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A Silurian short-great-appendage arthropod
Derek J. Siveter, Derek E. G. Briggs, David J. Siveter, Mark D. Sutton, David Legg and Sarah
Joomun
Proc. R. Soc. B 2014 281, 20132986, published 22 January 2014
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A Silurian short-great-appendage
arthropod
rspb.royalsocietypublishing.org
Derek J. Siveter1,2, Derek E. G. Briggs3, David J. Siveter4, Mark D. Sutton5,
David Legg1 and Sarah Joomun1
1
Earth Collections, University Museum of Natural History, Parks Road, Oxford OX1 3PW, UK
Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3PR, UK
3
Department of Geology and Geophysics and Yale Peabody Museum of Natural History, Yale University, PO Box
208109, New Haven, CT 06520-8109, USA
4
Department of Geology, University of Leicester, Leicester LE1 7RH, UK
5
Department of Earth Sciences and Engineering, Imperial College London, London SW7 2BP, UK
2
Research
Cite this article: Siveter DJ, Briggs DEG,
Siveter DJ, Sutton MD, Legg D, Joomun S.
2014 A Silurian short-great-appendage
arthropod. Proc. R. Soc. B 281: 20132986.
http://dx.doi.org/10.1098/rspb.2013.2986
Received: 14 November 2013
Accepted: 19 December 2013
Subject Areas:
palaeontology, evolution, taxonomy
and systematics
Keywords:
Arthropoda, exceptional preservation,
Herefordshire Lagerstätte, Leanchoiliida,
Megacheira, Silurian
Author for correspondence:
Derek J. Siveter
e-mail: derek.siveter@oum.ox.ac.uk
Electronic supplementary material is available
at http://dx.doi.org/10.1098/rspb.2013.2986 or
via http://rspb.royalsocietypublishing.org.
A new arthropod, Enalikter aphson gen. et sp. nov., is described from the Silurian
(Wenlock Series) Herefordshire Lagerstätte of the UK. It belongs to the Megacheira (¼short-great-appendage group), which is recognized here, for the first
time, in strata younger than mid-Cambrian age. Discovery of this new Silurian
taxon allows us to identify a Devonian megacheiran representative, Bundenbachiellus giganteus from the Hunsrück Slate of Germany. The phylogenetic
position of megacheirans is controversial: they have been interpreted as stem
chelicerates, or stem euarthropods, but when Enalikter and Bundenbachiellus
are added to the most comprehensive morphological database available, a
stem euarthropod position is supported. Enalikter represents the only fully
three-dimensionally preserved stem-group euarthropod, it falls in the sister
clade to the crown-group euarthropods, and it provides new insights
surrounding the origin and early evolution of the euarthropods. Recognition
of Enalikter and Bundenbachiellus as megacheirans indicates that this major
arthropod group survived for nearly 100 Myr beyond the mid-Cambrian.
1. Introduction
Arthropods are the most diverse invertebrates throughout the Phanerozoic. They
originated in Ediacaran times, with the crown group present in lower Cambrian
strata [1]. The Silurian (Wenlock Series; ca 425 Myr BP) Herefordshire Lagerstätte
of the UK preserves invertebrates as calcitic void infills enclosed within carbonate
nodules in a volcaniclastic deposit [2–4]. Since its discovery in 1994, this exceptional preservation deposit has yielded, among various invertebrates, a wide
variety of remarkable arthropods that have contributed substantially to our
knowledge of the palaeobiology and early history of the phylum. These include
a pycnogonid [5], two synziphosurine chelicerates [6–8], a marrellomorph [9],
a putative stem lineage crustacean [10], four myodocopid ostracodes [11–14], a
phyllocarid [15] and a barnacle [16].
Some so-called short-great-appendage arthropods (¼Megacheira [17]),
such as leanchoiliids, are characterized by a first (great) head appendage
with a short peduncle connected by a knuckle/elbow joint to a distal ‘claw’,
the three podomeres of which each extends distally into a long flagellum
[18,19]. Megacheirans have only been recorded from Cambrian deposits.
Here, we describe a new genus and species of megacheiran with such a
great-appendage morphology: Enalikter aphson from the Silurian Herefordshire
fauna, representing another major arthropod group to be recognized from this
Lagerstätte. Fossils from exceptionally preserved lower Palaeozoic biotas, such
as the Herefordshire example, have the greatest potential for revealing the earliest stages of arthropod diversification, the stem region of the arthropod
phylogenetic tree. Phylogenetic analysis of Enalikter and the re-evaluated
& 2014 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution
License http://creativecommons.org/licenses/by/3.0/, which permits unrestricted use, provided the original
author and source are credited.
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2. Material and methods
3. Systematic palaeontology
Phylum: Arthropoda von Siebold, 1848 [27].
Class: Megacheira Hou and Bergström, 1997 [17].
Order: Leanchoiliida Størmer, 1944 [28].
Family: Enaliktidae fam. nov.
Type genus: Enalikter gen. nov.
Other genus: Bundenbachiellus Broili, 1930 [29].
Family diagnosis: Leanchoiliida with subrectangular or
semicircular head shield; head with three appendage pairs, the
first limb uniramous with three flagella, non-geniculate between
peduncle and flagella, the second and third limbs biramous; eyes
absent; trunk very long and narrow, lacking paratergal folds,
comprising 12 segments; trunk appendages biramous; telson
with two pairs of long posterior processes/spines.
Enalikter aphson gen. et sp. nov.
Etymology: Greek, Enalios (of the sea) þ mastikter (scourger), alluding to the whip-like process borne ventrally on
the head; aphares (naked) þ soma (body) þ gyion (limb), referring to the exposed trunk limbs.
Holotype: Oxford University Museum of Natural History
(OUMNH C.29631) complete outstretched specimen, length
24.4 mm from anterior margin of cephalic shield to posterior
margin of telson (figure 1a–c,k,o,x).
Other material: two specimens: OUMNH C.29632 and
OUMNH C.29633.
Datasets from serial-grinding tomography of the specimens are housed in the Oxford University Museum of
Natural History.
Horizon and locality: Wenlock Series, Silurian System,
Herefordshire, UK.
Other species. None.
Generic and specific diagnosis. Head shield subrectangular,
lacking a narrow, raised margin. Head bearing a boss-like
2
Proc. R. Soc. B 281: 20132986
Specimens of Enalikter were serially ground at 20 mm intervals.
Each ground surface was captured digitally and, through using
the SPIERS software suite, the resulting tomographic dataset
was rendered and studied as a three-dimensional virtual fossil
[20,21]. Interpretation on-screen of the virtual fossils was facilitated by variable magnification, unlimited rotational, virtual
dissection and stereoscopic-viewing capabilities; they were also
examined through hard-copy images.
Analysis of the phylogenetic position of Enalikter and
Bundenbachiellus was performed using a modified version (see
the electronic supplementary material, note S1) of the panarthropod character matrix of Legg et al. [22], which represents the
most comprehensive morphological matrix available. The Legg
et al. analysis included recent re-interpretations of head appendage
innervation [23,24], added to which we have now also taken into
account the subsequently published conclusions of Tanaka et al.
[25]. A dataset of 314 taxa and 753 characters was analysed
using maximum-parsimony in TNT v. 1.1 [26], which generated
36 most parsimonious trees (MPTs). The strict consensus tree is
provided (see electronic supplementary material, figure S1), and
also a summary of the topologies from the phylogenetic analyses
(figure 2; electronic supplementary material, figure S2).
structure ventromedially, extending anteriorly into a curved
whip-like process. Trunk limb exopods with long, narrow,
non-overlapping filaments lacking spines. Telson with a
needle-like process medially, and two pairs of blade-like
processes laterally.
Description. The head shield is about 1.5 times as long as
wide, subrectangular in outline and dorsoventrally shallow,
partially covering the first trunk segment (figure 1e,j). Surface
sculpture is apparently lacking.
Appendage 1 originates at about 20% of the head length
(figure 1h). It is uniramous, comprising a short peduncular section of probably two podomeres, plus three closely originating
and tapering flagella ( podomere numbers unresolved). One
flagellum is about half as long as the other two—the ventralmost on both the best-preserved, outstretched specimens
(figure 1e,h); an elbow/knuckle joint is lacking between peduncle and flagella. Appendage 2 is biramous and originates at
about 55% of the head length. The limb base is very short, anteroposteriorly flattened, and bears a conspicuous spine-like
endite. The endopod is finger-like, evenly tapered, and comprises at least three podomeres; the exopod is similar but
much more slender ( podomeres unresolved), and slightly
shorter (figure 1p). Appendage 3 arises at about 85% of the
head length. It is biramous and similar to appendage 2 but
slightly larger, with a more robust, blunter endite; the first of
the (at least four or five) podomeres of the endopod bears a
median ridge (figure 1s).
Eyes are absent. Ventromedially, a boss-like structure
(figure 1d,h,r) extends anteriorly into a recurved, whip-like
process that is subconical proximally, more slender and tapering distally, and presumably flexible, although in all three
specimens it ends beneath the mouth. The more ventral part
of the boss is subcylindrical and terminates in a flat, disc-like
surface with a central subcircular mouth that faces posteroventrally. A short, narrow, sediment-filled space immediately
inside the mouth is interpreted as a buccal cavity and/or
very short oesophagus (figure 1r); it connects sharply with a
broader, sediment-filled cavity, interpreted as the stomach.
The latter is directed dorsally before bending posteriorly in a
J-shape into the intestine/midgut (figure 1q,r,b1).
The rest of the body, comprising a trunk and a telson, is
about 14 times as long as wide. The trunk, which consists of
12 segments, is roughly parallel-sided, and is subcircular in
transverse section in OUMNH C.29632 (figure 1m,u–w,a1),
though both outstretched specimens display dorsoventral
compression (see Discussion). Each tergite is dome-like
(figure 1t,v) and lacks paratergal folds (tergopleurae). The sternite is a subcircular to subrectangular button-like structure,
with a central node and a tuberculate marginal rim (figure
1f,i,u). At the anterior and posterior margin of each tergite
and its associated sternite, there is a prominent, transverse, tuberculate ridge that encircles the trunk. In between these occur
weaker, less persistent ridges (figure 1m,u,v,a1) representing
articulations, which in places display a wedged concertina-like
form, indicating segment pinching (figure 1m,u). These areas presumably represent arthrodial tissue, which enabled lateral
flexure up to at least 908 between segments (figure 1t–v). Evidence of vertical trunk flexure is limited, and is at most gently
upwards posteriorly (figure 1b). The gut is preserved discontinuously along the narrow trunk, but there is no evidence of midgut
glands. Transverse, soft-tissue traces are evident posteriorly,
some (? tendinous bars) coinciding with segment boundaries
(figure 1x).
rspb.royalsocietypublishing.org
Devonian taxon Bundenbachiellus refines the topology of
this stem region, providing new insights into immediately
pre-euarthropod crown-group morphologies.
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The preservation of Enalikter (figure 1; electronic supplementary material, figure S4) in full three-dimensional form is
unique for a stem euarthropod. The trunk of OUMNH
C.29632 (figure 1 m,t –w,y,z,a1) is subcircular in cross-section,
it bends laterally through 1808, and the exopod filaments
curve around to hug the bend, in a lowered, presumed ‘in
repose’ position (figure 1t,y). The other two specimens
(figure 1a,g) have a flatter trunk section, yet retain upstanding
to outstretched limbs, with straight to slightly sinuous, vertically radiating exopod filaments (figure 1k,o). Operation of
the trunk and filaments by hydraulic pressure might account
for such differences of inflation and disposition, though
equally it might reflect the early onset of decay.
The pyritized but much larger arthropod (up to 228 mm
[30]) Bundenbachiellus giganteus [29] (¼ Eschenbachiellus wuttkensis [31]; see [30]) from the Lower Devonian Hunsrück
Slate is close in overall morphology to Enalikter. Insights
3
Proc. R. Soc. B 281: 20132986
4. Discussion
from the new Silurian taxon are used here to reinterpret the
younger Devonian form. Only one of the two specimens of
Bundenbachiellus preserves the head ([31], text figures 11– 13;
electronic supplementary material, figure S3), which was
previously interpreted as bearing five appendages.
A comparison with the better-preserved Enalikter indicates
that the structures interpreted by Briggs & Bartels ([31],
p. 293) as a uniramous first (evident only on the left side)
and a biramous second appendage, together represent a
single triflagellate limb. It is likely that the following two
(more posterior) head appendages of Bundenbachiellus were
biramous, although only the endopod is clearly evident
(see electronic supplementary material, figure S3). Comparison
with the head of Enalikter suggests that the appendage interpreted as a fifth head limb in Bundenbachiellus may belong
to the trunk. There would then be 12 pairs of biramous
appendages in the trunk of Bundenbachiellus (although their
correspondence to tergites is uncertain), as in Enalikter, and
the posteriormost spines/appendages could be interpreted as
telson processes (rather than a pair of spines and a caudal
furca) such as those in Enalikter. Bundenbachiellus differs from
Enalikter, however, in a number of ways: the head shield was
semicircular (not subrectangular), surrounded by a narrow
raised margin; there is no evidence of a whip-like process
ventrally on the head; the trunk exopod filaments are leaflike (not linear) structures with fine spines on their inner
margins; and there is no evidence of a medial, needle-like process on the telson. Additionally, the Devonian species is an
order of magnitude larger than the Silurian one.
Enalikter and Bundenbachiellus fall in a clade of short-greatappendage (¼megacheiran) arthropods [32] that includes
Leanchoilia from the lower Cambrian of Chengjiang and the
middle Cambrian Kaili Lagerstätte, China, and the Burgess
Shale, Spence Shale and Marjum Formation of North America;
Alalcomenaeus from Chengjiang and the Burgess Shale; Actaeus
from the Burgess Shale; and Oestokerkus from the lower
Cambrian Emu Bay Shale, Australia [32–37] (figure 2;
electronic supplementary material, text S1 and figure S1).
Specifically, Enalikter is recovered in a clade (Enaliktidae)
together with Bundenbachiellus. More broadly, it falls under a
clade that is the most derived in the euarthropod stem and
sister to Euarthropoda, and which also includes the megacheirans Haikoucaris and Parapeytoia from Chengjiang, and
Yohoia from Burgess.
Our analysis supports the interpretation of all short-greatappendage arthropods as stem euarthropods [17,22,38 –43]
rather than as stem chelicerates [18,19,32,44–47].
While the tergopleurae are reduced in some stem euarthropods—for example Haikoucaris [18]—enaliktids appear
to be unique among stem euarthropods in lacking them
entirely. Enaliktids are also distinguished among megacheirans in their lack (loss) of the knuckle/elbow joint between the
peduncle and podomeres of the ‘claw’ (flagella), a hallmark
of other megacheirans [48] (although this feature is only
weakly developed in at least one other purported megacheiran, Occacaris [19]). A remarkable feature of Enalikter is
the long, posteriorly recurved, whip-like anterior process on
the head, which may be analogous to the spinose hypostomal
structure found in parasitic eucrustaceans [49] (electronic
supplementary material, text S2). The ventromedial, subventrally projecting boss-like feature to which the process
is attached recalls similar structures interpreted as hypostomal homologues in the stem mandibulates Agnostus,
rspb.royalsocietypublishing.org
The first trunk appendage (figure 1d,e,h) is biramous,
with a short, stout, simple limb base that lacks endites. The
endopod is stenopodous, similar but larger than that of
head appendage 3, with at least six or seven podomeres,
the second(?) of which is raised medially. The exopod consists of a slender, tapering shaft bearing at least eight
filaments (each probably from a separate podomere). The filaments are long, slender, non-overlapping and apparently
suboval in section; the most proximal is the stoutest, and
they become shorter distally. Trunk segments 2–12 each
bear a biramous appendage pair similar to the first trunk
appendage (figure 1a– c). Some endopods preserve two
slender spinose/setal terminal projections, which were presumably present on all trunk limbs. The exopods are
recurved dorsomedially in both outstretched specimens.
They preserve from 11 to 17 filaments (see figure 1o for a typical biramous limb). These filaments are long enough to
overlap at least partially those of the following appendage
(figure 1b). The trunk appendages increase in size from the
first to about the fifth, and are similar in length on successive
segments (figure 1a,g). The endopods of the more posterior
trunk appendages are slightly more slender.
The telson is ovoid in dorsal view (figure 1a,l) and about 1.3
times as long (medially) as wide; in lateral view, it is wedgelike, increasing in height posteriorly (figure 1b,n,w). Ventrally,
a slightly raised, posteriorly narrowing subtriangular axial
region is bounded by a very weak abaxially convex furrow
(figure 1u). A narrow, prominent tuberculate ridge and parallel
furrow, similar to those on the trunk segments, encircle the
anterior margin of the telson. Posteriorly, the telson bears
two pairs of long, blade-like processes (figure 1l,n); each originates adjacent to the midline, tapers to a point, and is
laterally flattened and suboval in section. The dorsal processes
project posterodorsally at about 308. The ventral ones curve
evenly dorsally through about 608, their tips crossing immediately outside those of the dorsal pair. There is no evidence for
or against mobility in any of these processes. A medial, needlelike process projects posterodorsally from between the ventral
pair. The anus lies posteroventrally, as indicated by a faecal
stream (figure 1w,z,a1). The telson extends parallel to the
trunk (figure 1w) or may be inclined upwards at about 308
(figure 1b).
Downloaded from rspb.royalsocietypublishing.org on January 22, 2014
(a)
h1 ap hs
h2, h3
t1 t1
(c)
h1
ap
h1
h2, h3
hs
(d)
h3
4
h2
ap
mo
h1
t1
(g)
t4
t4
t4
vb
dos
t1 mo
(e)
h1
(f)
tr
t8
Proc. R. Soc. B 281: 20132986
t8
h1
t8
t9
ap
h2
h3
t11
t12
t12
t
(h)
bs
( j)
(i)
t12
t1
t
t
t2
t
(k)
h1
cn
dos
h2
h3
h1
ap
mo
vb
hs
(r)
t1
(n)
t2
(l)
ptr
t11ef (p)
(o)
t
ap
(q)
h2ex
h1
dos
h2
h2
t
wr
dpp
h2en
(m) vpp
mnp
ap
h1
h2
t12en
h3
hs
t
mnp
ap h1
tef
h1
t11ex
t1en
hs
(z)
cn
tst
t
tef
vpp
dpp
fs
(b1)
t9
tst?
(u)
ptr
an
h1
t8
ptr
(t)
(a1)
vpp
(y)
t4en
wr
h3
gut
h3ex t
t
dt
sar
s
dpp
gut
hs
oe
mo
h3
t11en (s)
vpp
dpp
fs
h3en
an
gut
(v)
t8en
(w)
(x)
t11en
Figure 1. (Caption opposite.)
Henningsmoenocaris and Martinssonia [50]; as in those taxa, a
discrete, fully sclerotized hypostome is lacking in Enalikter.
The flat, wide, circumoral disc-like surface in Enalikter bears
comparison, variously, with the mouth/‘Peytoia’ cone of the
panarthropod lobopodians Pamdelurian and Opabinia, and
stem euarthropod radiodontids such as Anomalocaris and
Peytoia, and the great appendage arthropod Parapeytoia
[51 –55] (electronic supplementary material, figure S1). In
those taxa, however, the oral cone surface is rigid and plated,
unlike the disc surface of Enalikter, which lacks evidence of
rspb.royalsocietypublishing.org
(b)
ap
plates and was presumed fleshy (see electronic supplementary
material, text S2). Enalikter inhabited the outer shelf/upper
slope of the Anglo-Welsh basin, where water depths might
have been up to some 200 m [2]. It is likely to have been a
benthic or nektobenthic scavenger/detritivore (see electronic
supplementary material, text S2).
Recognition of Enalikter and Bundenbachiellus in Silurian
and Devonian rocks indicates that members of the stem
clade Leanchoiliida survived for nearly 100 Myr (75 and 97
Myr, respectively [56]) after the mid-Cambrian Leanchoilia?
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'Megacheira'
(in partim)
Leanchoiliidae
Enaliktidae
Leanchoiliida
incertae sedis:
Actaeus
Alalcomenaeus
Chelicerata
(total group)
Mandibulata
(total group)
Euarthropoda
Figure 2. A summary of the phylogenetic relationships and of topologies produced during phylogenetic analyses of Enaliktidae, which were consistent over all of
them (see electronic supplementary material, text S1 and figure S1 for details).
sp. of the Marjum Formation (ca 500 Myr BP [34]), the hitherto
stratigraphically youngest known short-great-appendage
arthropod. Enalikter and Bundenbachiellus are some 55 and
77 Myr, respectively, younger than the next youngest stem
euarthropod, anomalocaridids from the lower Ordovician
Fezouta Lagerstätte (ca 480 Myr BP) of Morocco [57]; and the
enaliktids represent only the second record of stem euarthropods in Silurian or Devonian strata, the other being that of
Schinderhannes from the Hunsrück Slate [47]. Data on Enalikter
and Bundenbachiellus highlight the importance of rare Silurian
and Devonian Konservat–Lagerstätten for revealing the much
later, mid- and upper Palaeozoic history of groups such as
megacheirans that have previously been considered to be
restricted to the Cambrian; more accurate knowledge of their
true stratigraphic range is dependent on these critical taphonomic windows. Our study also highlights the advantage
available in combining morphological data from different
types of exceptional-preservation deposits.
Acknowledgements. We thank the Natural Environmental Research
Council (NERC grant no. NE/F018037/1) and English Nature for
financial support, K. Saunders, K. Davies and C. Lewis for technical
support, David Edwards and other staff of Tarmac Western for
general assistance, and two anonymous reviewers for valuable comments on the manuscript.
Data accessibility. Virtual models of the three Enalikter specimens in
VAXML format (see [21]): Dryad doi:10.5061/dryad.jb0t7. Phylogenetic data matrix in NEXUS format: electronic supplementary
material, table S1.
Proc. R. Soc. B 281: 20132986
outgroups
5
rspb.royalsocietypublishing.org
Figure 1. (Opposite.) Enalikter aphson, virtual reconstructions. (a – c,k,o,x) Holotype, OUMNH C.29631, outstretched specimen, trunk somewhat dorsoventrally compressed; (a – c,k) complete specimen, (a) dorsal stereo pair, (b) left lateral view, (c) ventral view, (k) anterior-oblique stereo pair, (o) trunk appendage 11,
posteroventral view, (x) trunk between appendages 8 and 9, cuticle translucent, dorsal view. (d – j,l,n,p – s,b1) OUMNH C.29633, almost complete outstretched
specimen, trunk somewhat dorsoventrally compressed, estimated length 15.9 mm; (d,e,h,j,b1) head and anterior-most part of trunk, (d ) ventral posterior-oblique
view, (e) dorsal view, (h) ventral stereo pair, (i) lateral view, (b1) posterior view, ( f ) trunk between trunk appendages 6 and 11, ventral view, (g) complete
specimen, (i) trunk between trunk appendages 11 and 12, ventral view, (l,n) telson, (l ) dorsal stereo-pair, (n) lateral view, ( p,s) head appendages 2, and 3,
posteroventral views, (q) head, with head shield and soft tissue around the gut removed, dorsal view, (r) head, with head shield translucent, lateral view.
(m,t – w,y,z,a1) OUMNH C.29632, complete, laterally flexed specimen, estimated length 11.0 mm; (m) trunk between trunk appendage 10 and anterior part of
telson, ventrolateral stereo pair, (t – w,y) complete specimen, (t,v) with exopods, and with exopods removed, dorsal stereo pair, (u) ventral stereo-pair, (w) exopods
removed, lateral view, ( y) posterior-oblique view, (z) telson, posterior view, (a1) telson and posterior part of trunk, posterodorsal view. Scale bars are all 1 mm. an,
anus; ap, anterior process; bs, button-like sternite; cn, central node; dpp, dorsal posterior process; dos, disc-like oral surface; dt, dome-like tergite; fs, faecal stream;
gut, midgut/intestine; h1, h2, h3, head appendages 1, 2 and 3; h2en, head appendage 2 endopod; h2ex, head appendage 2 exopod; h3en, head appendage 3
endopod; h3ex, head appendage 3 exopod; hs, head shield; mnp, medial needle-like process; mo, mouth; oe, oesophagus; ptr, prominent trunk ridge; sar, subtriangular axial region; s, stomach; t, telson; t1, t2, t4, t8, t9, t11, t12, t14, trunk appendages 1, 2, 4, 8, 9, 11, 12 and 14; t1en, trunk appendage 1 endopod; t4en,
trunk appendage 4 endopod; t8en, trunk appendage 8 endopod; t11en, trunk appendage 11 endopod; t11ex, trunk appendage 11 exopod; t11ef, trunk appendage
11 exopod filaments; t12en, trunk appendage 12 endopod; tef, trunk appendage exopod filaments; tr, trunk ridge(s); tst, transverse soft tissue; tst?, transverse soft
tissue?; vb, ventral boss; vpp, ventral posterior process; wr, wedge-like region.
Downloaded from rspb.royalsocietypublishing.org on January 22, 2014
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