, 35/715-717 (all oc 67/bd 12); 35/718,
35/720 (both oc 41/bd 12), 35/721-734, 35/736-749
(all oc 67/bd 12), 35/750-768 (all oc 1/bd 21); 35/769777 (all oc 41/bd 12); 35/778 (oc 40/bd 6).
G EOGRAPHICAL AND STRATIGRAPHICAL DISTRIBU TION. — All material is from the Severnaya Zemlya
Formation (Lower Devonian [lower Lochkovian]),
October Revolution Island.
DIAGNOSIS. — As for the genus.
357
Dunlop J. A.
DESCRIPTION
No. 35/336 (holotype)
Specimen Nos. 35/336 and 35/337 are preserved
together in one nodule (Figs 6B; 7B) along with
a poorer specimen, 35/338, not described here.
No. 35/336 very well preserved as external
mould showing dorsal surface. Total length 25,
carapace length 7, preabdomen length 8, postabdomen length 10, including telson. Maximum
width 14. Carapace subquadrate with approximately straight posterior margin merging into a
genal spine on the right side. Middle of carapace
somewhat deformed, but with evidence for at
least one median eye in the centre of the carapace. Reniform lateral eye tubercles present. All
13 opisthosomal segments preserved. Fragments
of three appendages preserved beyond margin of
left side of carapace. Opisthosomal tergite 1 short,
tuberculate and occupying full width of carapace. Preabdominal tergites 2-4 large with curving posterior margins and distinct ornament of
tubercles. Postabdominal segments taper
smoothly towards telson. Segment 5 longest,
but successive tergites strongly telescoped
together. Postabdominal segments with slight
posterior curvature; ornamentation absent.
Pretelson with pair of short, styliform epimera.
Telson short and styliform.
No. 35/324
Excellent specimen preserved as external mould
in dorsal view (Figs 6A; 7A). Total length 34,
carapace length 9, preabdomen length 7, postabdomen length 18. Maximum width 13. Carapace
well preserved, rounded anteriorly and showing
both median and lateral eyes. Median eyes small
and located in slight triangular depressions on
posteriorly tapering tubercle in centre of carapace. Broad depressions present either side of
median eyes. Lateral eye tubercles reniform,
each located at the posterior end of what
appears to be an ophthalmic ridge. Slight ridge
preserved close to anterior margin of the carapace where it follows the carapace curvature.
Small prosomal limb fragment preserved on left
side of specimen beyond carapace margin. All
13 opisthosomal segments preserved. Tergite 1
358
short, but lateral tuberculation evident. Tergites
2-4 large, forming the preabdomen, with tergite
4 longest. Preabdomen with strong sculpture of
tubercles, with most tubercles concentrated at
the lateral margins. Postabdomen elongate with
all segments approximately the same length and
narrowing gradually posteriorly. Postabdominal
segments lack ornamentation. Terminal
segment with poorly preserved pair of epimera
surrounding a short telson. Telson with slight
median depression.
No. 35/337
Specimen preserved in dorsal view as an internal
mould (Figs 6B; 7B). Total preserved length 18,
carapace length 8, preabdomen length 5, postabdomen length 5, though terminal end of postabdomen missing. Maximum width 12.
Carapace rounded anteriorly, highest in the centre where the median eyes would be expected,
but morphological details lacking. Preabdomen
distorted and lacks detail, though tuberculation
at the lateral margins well preserved.
Postabdomen telescoped with short segments
tapering posteriorly. Terminal end of postabdomen and telson absent.
No. 35/339
Specimen preserved as internal mould in dorsal
view (Figs 8C; 9C). Total length 20, carapace
length 8, preabdomen length 7, postabdomen
length 5. Maximum width 12. Carapace rounded
anteriorly and with straight posterior margin,
but shows few morphological details. Two
appendages, possibly prosomal limbs 3 or 4,
preserved beyond carapace margin. Limbs
slender with evidence for division into short
podomeres. All 13 opisthosomal segments
preserved. Tergite 1 distinct and short.
Preabdomen poorly preserved with much of
tergites 2-4 missing to reveal the featureless
ventral plate beneath them. Postabdomen highly telescoped and bends down towards the
matrix at the distal end such that posterior tip
is absent. First postabdominal segment (segment 5) broken to reveal full length of next
segment tucked under it.
GEODIVERSITAS • 2002 • 24 (2)
Severnaya Zemlya arthropods
A
B
C
D
FIG. 6. — Octoberaspis ushakovi n. gen., n. sp.; A, No. 35/324, 34 × 13 mm; B, Nos. 35/336 (holotype) (above), 24 × 14 mm, and
35/337 (below), 18 × 12 mm; C, No. 35/711, 33 × 9 mm; D, No. 35/712, total length 30 mm.
GEODIVERSITAS • 2002 • 24 (2)
359
Dunlop J. A.
No. 35/379
Specimen probably in ventral view, but with
prosoma missing (Figs 8E; 9E). Total preserved
length 27, preabdomen length 9, postabdomen
length 18. Maximum width 12. Preabdomen
composed of broad, mostly featureless, but
slightly folded area (the ventral plate) overlying
tuberculated tergites visible on right margin of
preabdomen. Postabdomen well preserved,
though segments have become slightly disarticulated and separated by thin areas of matrix.
Postabdomen tapers posteriorly with all segments approximately the same length.
Postabdominal segments lack ornamentation.
Thirteenth segment with clear pair of short,
styliform epimera. Short, styliform telson lies
between these epimera.
No. 35/707
Specimen preserved in dorsal view (Figs 8B; 9B).
Total length 26, carapace length 9, preabdomen
length 7, postabdomen length 11. Maximum
width 12. Carapace rounded anteriorly with distinct ridge close to carapace margin and median
eyes clearly visible as a pair of tiny circles in the
centre of the carapace set into slight triangular
depressions. Lateral eyes not preserved.
Appendage fragments preserved on right side of
specimen, including a poorly preserved paddle.
Tergite 1 not preserved. Preabdomen poorly
preserved with much of the segmentation missing to reveal the ventral plate underneath.
Postabdomen quite well preserved, though segmentation indistinct. Postabdomen tapers
towards the fragmentory pretelson segment and
telson.
served. Opisthosoma without characteristic
broad preabdomen visible in dorsal view.
Segment 1 (the metastoma) not clearly preserved. Segments 2-4 expressed as sclerites
(probably operculae). Distinct genital
appendage preserved on the midline in positive
relief, length 4, originating from segment 2 and
extending down to posterior margin of segment 3. Genital appendage relatively squat,
apparently consisting of a single section,
widening posteriorly with a distinctly trilobed
terminal end. Postabdomen tapers posteriorly
with all segments approximately same length.
Epimera on segment 13 poorly preserved, but
short, styliform telson present.
No. 35/712
Specimen preserved in ventral view (Figs 6D;
7D). Total length 30, prosoma length 6, preabdomen length 5, postabdomen length 19.
Prosoma fairly poorly preserved, but with distinct shield-shaped metastoma representing
opisthosomal segment 1. Opisthosomal segments 2-4 expressed as sclerites (probably opercula). Anterior two sclerites strongly curved
posteriorly and sculptured with a small number
of tubercles. Sclerites 2 and 3 approximately
same length, sclerite 4 longer. Sclerite 2 with
indeterminate, squat median structure reaching
entire length of plate. Median structure apparently composed of rectangular basal section and
triangular distal section, again interpreted as a
genital appendage. Postabdomen tapers posteriorly, curving slightly to the right, with all segments approximately the same length. Segment
13 with poorly preserved pair of short, styliform
epimera. Telson short and styliform.
No. 35/711
Specimen preserved in ventral view, mostly as
an external mould, but with some structures in
positive relief (Figs 6C; 7C). Total length 33,
prosoma length 7, opisthosoma length 26,
tagmosis into preabdomen and postabdomen
not clearly demarcated. Maximum width 9.
Prosoma poorly preserved, but may show
areas corresponding to coxae of prosomal
appendages. Prosomal appendages not pre-
360
No. 35/713
Small, probably juvenile specimen in dorsal
view (Figs 8F; 9F). Length 11, carapace length
4, opisthosoma length 7. Differentiation into
preabdomen and postabdomen not so distinct
as in adults. Maximum width 4. Carapace
rounded with proportionally large median and
lateral eye tubercles. Opisthosoma apparently
with full complement of 13 segments, postab-
GEODIVERSITAS • 2002 • 24 (2)
Severnaya Zemlya arthropods
A
B
cr
me
le
ap
ap
cp
gs
1
2
pra
tb
pra
3
5
6
7
8
9
poa
4
poa
tb
ep
tl
10
11
12
13
tl
C
D
ms
ga
op
pra
ga
poa
FIG. 7. — Octoberaspis ushakovi n. gen., n. sp., interpretative drawings of the specimens shown in Fig. 6; A, No. 35/324; B, Nos.
35/336 (holotype) (above) and 35/337 (below); C, No. 35/711; D, No. 35/712. Abbreviations: ap, prosomal appendage; cp, carapace;
cr, marginal carapace rim; ep, epimera of pretelsonic segment; ga, genital appendage; gs, genal spine; le, lateral eye tubercle;
me, median eye tubercle; ms, metastoma; op, opisthosomal operculae; pra, preabdomen; poa, postabdomen; tb, preabdominal
tuberculation; tl, telson, opisthosomal segments numbered. Scale bar: 5 mm.
GEODIVERSITAS • 2002 • 24 (2)
361
Dunlop J. A.
domen tapering posteriorly, though posteriormost segments difficult to distinguish. Telson
slender and proportionally longer than in
adults.
No. 35/719
Small, probably juvenile, specimen preserved
mostly in outline and in dorsal view (Figs 8G; 9G).
Total length 8, carapace length 3, preabdomen
length 3, postabdomen length 3. Maximum width
3.5. Carapace rounded anteriorly but lacks detail.
Eyes and appendages not preserved. Preabdomen
rectangular and segmentation not evident.
Postabdomen tapers distally where segment
boundaries are faintly visible. Telson not preserved.
Chasmataspida incertae sedis
No. 35/714
Poor specimen lacking details, but appears to
show different proportions of the postabdomen in comparison to the other material
(Figs 8D; 9D). Total preserved length 29, carapace length 8, preabdomen length 7, postabdomen length 14, though not all postabdominal
segments preserved. Maximum width 12.
Carapace rounded anteriorly, but lacking
detail. Appendages absent. Preabdomen
appears broader than long but segmentation
and ornamentation not preserved. At least
seven postabdominal segments preserved, all
very narrow compared to preabdomen, average
width 3. These segments do not clearly taper
posteriorly.
No. 35/735
Relatively poor specimen in dorsal view in centre of nodule surrounded by a number of additional fragments (Figs 8A; 9A). Total preserved
length 27, carapace length 8, preabdomen length
8, postabdomen length 11, though terminal end
missing. Maximum width 12. Carapace broadly
rounded anteriorly with genal spine on right side,
but details lacking. Small paddle clearly preserved on left side at the posterior left corner of
the carapace. Terminal three podomeres of paddle preserved, a broad proximal segment, length
3, widening distally and an oval distal segment,
length 3. Small diamond-shaped podomere articulates between them. Preabdomen with somewhat indistinct segmentation. Tergites on left
side removed during preparation to reveal ventral plate underlying the tergites. Scratches on
ventral plate are preparation artefacts. Left side
of ventral plate itself further removed to reveal
ventral surface of preabdomen at its lateral margins where it forms a doublure tucking under to
form ventro-lateral region of opisthosoma. This
ventral surface matches outline of dorsal surface
and has marginal, ventral tuberculation similar to
the marginal dorsal tuberculation. Ventral opercula not revealed during preparation. Postabdomen telescoped, but poorly preserved and
segmentation indistinct. Telson faintly preserved.
362
MORPHOLOGICAL RECONSTRUCTION
Carapace
The carapace is broadly rounded anteriorly
with a distinct ridge, or rim, close to the anterior margin (No. 35/324, Figs 6A; 7A). The
posterior margin of the carapace is approximately straight, though in the more complete
carapaces small genal spines are present at the
postero-lateral corners of the carapace (e.g.,
No. 35/336, Figs 6B; 7B). Genal spines are also
known from Chasmataspis and though not
explicitly described from Diploaspis and
Heteroaspis, appear to be present based on
Størmer’s (1972) plates and figures of the
Alken an der Mosel fossils. The carapace lacks
any obvious ornament, like that seen on the
preabdomen. The detailed shape of the carapace in life is difficult to reconstruct since
there is evident compression of the material,
though there appears to be slight lobes on the
carapace between the median and lateral eyes.
This, however, would be difficult to interpret
as a xiphosuran-like cardiac lobe. There are
lateral depressions of the carapace delineated
by curving lines posterior to the lateral eyes
(No. 35/324, Figs 6A; 7A, No. 35/713, Figs 8F;
9F; 11). A similar morphology has been reconstructed for some eurypterid carapaces (e.g.,
Størmer 1955: fig. 17).
GEODIVERSITAS • 2002 • 24 (2)
Severnaya Zemlya arthropods
A
B
C
F
D
E
G
FIG. 8. — A-C, E-G, Octoberaspis ushakovi n. gen., n. sp.; A, No. 35/735, 27 × 12 mm; B, No. 35/707, 26 × 12 mm; C, No. 35/339, 20
× 12 mm; E, No. 35/379, 27 × 12 mm; F, No. 35/713 (juvenile), 11 × 4 mm; G, No. 35/719 (juvenile), 8 × 3.5 mm; D, Chasmataspida
incertae sedis, No. 35/174, 29 × 12 mm.
Eyes
The median eyes, or ocelli, are tiny structures
located in V-shaped depressions on a small,
GEODIVERSITAS • 2002 • 24 (2)
posteriorly tapering tubercle in the centre of
the carapace (Nos. 35/324, 35/707, Figs 6A, B;
7A, B). Lateral eye tubercles can be clearly
363
Dunlop J. A.
seen in 35/324 as well (Figs 6A; 7A). The
lateral eyes are fairly small, kidney-shaped and
located close to the lateral margins of the carapace at about the same level as the median
eyes. The lateral eyes appear to be situated on
slight ridges, at least anterior to the eye tubercle itself, where a short ridge curves laterally
towards the anterior carapace rim (No. 35/324,
Figs 6A; 7A). These are similar to the ophthalmic ridges of xiphosurans, though the
presence of this character should be treated
with caution as it has been considered an
autapomorphy of the latter group (Anderson
& Selden 1997) and is not obvious in
Chasmataspis, Diploaspis or Heteroaspis.
Prosomal appendages
The prosomal appendages are poorly preserved.
Apart from the metastoma (see below), the morphology of the coxal region and chelicerae (if
present) are unknown. However, a number of
specimens (e.g., Nos. 35/324, 35/336, 35/707,
35/339, Figs 6A, B; 7A, B; 8B, C; 9B, C) show
fragments of narrow, pediform appendages projecting beyond the carapace. Similar, though
dissociated, appendages were described in
Diploaspis by Størmer (1972). The present material, with the legs in situ, suggests they were
somewhat eurypterid-like, since in xiphosurans
the appendages are held entirely beneath the
carapace. Unfortunately the detailed morphology of these pediform chasmataspid appendages
remains unknown and the fossils do not show
the number of podomeres, whether or not they
bore spines and whether or not they were
chelate (like at least one leg described by Caster
& Brooks [1956] from Chasmataspis).
In addition to these pediform appendages, this
new material includes two examples of paddles
(Nos. 35/735, 35/707, Figs 8A, B; 19A, B) similar
in size and position to those figured by Størmer
(1972) in Diploaspis. These paddles probably
represent the modified sixth prosomal appendage, though with an incomplete coxal region
this cannot be proven here. Three podomeres of
the paddle can be seen in this material, two larger
podomeres with a smaller, diamond-shaped
364
podomere in between them (No. 35/735,
Figs 8A; 9A) which appears to act as some sort of
articulation between the “blades” of the paddle.
Though strongly resembling the paddles of many
eurypterids, and Recent portunid crabs, this
morphology differs slightly to the eurypterid
paddle (e.g., Størmer 1955), which does not have
a diamond-shaped podomere in this somewhat
lateral position. Furthermore there is no obvious
distal “claw” in the chasmataspid paddle, something that is seen in eurypterids.
Opisthosomal segmentation
This new material, along with an ongoing revision of the Chasmataspis type material, indicates
that chasmataspids had 13 opisthosomal segments (e.g., Anderson & Selden 1997), not 12 as
reported by Caster & Brooks (1956) and most
subsequent authors. The extra-segment comes
from the recognition of a short tergite between
the carapace and the preabdomen which can be
seen in Nos. 35/324 and 35/336 (Figs 6A, B; 7A,
B). This is a genuine sclerite, and not just a space
where the prosoma and opisthosoma have disarticulated, because in both specimens there are
traces of tuberculation on this first tergite.
Following this, tergites 2-4 are broad sclerites
that form the dorsal surface of the preabdomen
(e.g., No. 35/324, Figs 6A; 7A), or “buckler” to
use Caster & Brooks’ (1956) term. These preabdominal tergites are associated with three ventral
opercula (see below). Following the preabdomen, segments 5-13 are interpreted as ringlike segments with fused tergites and sternites,
and form the nine-segmented postabdomen (e.g.,
Nos. 35/324, 35/379, Figs 6A; 7A; 8E; 9E). The
short telson is not considered a true body segment.
Preabdomen
Dorsally the preabdomen is slightly wider than
long, somewhat rounded in appearance and is divided into three tergites (tergites 2-4) with slightly curved posterior margins (e.g., Nos. 35/324,
35/336, Figs 6A, B; 7A, B). Tergite 4 is the
longest and has a pair of broad projections forming the postero-lateral corners. In Chasmataspis
the “buckler” of tergites appears to be a fully
GEODIVERSITAS • 2002 • 24 (2)
Severnaya Zemlya arthropods
A
B
cr
me
ap
C
ap
gs
ap
pd
pd
od
vp
vp
A, B, E, F
C, D, G
F
me
D
le
E
cp
vp
pra
poa
tl
pra
poa
G
cp
pra
ep
tl
poa
FIG. 9. — Interpretative drawings of the specimens shown in Fig. 8; A-C, E-G, Octoberaspis ushakovi n. gen., n. sp., A, No. 35/735; B, No.
35/707; C, No. 35/339; E, No. 35/379; F, No. 35/713 (juvenile); G, No. 35/719 (juvenile); D, Chasmataspida incertae sedis, No. 35/714.
Abbreviations: ap, prosomal appendage; cp, carapace; cr, marginal carapace rim; ep, epimera of pretelsonic segment; gs, genal spine;
le, lateral eyes; me, median eyes; od, opisthosomal doublure where tergites tuck round onto ventral surface; pd, paddle (prosomal
appendage VI?); pra, preabdomen; poa, postabdomen; tl, telson; vp, ventral plate underlying tergites. Scale bars: 5 mm.
GEODIVERSITAS • 2002 • 24 (2)
365
Dunlop J. A.
fused structure similar to the xiphosurid thoracetron, but fusion appears less likely in this
new material as there is a distinct overlap between the tergites. The preabdomen has a distinct
ornamentation of tubercles (see also Systematics)
with rows of larger, oval median tubules merging
into a higher concentration of smaller tubules at
the lateral margins of the tergites.
Ventrally, there are three preabdominal plates
(Nos. 35/711, 35/712, Figs 6C, D; 7C, D), presumably corresponding to tergites 2-4 (see also
below). This is a significant observation. Both
Caster & Brooks (1956) and Størmer (1972)
described their chasmataspids as having a preabdomen with a single, fused ventral plate and
interpreted this as having a pair of long, posterior slits opening respectively into a pair of
pouches containing the gills. This would be very
different to what is known from other aquatic
chelicerates, i.e. xiphosurans and eurypterids,
where there are a series of ventral, plate-like
operculae derived from the opisthosomal
appendages. Simonetta & Delle Cave (1981)
noted the obvious problems of functional morphology in getting water through narrow slits
without some sort of pumping mechanism.
Their suggestions about how the chasmataspid
gills were oxygenated were ingenious, though
not entirely convincing, while both they and
Størmer (1972) further speculated that the large
ventral plate might protect the gills from drying
and imply that chasmataspids were at least partially terrestrial.
The new Severnaya Zemlya material suggests
that chasmataspids were in fact more typical
chelicerates and did have a series of plate-like
opercula on the ventral surface of the opisthosoma. Furthermore, unpublished studies by the
author in collaboration with Simon Braddy and
Lyall Anderson have interpreted some late
Cambrian material from Texas as ventral
impressions of a Chasmataspis-like animal
which also appears to have had a series of preabdominal operculae (see also Dunlop et al.
1999). Unfortunately chasmataspid respiratory
organs, and which segments they belonged to,
are unknown. Meanwhile, two questions remain
366
from the preabdomen of the fossils described
here. Firstly, the ventral operculae are not as
broad as the corresponding tergites. This implies
a similar situation to Recent xiphosurans in
which the tergites (here fused into a thoracetron) continue round onto the ventral surface as a broad doublure (e.g., Størmer 1955),
while the gill operculae occupy a trapezoidal
space set into this doublure. This interpretation
is supported by No. 35/735 (Figs 8A; 9A),
which shows essentially a dorsal view but where
successive layers of the preabdomen have been
removed by preparation. This specimen reveals
the ventral surface of the preabdomen at its lateral margins which retains the outline of the
dorsal surface, and was likewise tuberculated.
This interpretation is made with some reservations as no single specimen shows the entire
ventral preabdomen, but the latter feature is
shown in the reconstruction (Fig. 10).
Secondly, this specimen, along with Nos. 35/707,
35/339 and 35/379 (Figs 8B, C, E; 9B, C, E) preserves a broad, featureless area within the postabdomen that lacks obvious segmentation.
Preparing down through a specimen, removing
first the tergites and then this ventral plate, indicated that the plate is sandwiched between the
dorsal tergites and the ventral preabdomen (see
No. 35/735, Figs 8A; 9A). This plate is unusual
for chelicerates and furthermore it is probably
the same structure as the single ventral plate described by both Caster & Brooks (1956) and
Størmer (1972) in their chasmataspids. Its presence is confirmed in this new material, but its interpretation remains difficult. The simplest
explanation is that it is the ventral wall of the preabdomen from which the operculae hang.
Nothing directly comparable is known from fossil eurypterids or xiphosurans. However examination of Recent xiphosuran material from
which the gill operculae were removed showed
that these operculae do overly a single plate, essentially the fused true sternites of the opisthosoma, although unlike the chasmataspid plate
observed here this has a relatively large space anteriorly where the operculae attach. Re-examination of both Caster & Brooks’ (1956) and
GEODIVERSITAS • 2002 • 24 (2)
Severnaya Zemlya arthropods
FIG. 10. — Reconstruction of Octoberaspis ushakovi n. gen., n. sp. in both dorsal and ventral view. Chelicerae and leg coxae hypothetical, based on comparisons with eurypterids and xiphosurans. Scale bar: 5 mm.
Størmer’s (1972) material will probably be needed before this structure can be fully explained.
Genital appendage
One specimen, No. 35/711 (Figs 6C; 7C), shows a
structure on the midline of the ventral preabdomen which originates on the anteriormost
operculum and extends down to the posterior
margin of the next sclerite. Here it widens and divides into three parts. This structure is very similar
in size, shape and position to the longer, type A,
genital appendage of eurypterids, a character previously thought to be unique (i.e. autapomorphic)
GEODIVERSITAS • 2002 • 24 (2)
for this group. A second specimen, No. 35/712
(Figs 6D; 7D), shows a short, squat structure,
again on the midline and located on the anteriormost preabdominal operculum. The structure
here consists of a rectangular basal region and a
triangular distal region pointing posteriorly. This
is not unlike the shorter, type B, genital appendage
in eurypterids; see Braddy & Dunlop (1997) for a
recent study of eurypterid genital appendages.
A genital appendage in chasmataspids would be a
significant discovery, but we must exclude the
possibility that Nos. 35/711 and 35/712 are simply misidentified eurypterids. Firstly, there are
367
Dunlop J. A.
no other specimens from this locality which can
unequivocally be interpreted as eurypterids.
Secondly, these two specimens do not show the
eurypterid body plan of a preabdomen of seven
segments and a postabdomen of five segments.
They are much more consistent in overall appearance with chasmataspids and their ninesegmented postabdomen plus very short telson;
the telsons of most eurypterids are elongate by
comparison. Finally, the genital appendage of
eurypterids is associated with a large genital
operculum, probably a fusion of two operculae
(Braddy & Dunlop 1997). There is no large genital operculum in these Severnaya Zemlya
Devonian specimens, which can therefore be regarded with some confidence as chasmataspids
having a genital appendage. This structure also
supports the pattern of opisthosomal segmentation presented herein. If the metastoma belongs
to opisthosomal segment 1 (see below) then the
next segment should be the genital segment in
chelicerates (number 2), and indeed the structure
interpreted as the genital appendage of chasmataspids originates on this segment.
Metastoma
As well as the genital appendage, No. 35/712
(Figs 6D; 7D) has a shield-shaped plate on the
ventral prosoma. This is pointed posteriorly and
has clear margins preserved on the anterior and
right sides. This plate is very similar in size,
position and appearance to the eurypterid
metastoma, a plate covering the posterior
gnathobases in these animals. Like the genital
appendage, a metastoma was thought to be an
eurypterid autapomorphy (e.g., Dunlop &
Selden 1997), but this new material shows that it
was present in chasmataspids too. The metastoma is believed by many authors (e.g., Størmer
1955) to represent the fused appendages of
opisthosomal segment 1 and this interpretation
would fit the pattern of chasmataspid opisthosomal segmentation presented above.
Postabdomen
The postabdomen comprises nine ring-like segments which originate beneath the preabdominal
368
tergites (e.g., No. 35/379, Figs 8E; 9E). The first
postabdominal segment, segment 5, is usually
longest, with successive segments approximately
the same length. The whole postabdomen tapers
posteriorly towards the telson. However, there
are also a number of “short” specimens in which
there is considerable telescoping of the postabdomen (e.g., Nos. 336-7, Figs 6B; 7B).
Initially, it was suspected that the “short” morphotype represented sexual dimorphism, or perhaps even a different species, but careful
observation of these “short” forms in which
overlying segments were either removed by
preparation or naturally broken revealed complete, “long” segments underneath (e.g., No.
35/339, Figs 8C; 9C). Furthermore, the degree of
shortening is not consistent between specimens.
This type of postabdominal telescoping is known
from eurypterids (Simon Braddy pers. comm.),
and is presumably taphonomic in origin.
The postabdominal segments lack the preabdominal ornamentation of large tubercles and
have a slight curve to their posterior margins.
Where the full length of the postabdomen is preserved there is a slight bend in some examples
(e.g., No. 35/712, Figs 6D; 7D), which suggests
that there was some flexibility in this structure.
The thirteenth (pretelsonic) opisthosomal segment has a pair of short, tapering, posteriorly directed, epimera (No. 35/379, Figs 8E; 9E). The
telson originates between these epimera and is
similarly short and tapering, though a little
longer than the adjacent epimera. This gives the
end of the opisthosoma a trifurcate appearance.
The telson itself has a slight median dorsal depression (No. 35/324, Figs 6A; 7A).
Juveniles
Specimen No. 35/719 (Figs 7G; 9G) is a very
small, probably juvenile example, about 8 mm
long, preserved with a truncated postabdomen.
More interesting is No. 35/708 (Figs 8F; 9F),
another small specimen, about 11 mm long. Its
status as a juvenile is suggested by the proportionally large median and lateral eye tubercles
relative to the rest of the carapace. It also shows
that at least some juveniles had the full comple-
GEODIVERSITAS • 2002 • 24 (2)
Severnaya Zemlya arthropods
ment of 13 opisthosomal segments (it has been
reported [e.g., Størmer 1955], probably erroneously, that juvenile eurypterids had less segments than adults) and that the telson was
relatively long in early instars. This suggests that
the short telson of the adult form is somewhat
derived relative to a plesiomorphic condition
still expressed in juveniles. A number of other
small, but poorly preserved specimens in this
material probably represent juveniles too.
Incertae sedis
One curious specimen remains among the
Severnaya Zemlya material (No. 35/714, Figs 8D;
9D). Although preserved in outline, there are
few morphological details. However, there are
at least seven postabdominal segments preserved
that appear to be rather narrower than the
postabdomens of the other chasmataspid material from this locality. In fact, it is similar in
overall appearance to Chasmataspis. Since the
specimen is not well preserved I am reluctant to
create a new taxon for what may be a preservational variant, but the specimen is included here
for completeness.
PHYLOGENY
Four chasmataspids have been previously
described: Chasmataspis, from the Ordovician
of the USA (Caster & Brooks 1956), Diploaspis
and Heteroaspis, both from the Lower
Devonian of Germany (Størmer 1972), and
Forfarella from the Lower Devonian of
Scotland (Dunlop et al. 1999). A Middle
Devonian example has also been recognised
from Scotland (Anderson et al. 2000). Two
additional taxa, Borchgrevinkium taimyrensis
Novojilov, 1959 and “Eurypterus” stoermeri
Novjilov, 1959, originally described as
eurypterids, may also be chasmataspids
(Størmer 1972), but this requires confirmation
from the original material.
As a group, chasmataspids are undoubtedly chelicerates (Dunlop & Selden 1997), but until now
their position within Chelicerata was less certain. Chasmataspids were originally interpreted
as an order of the Xiphosura (Caster & Brooks
GEODIVERSITAS • 2002 • 24 (2)
1956; Stormer 1972; Selden & Siveter 1987).
Subsequently, Eldredge (1974) suggested chasmataspids were related to eurypterids based on
similar appendages. Though not strictly an
Eldredge character, the present study suggests
that the paddles of chasmataspids and
eurypterids are probably a convergent development in both groups (see also Bergström 1975).
Meanwhile, Bergström (1975, 1979, 1980) questioned chasmataspid monophyly, restricting
Chasmataspida to Chasmataspis, placing
Heteroaspis as Chasmataspida incertae sedis and
Diploaspis, with its appendages unlike those of
other xiphosurans, close to the ancestry of
arachnids. Simonetta & Delle Cave (1981) and
Delle Cave & Simonetta (1991) regarded chasmataspids as one of the chelicerate branches
radiating from among the “emeraldellids”, i.e.
Emeraldella and similar looking “Burgess
Shale” type arthropods.
Anderson & Selden (1997) excluded chasmataspids from Xiphosura due to differences in
the segmentation and tagmosis of the opisthosoma, correctly recognising the presence of 13
opisthosomal segments. Dunlop & Selden
(1997) recognised Chasmataspida as a monophyletic group within Chelicerata, defined by
their autapomorphic opisthosomal tagmosis, i.e.
a preabdomen of three segments and a postabdomen of nine. Dunlop & Selden (1997) further
recognised the clade (Chasmataspida
(Eurypterida + Arachnida)), with xiphosurans
as the outgroup. However, this clade was based
on a synapomorphy of an opisthosoma of
12 segments, a character we now know to be
incorrect for chasmataspids (Anderson & Selden
1997; Dunlop et al. 1999; this study), and which
may be incorrect for eurypterids and scorpions
too (Dunlop & Webster 1999).
The new Severnaya Zemlya fossils are significant in resolving the phylogenetic position of
chasmataspids. The genital appendage and
metastoma seen in this material represent two
strong synapomorphies for Eurypterida and
Chasmataspida. These characters were previously autapomorphic for eurypterids, and their
presence, and significance, in chasmataspids was
369
Dunlop J. A.
A
B
C
FIG. 11. — Elymocaris urvantsevi n. sp.; A, No. 35/377, 40 × 19 mm; B, No. 35/373, 14 × 8 mm; C, No. 35/372 (holotype), total length
51 mm.
noted by Dunlop (1997) and Dunlop et al.
(1999). The genital appendage and metastoma
could be seen as grounds for referring
Chasmataspida to Eurypterida, although chasmataspids have genal spines and the material
described here has lateral eyes associated with
poorly-defined carapace ridges (both absent in
eurypterids). Chasmataspids also have a different pattern of opisthosomal tagmosis. The position of the scorpions, which have been
considered as the sister group of eurypterids by
some authors (see Dunlop & Webster 1999),
may complicate this issue. Chasmataspids share
a number of characters both with xiphosurans
(e.g., genal spines, opisthosomal doublure,
?ophthalmic ridges) and eurypterids (genital
370
appendage, metastoma, ?longer legs) and will
eventually need to be placed as part of a broader
revision of chelicerate phylogeny.
MODE OF LIFE
Størmer (1972) considered Diploaspis and
Heteroaspis to be primarily aquatic, especially
given the paddles in Diploaspis which suggest an
animal that could swim. This interpretation seems
reasonable for the Severnaya Zemlya fossils which
have paddles too. Like these new Russian forms,
Diploaspis and Heteroapsis are thought to have
been deposited in quiet, possibly lagoonal waters.
Both Størmer (1972) and Simonetta & Delle Cave
(1981) speculated that chasmataspids were capable
of terrestrial activity, though this proposal was
GEODIVERSITAS • 2002 • 24 (2)
Severnaya Zemlya arthropods
A
B
ab
ab
fu
at
cp
tl
A, C
C
st
tl
fu
ab
cp
st
FIG. 12. — Elymocaris urvantsevi n. sp., interpretative drawings of the specimens shown in Fig. 11; A, No. 35/377; B, No. 35/373;
C, No. 35/372 (holotype). Abbreviations: ab, abdomen; at, anterior tubercle; cp, carapace pleural fold; fu, furca; st, striae; tl, telson.
Scale bars: 5 mm.
partly based on the belief that these animals had a
ventral opisthosoma covered by a single plate
which could protect the gills on land (see discussion above). There is no convincing evidence for
chasmataspids having special opisthosomal structures allowing them to be habitually terrestrial;
although an amphibious mode of life has been
proposed for eurypterids (see Manning & Dunlop
1995 for a review) and even xiphosurans emerge
onto land to breed.
With regards to feeding, most chelicerates are
predators and though we have no details of the
chasmataspid mouthparts it seems likely that they
were essentially benthic predators with a mode of
life analogous to xiphosurans. Small phyllocarids
and the ostracodes recorded from the same for-
GEODIVERSITAS • 2002 • 24 (2)
mation as the chasmataspids represent a possible
source of food, although xiphosurans also dig
soft-bodied prey such as worms out of the substrate. Paddles suggest these chasmataspids could
swim, although whether they swam to catch prey
or to avoid predators (i.e. acanthodian fish) remains speculation. Paddles might also be used to
help bury themselves in the substrate.
Subclass PHYLLOCARIDA Packard, 1879
REMARKS
Approximately 70 phyllocarid crustaceans are
present among this Severnaya Zemlya material
and these were provisionally referred to
371
Dunlop J. A.
Ceratiocaridae by Novilskaya et al. (1983). Some
additional arthropod fragments also probably
represent crustaceans. The fossils are evidently
phyllocarids, having a large, bivalved carapace
comprising a cephalothoracic shield formed
from two pleural folds and an anteriorly projecting rostral plate, and an abdomen projecting
from this carapace ending in a telson with a pair
of furcal rami. The best specimens are described
and figured here (Nos. 35/372-3, 35/375, 35/377,
35/778-9, 35/781-2) (Figs 11-14).
him as having subcircular carapace valves, and as
such this new Severnaya Zemlya material can be
referred to Rhinocarididae with some confidence.
Genus Elymocaris Beecher, 1884
TYPE SPECIES. — Elymocaris siliqua Beecher, 1884.
ADDITIONAL SPECIES. — Elymocaris urvantsevi n. sp.
DIAGNOSIS. — Like Rhinocaris but carapace valves
without posteroventral spine; no mesolateral carina,
anterior tubercle present; rim with oblique, posteriorly imbricating ridges; rostral plate folded ventrally
along two lateral anteriorly converging carinae; telson
with broad median ridge (Rolfe 1969).
Order ARCHAEOSTRACA Claus, 1888
REMARKS
REMARKS
Rolfe (1969) diagnosed this order on the presence of a hinge line on the carapace, an elongate
seventh abdominal (i.e. pretelsonic) segment and
a telson produced between two furcal rami, all
of which are present in this new Severnaya
Zemlya material.
Suborder RHINOCARINA Clarke
in Eastman & Zittel, 1900
DIAGNOSIS. — Carapace with a median dorsal plate
separating the valves behind rostral plate; last abdominal somite elongated (Rolfe 1969).
REMARKS
Although tentatively referred to Ceratiocarididae (a family of suborder Ceratiocarina)
by Novilskaya et al. (1983), the new Severnaya
Zemlya material appears to have a median dorsal
plate and so can be referred to the suborder
Rhinocarina sensu Rolfe 1969.
The Severnaya Zemlya material is referred to
Elymocaris with some reservations. It lacks posteroventral spines on the carapace, which
excludes it from Rhinocaris Hall & Clarke, 1888,
and Dithyrocaris Scouler, 1835, and it has an
anterior tubercle, which was reported absent by
Rolfe (1969) in Tropidocaris Beecher, 1884, the
other rhinocaridid genus lacking posteroventral
spines. There is also a ridge on the telson in this
new material. However, some specimens shows
structures which could be interpreted as lateral
carina, but could equally well be taphonomic
folding of the carapace. Furthermore, the folding of the rostral plate, a diagnostic character of
Elymocaris, is not obvious in this Severnaya
Zemlya material. I am reluctant to create a new
genus unnecessarily and so have referred this
material to the genus which it most closely
resembles, Elymocaris, though I note Rolfe’s
(1969) and Racheboeuf’s (1994) comments that
many supposed generic characters may only be
of specific value. As such, considerable revision
of the family may be required.
Family RHINOCARIDIDAE Hall & Clarke, 1888
DIAGNOSIS. — Carapace valves elongate, subovate;
median dorsal plate narrow and with chevron ornament; rostral plate and median dorsal plate slightly
bent along median carina (Rolfe 1969).
REMARKS
The other rhinocaridid family mentioned by
Rolfe (1969), Ohiocarididae, was diagnosed by
372
Elymocaris urvantsevi n. sp.
(Figs 11-15)
Ceratiocaridae – Novilskaya et al. 1983: 94, fig. 4.
HOLOTYPE. — GIL No. 35/372 (oc 1/bd 21).
ADDITIONAL MATERIAL. — GIL Nos. 35/373 (oc 1/bd
21), 35/375 (oc 41/bd 12), 35/377-378, 35/779 (all oc
67/bd 12), 35/781 (oc 1/bd 21), 35/782.
GEODIVERSITAS • 2002 • 24 (2)
Severnaya Zemlya arthropods
ADDITIONAL MATERIAL (NOT FIGURED). — GIL Nos.
35/374, 35/376 (both oc 41/bd 12), 35/780 (oc 1/bd
21), 35/783-790 (all oc 67/bd 12), 35/791-817 (all oc
1/bd 21), 35/818-837 (all oc 41/bd 12).
E TYMOLOGY . — In honour of N. N. Urvantsev, a
geologist who participated in the first expedition to
Severnaya Zemlya in 1930-1932.
G EOGRAPHICAL AND STRATIGRAPHICAL DISTRIBU TION. — All material is from the Severnaya Zemlya
Formation (Lower Devonian [lower Lochkovian]),
October Revolution Island.
D I A G N O S I S . — Elymocarid with a downwardcurving, hook-shaped rostral plate.
DESCRIPTION
No. 35/372 (holotype)
Almost complete specimen in lateral view
(Figs 11C; 12C). Total length 51, carapace
length 29, abdomen length, including telson, 22.
Maximum carapace width 13. Rostral plate faint,
hook-shaped. Cephalothoracic shield preserved
as impression of left pleural fold, broadly oval,
straighter along the dorsal surface and with a
distinct, narrow margin; posterior end of shield
obscured. Pleural fold with broad, oblique folds
(?carina) below which is a large depression in
the antero-ventral region. Pleural fold ornamented with fine antero-posterior striae, closely
packed near the rostal plate, more widely spaced
in the posterior ventral region of the shield.
Abdomen tapers posteriorly, individual
pleomeres difficult to distinguish. Styliform telson preserved along with one shorter furcal
ramus. Appendages not preserved.
No. 35/373
Disarticulated carapace and abdomen (Figs 13A;
14A). Maximum carapace length 14, though
posterior end of carapace not preserved.
Maximum carapace width 8. Rostral plate
apparently hook-shaped, but detached from rest
of carapace and surrounded by other cuticle
fragments. Right pleural fold of cephalothoracic
shield preserved, broadly oval in outline with
narrow margin, but details of morphology poor.
Abdomen detached from carapace, maximum
length including telson 17. Five abdominal
pleomeres preserved, possibly in ventral view,
GEODIVERSITAS • 2002 • 24 (2)
plus telson. Pleomere lengths: 3, 2, 1.5, 2, 3.5,
average width 4, but narrowing posteriorly.
First pleomere poorly preserved, second with
faint ridge, third with large tubercle, fourth and
fifth with distinct ventral ridges. Telson styliform with median groove, length 6. Both styliform furcal rami preserved, length 3.
Appendages not preserved.
No. 35/375
Terminal end of abdomen in dorsal view
(Figs 11B; 12B). Maximum length 23. Division
of abdomen into pleomeres indistinct, but
abdomen tapers posteriorly. Telson styliform,
length 15, with broad median depression. Furcal
rami also styliform and with median depression,
but shorter than telson, length 8. Right ramus
better preserved than left ramus.
No. 35/377
Large carapace preserved in positive relief
(Figs 11A; 12A). Maximum preserved length 40,
though posterior end of carapace absent.
Maximum width 19. Rostral plate absent.
Cephalothoracic shield preserved as internal
mould of right pleural fold. Cephalothoracic
shield broadly oval in outline though straighter
along dorsal margin where a slight fold indicates a
median dorsal plate was present, visible more
clearly when specimen is examined from above.
Pleural fold with narrow margin and oblique fold
running from near where the rostal plate would
be to the ventral margin about two thirds along
the length of the carapace. Depression occurs
below this fold, but with rounded, raised anterior
tubercle nearer the rostral end of the cephalothoracic shield. Posterior margin of pleural fold broken to reveal underlying abdominal pleomeres.
Rest of abdomen and appendages not preserved.
No. 35/378
Almost complete specimen in lateral view
(Figs 13D; 14D). Maximum preserved length
31, carapace length 17, abdomen length, including telson, 14. Maximum carapace width 9.
Rostral plate obscured. Cephalothoracic shield
preserved as cast of left pleural fold, but with
373
Dunlop J. A.
the ventral half restricted to an impression of
the carapace. Large area dorsal to specimen
probably deformed right pleural fold and
joined to left pleural fold by median dorsal
plate. Pleural fold broadly oval, straighter along
the dorsal surface with a distinct, narrow margin; ornamentation not preserved, but broad,
slightly oblique ridge runs dorso-ventrally.
Posterior margin of shield slightly broken to
reveal at least three underlying pleomeres; anterior two short, posterior one longer. Abdomen
tapers posteriorly. Styliform telson and one
poor furcal ramus preserved. Appendages not
preserved.
No. 35/779
Carapace preserved in lateral view (Figs 13E;
14E). Maximum preserved carapace length 25.
Maximum carapace width 14. Rostral plate
hook-shaped, curving ventrally, but slightly displaced. Cephalothoracic shield preserved as
crystaline internal mould of right pleural fold,
though crystaline part missing from ventral
region. Pleural fold broadly oval, straighter
along the dorsal surface and with a distinct, narrow margin. Part of median dorsal plate clearly
preserved above the specimen. Oblique line
originating close to the rostral plate creates a
depression in the anterior ventral part of the
cephalothoracic shield, though this effect may
be exaggerated by the loss of the crystaline part
in this region noted above. A second oblique
line runs close to the margin of the cephalothoracic shield. Posterior part of pleural fold broken to reveal three underlying pleomeres.
Remainder of abdomen and appendages not
preserved. Additional, poorer specimen (not
figured) preserved in same nodule.
No. 35/781
Specimen preserved in lateral view (Figs 13C;
14C). Maximum preserved length 30, carapace
length 19, abdomen length 11, though not all of
abdomen preserved. Maximum carapace width
9. Rostral plate well preserved, hook-shaped,
curving ventrally. Cephalothoracic shield preserved as impression of left pleural fold. Pleural
374
fold broadly oval with distinct, narrow margin.
Fine striae preserved near postero-dorsal region
of the pleural fold, above where the abdomen
originates. Anterior tubercle clearly preserved.
Dark area below rostal plate, adjacent to margin
of cephalothoracic shield, could be remains of
an eye. Abdomen originates approximately a
third of the way into the carapace, though individual pleomeres not discernable. Possible dark
gut trace preserved within abdomen. Telson and
furcal rami indistinct. Appendages not preserved.
No. 35/782
Relatively poor specimen in lateral view
(Figs 13B; 14B). Maximum length 33, carapace
length 20, abdomen length 13. Maximum carapace width 10. Rostral plate hook-shaped,
curving ventrally. Cephalothoracic shield preserved as impression of left pleural fold with
narrow margin, but details of morphology
poor. Pleural fold broadly oval with oblique
striae present near dorsal margin. Grey area
below rostral plate adjacent to margin of
cephalothoracic shield may be remains of an
eye. Abdomen possibly displaced, emerging
rather ventrally from the carapace and individual pleomeres not discernable. Telson styliform, apparently with shorter, styliform furcal
ramus preserved dorsal to the telson. Appendages not preserved.
MORPHOLOGICAL RECONSTRUCTION
The crustaceans range in size from very large,
No. 35/789 (not figured) lacks detail but is at
least 11 cm long, through to small specimens
of about 1 cm. Morphological terminology is
based on that used by Vannier et al. (1997)
and Rolfe (1969). Appendages are not preserved in the Severnaya Zemlya Devonian
material, but in phyllocarids they comprise
two pairs of large antennae, thoracic
appendages typically held within the carapace,
and a series of abdominal pleopods projecting
beyond the carapace and used in swimming. A
reconstruction of the animal in life attitude is
presented in Fig. 15.
GEODIVERSITAS • 2002 • 24 (2)
Severnaya Zemlya arthropods
A
C
D
B
E
FIG. 13. — Elymocaris urvantsevi n. sp.; A, No. 35/375, maximum length 23 mm; B, 35/782, maximum length 33 mm; C, 35/781,
maximum preserved length 30 mm; D, No. 35/378, maximum preserved length 31 mm; E, No. 35/779, maximum preserved carapace length 25 mm, maximum carapace width 14 mm.
Carapace
The overall morphology of the carapace is best
seen in specimen Nos. 35/377, 35/372 35/781,
35/378, 35/779 (Figs 11A, C; 12A, C; 13C-E;
14C-E). The paired pleural folds are broadly
elongate and are curved both dorsally and ventrally, giving the whole carapace an oval shape in
GEODIVERSITAS • 2002 • 24 (2)
lateral view. The pleural folds have a distinct
marginal rim (No. 35/781, Figs 13C; 14C) and
are connected by a median dorsal plate behind
the rostral plate (No. 35/779, Figs 13E; 14E).
This plate appears to be strongly attached to the
pleural folds; the division between the two is not
well preserved in this material (Nos. 35/782,
375
Dunlop J. A.
C
cp
gt
st
ab
rp
A
at
cp
rp
D
3
mpd
4
ab
5
tl
6
7
fu
fu
tl
B
cp
cp
E
mpd
rp
rp
ab
cp
FIG. 14. — Elymocaris urvantsevi n. sp., interpretative drawings of the specimens shown in Fig. 13; A, No. 35/375; B, 35/782;
C, 35/781; D, No. 35/378; E, No. 35/779. Abbreviations: ab, abdomen; at, anterior tubercle; cp, carapace pleural fold; gt, possible
gut trace; fu, furca; mdp, median dorsal plate; rp, rostral plate; st, striae; tl, telson; abdominal segments numbered (note longer
segment 7). Scale bars: 5 mm.
35/781, Figs 13B, C; 14B, C). The rostral plate
has a distinct, downward-curving hook shape
(see also Systematics). It is difficult to say
whether or not lateral carina (i.e. strong ridges
on the pleural folds) were present. Two specimens show what appear to be folds or carina on
the carapace (Nos. 35/372, 35/375, Figs 11C;
12C; 13A; 14A), but these are not in the same
position and could be taphonomically induced
as they are not seen at all in other material.
Similarly, two specimens show strong evidence
for an anterior tubercle on the pleural fold
(Nos. 35/377, 35/781, Figs 11A; 12A; 13C; 14C)
376
although it is not unequivocally present in every
specimen. Some specimens also suggest that the
antero-ventral region of the carapace had a
slight oblique fold below which it was slightly
depressed (e.g., No. 35/377, Figs 11A; 12A).
When examined in detail the carapaces of
Nos. 35/372 and 35/781 (Figs 11C; 12C; 13C;
14C) clearly show a fine ornament of ridges, or
striae. These striae are short, fine and closely
packed near the rostral plate where they run
approximately antero-dorsally (Figs 11C; 12C).
Meanwhile longer, oblique striae running postero-dorsal to antero-ventral can be seen near
GEODIVERSITAS • 2002 • 24 (2)
Severnaya Zemlya arthropods
FIG. 15. — Reconstruction of Elymocaris urvantsevi n. sp. in lateral view. Appendages reconstructed by comparison to other phyllocarids. Scale bar: 5 mm.
the median dorsal plate (Figs 13C; 14C). Larger
and more widely spaced striae are present in the
ventral part of the carapace where they run longitudinally, following the ventral curvature of
the pleural fold (Figs 11C; 12C). Similar striae
have been reported in fossil phyllocarids by
Racheboeuf (1994) and Vannier et al. (1997)
where they have been interpreted as an aid to
burrowing (see below).
Abdomen
The phyllocarid abdomen is composed of a
series of seven segments, or pleomeres, the anteriormost of which originates beneath the carapace. The abdomen can be seen overlapped by
the carapace, e.g., No. 35/781 (Figs 13C; 14C)
where the abdomen occupies the posterior third
of the space beneath the carapace. Up to five of
the seven pleomeres can be seen in this material
(No. 35/375, Figs 14A; 15A). No. 35/781
(Figs 13C; 14C) preserves a possible gut trace
running along the length of the abdomen. The
abdomen tapers posteriorly and in No. 35/375
(Figs 13A; 14A), which appears to be in ventral
view judging from the position of the emerging
furcal rami, there is ornamentation in the form
of ridges on the posterior abdominal segments.
The pretelsonic pleomere (segment 7) is longer
than the preceeding pleomeres (No. 35/375, Figs
13A; 14A). The telson is elongate and styliform
(e.g., No. 35/373, Figs 11B; 12B) with a pair of
furcal rami either side. The furcal rami are also
styliform, but are shorter and originate ventrally
close to the base of the telson (No. 35/378,
GEODIVERSITAS • 2002 • 24 (2)
Figs 13D; 14D). Both the telson and furcal rami
are ornamented with ridges (No. 35/373,
Figs 11B; 12B).
PHYLOGENY
There are morphological differences and inconsistencies among the specimens described here.
However, since these morphological differences
do not divide the material neatly into two or
more groups, and since there has been taphonomic distortion, all the material is referred to a
single species. The systematics and phylogeny of
phyllocarids remain in a somewhat unsatisfactory state (see comments by Rolfe [1969]) with
no clear indication of polarity for the characters
used to differentiate phyllocarid taxa. Such a
revision is beyond the scope of the present
work.
MODE OF LIFE
Vannier et al. (1997) studied living phyllocarids
in an attempt to determine the palaeobiology of
the Palaeozoic forms. They concluded that most
Palaeozoic phyllocarids were capable of swimming, both as pelagic and nekto-benthonic
forms, but that they also showed adaptations for
burrowing, such as the striae on the carapace
observed in the new Severnaya Zemlya material.
These striae may have helped channel the sediment past the carapace during burrowing.
Vannier et al. (1997) speculated that some
Palaeozoic phyllocarids burrowed during the
day to avoid predatory fish, emerging at night to
feed in the water column. Considering the
377
Dunlop J. A.
predatory acanthodians present in the Severnaya
Zemlya Formation, this mode of life seems
appropriate for the new phyllocarid material
too.
Acknowledgements
I am very grateful to Dr V. Talimaa (GIL) for
the opportunity to work on this material, for
her hospitality during my visit to Vilnius and
for information on the geology of the original
localities. I also thank Dr V. Relys for hospitality in Vilnius. Dr S. Braddy, Dr L. Anderson,
Dr P. Selden and M. Poschmann are thanked for
valuable discussions. I thank Dr P. Männik for
advice on the geological setting of this material,
E. Jantscher for proofreading the manuscript
and Drs D. Siveter and F. Schram for revising
the text. This work was initiated under a NERC
postdoctoral fellowship at the University of
Manchester.
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Submitted on 9 November 1998;
accepted on 4 November 1999.
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