ORIGINAL PAPER

Evolution and development of the synarcual in early vertebrates

Zerina Johanson • Kate Trinajstic •

Robert Carr • Alex Ritchie

Received: 14 March 2012 / Revised: 20 June 2012 / Accepted: 20 June 2012 / Published online: 17 July 2012

� Springer-Verlag 2012

Abstract The synarcual is a structure incorporating the

anterior vertebrae of the axial skeleton and occurs in ver-

tebrate taxa such as the fossil group Placodermi and the

Chondrichthyes (Holocephali, Batoidea). Although the

synarcual varies morphologically in these groups, it rep-

resents the first indication, phylogenetically, of a differ-

entiation of the vertebral column into separate regions.

Among the placoderms, the synarcual of Cowralepis

mclachlani Ritchie, 2005 (Arthrodira) shows substantial

changes during ontogeny to produce an elongate, spool-

shaped structure with a well-developed dorsal keel.

Because the placoderm synarcual is covered in perichon-

dral bone, the ontogenetic history of this Cowralepis

specimen is preserved as it developed anteroposteriorly,

dorsally and ventrally. As well, in the placoderm Mater-

piscis attenboroughi Long et al., 2008 (Ptyctodontida),

incomplete fusion at the posterior synarcual margin indi-

cates that both neural and haemal arch vertebral elements

are added to the synarcual. A survey of placoderm syn-

arcuals shows that taxa such as Materpiscis and Cowralepis

are particularly informative because perichondral ossifica-

tion occurs prior to synarcual fusion such that individual

vertebral elements can be identified. In other placoderm

synarcuals (e.g. Nefudina qalibahensis Lelievre et al.,

1995; Rhenanida), cartilaginous vertebral elements fuse

prior to perichondral ossification so that individual ele-

ments are more difficult to recognize. This ontogenetic

development in placoderms can be compared to synarcual

development in Recent chondrichthyans; the incorporation

of neural and haemal elements is more similar to the ho-

locephalans, but differs from the batoid chondrichthyans.

Keywords Vertebral fusion � Synarcual � Placodermi �Chondrichthyes � Holocephali � Batoidea � Vertebral

column

Introduction

In certain vertebrates, the axial skeleton is modified by

fusion of anterior elements into a structure known as the

synarcual. A synarcual is present in the fossil group Pla-

codermi (Figs. 1, 2, 3, 4, 5) and in certain chondrichthyans,

including the holocephalans and batoids (Figs. 6, 7). The

synarcual represents, phylogenetically, the first known

appearance of a differentiation of the axial skeleton into a

distinct anterior region, relative to the remainder of the

vertebral column. These synarcuals are non-homologous,

so it could be predicted that they (and the distinct anterior

Communicated by A. Schmidt-Rhaesa.

Z. Johanson (&)

Earth Sciences, Natural History Museum, Cromwell Road,

London SW7 5BD, UK

e-mail: z.johanson@nhm.ac.uk

K. Trinajstic

Western Australian Organic and Isotope Geochemistry Centre,

Department of Chemistry, Curtin University, Bentley,

WA 6845, Australia

K. Trinajstic

Department of Earth and Planetary Sciences,

Western Australian Museum, 49 Kew Street,

Welshpool, WA 6106, Australia

R. Carr

Department of Natural Sciences and Geography,

Concordia University Chicago, 7400 Augusta St.,

River Forest, IL 60305-1402, USA

A. Ritchie

Palaeontology, Australian Museum, Sydney 2010, Australia

123

Zoomorphology (2013) 132:95–110

DOI 10.1007/s00435-012-0169-9

regions they represent) developed in different ways. We

will test this hypothesis by comparing the development of

the synarcual in these early vertebrates. Vertebral elements

that can be incorporated into the synarcual include dorsal

(basidorsal/neural, interdorsal), ventral (basiventral/hae-

mal, interventral) and central elements (e.g. Gadow 1933,

see Arratia et al. (2001) for a review of the homology of the

centrum, including the arcocentra in placoderms). The

dorsal, ventral and central vertebral elements are consid-

ered to be homologous across the vertebrates.

Placoderms (extinct armoured fish; Fig. 1) are members

of the gnathostome stem group, resolved phylogenetically

to the basal nodes of the jawed vertebrate clade (Janvier

1996; Brazeau 2009). Among the placoderms, a synarcual

is preserved in the Rhenanida, Ptyctodontida and Ar-

throdira. A synarcual is also present in Stensioella heintzi

Broili 1933 (Gross 1962, 1965). Stensioella was described

as a placoderm (Gross 1962), but this has been questioned

due to the absence of placoderm characters in Stensioella

and similarity to the holocephalan Deltoptychius Morris

and Roberts 1862 (Coates and Sequiera 2001). A synarcual

was also reconstructed in the Bothriolepididae (Antiarchi),

based on the morphology of grooves and pits on the

internal surface of the trunkshield plate (Moloshnikov

2008). Dorsal parts of the synarcual were said to insert into

these grooves and pits, but the synarcual itself has never

been observed. Current phylogenies resolve the Placodermi

as a paraphyletic group (Brazeau 2009; Davis et al. 2012),

although Young (2010; also Goujet and Young 1995, 2004)

describes the Placodermi as monophyletic. In either

instance, the Rhenanida would be resolved as a sister taxon

to the Ptyctodontida and Arthrodira.

Among chondrichthyans, a synarcual is present in the

crown group chondrichthyans (sensu Pradel et al. 2011;

Euselachii ? Holocephali), but absent in stem group taxa.

In chondrichthyans, the synarcual supports the dorsal fin

spine in holocephalans such as Chimera monstrosa Lin-

naeus, 1758 (Fig. 6a), or the pectoral fin in skates and rays

(Gonzalez-Isais and Domınguez 2004). The function of the

synarcual in placoderms is more difficult to establish. The

pectoral fin and scapulocoracoid are associated with the

lateral and anterior trunkshield plates and separated from

the synarcual, which is positioned under the dorsal trunk-

shield and extends anteriorly to articulate with the brain-

case (Fig. 1). In this position, the synarcual is also

separated from any dorsal fin elements (but see Miles and

Young 1977: fig. 34), but acts to support the headshield

with the trunkshield.

We will review synarcual morphology in the Placodermi

and compare this to extant chondrichthyan synarcuals, in

order to compare developmental patterns in both groups

and the formation of this distinct anterior region of the

vertebral column. Normally, the synarcual of extant

chondrichthyans would be expected to provide more

developmental information than the synarcual of the fossil

placoderms. However, because the placoderm synarcual is

perichondrally ossified, the early ontogenetic history of the

synarcual can be well preserved, while it is lost due to

cartilage fusion in living chondrichthyans. Therefore, the

placoderm synarcual may illuminate some of the early

ontogeny of the chondrichthyan synarcual as well. For

example, new specimens of the arthrodire Cowralepis

mclachlani and the ptyctodont Materpiscis attenboroughi

preserve a substantial amount of ontogenetic information,

including early stages of development. This is due to the

deposition of perichondral bone around cartilaginous ele-

ments during development. By comparison, the mineral-

ized cartilage of recent chondrichthyan synarcuals is

largely fused; synarcual development is best observed

posteriorly, where vertebral elements are newly incorpo-

rated into the synarcual (e.g. Fig. 6a).

One feature that will be examined in detail is the

development of the ventral part of the placoderm synarc-

ual. This appears rounded or spool-like relative to the

flange or keel of the dorsal part of the synarcual and may

develop from a ventral expansion of the basiventrals (as in

most living skates and rays, Figs. 6, 7) or from direct

incorporation of separate ventral elements (as in the chi-

mera, Fig. 6a). For example, the synarcual of Materpiscis

attenboroughi indicates that placoderms are more

Fig. 1 Generalized placoderm (lateral view) showing limits of headshield and trunkshield. Axial skeleton in black, with synarcual anteriorly,

bridging the gap between head and trunkshield. After Dennis and Miles (1981: fig. 2) and Miles and Westoll (1968)

96 Zoomorphology (2013) 132:95–110

123

Fig. 2 a1, a2 Placoderm axial skeleton. NHMUK PV P. 50934.

Incisoscutum ritchiei, showing neural arches/spines and haemal

arches/spines. Anteriorly, haemal spines are notably reduced. a1 Inset,closeup of neural spines showing zygapophyses. b–f Placoderm syn-

arcuals. b1, b2 WAM 94.12.2, Compagopiscis croucheri (Eubrachytho-

racida; Arthrodira). b1 inset shows region of the synarcual indicated by

asterisk in main Figure (b2), rotated counter clockwise 90 degrees. c,

d WAM 11.9.1, Campbellodus decipiens (Ptyctodontida). c Synarcual

and more posterior axial skeleton, in lateral view. Smaller white arrowindicates rounded base of neural arch. Black box in c indicates area

highlighted in d. d Closeup of synarcual in Fig. 2c and more posterior

vertebral elements. Smaller white arrow indicates expanded dorsal

flange. e NHMUK PV P.57665, Austroptyctodus gardineri (Ptycto-

dontida), small white arrows indicate spool-shaped areas of the ventral

synarcual, including haemal elements. f, g WAM 07.12.1, Materpiscisattenboroughi (Ptyctodontida). f Lateral view. Smaller white arrowindicates top of spool-shaped part of the synarcual, composed of haemal

arch elements. g Medial view. White arrow indicates top of spool-

shaped part of the synarcual. 3–5 indicate the individually recognizable

haemal arches being added to the ventral synarcual. Abbreviations:haem. haemal arch/spine, kl median dorsal keel, na/n.sp neural arch,

neural spine, n.sp neural spine, syn synarcual, zyg, zygapophyses. Scalebars = a2, b2, e = 1 cm; f = 0.5 cm. Larger white arrows indicate

anterior in all figures

Zoomorphology (2013) 132:95–110 97

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comparable to the Holocephali in that separate ventral arch

elements are directly incorporated into the spool-shaped

part of the synarcual, with no ventral expansion.

Materials and methods

Fossil placoderm synarcuals were examined from the

groups Arthrodira [Incisoscutum ritchiei Dennis and Miles,

1981 (Dennis and Miles 1981), Compagopiscis croucheri

Gardiner and Miles, 1994 (Gardiner and Miles 1994),

Dunkleosteus sp., Cowralepis mclachlani (Ritchie 2005)],

Ptyctodontida [Campbellodus decipiens Miles and Young,

1977 (Miles and Young 1977), Austroptyctodus gardineri

Fig. 3 Placoderm synarcual a–c, CMC VP8544, Dunkleosteus sp.,

Devonian, Morocco. a Anterior view of articular condyle (articulating

with occipital region of braincase). b Lateral view. c Ventral view.

Abbreviations: art.cond articular condyles, na neural arch, ncnotochord, sp.n spino-occipital nerve foramina

Fig. 4 Placoderm synarcuals. a–d Cowralepis mclachlani (Devo-

nian, Merriganowry Quarry, New South Wales). a, b1, b2

AMF129164, smaller individual. a Ventral view showing internal

surface of head and trunkshield, occipital region of braincase and

synarcual, larger white arrow indicates anterior. b1, b2, closeup of

occipital and synarcual, stereopair. Black arrows indicate circular

growth rings of the neural bases in both the occipital and synarcual.

White arrows indicate location of spino-occipital nerves. c–d New

synarcual uniquely preserved in lateral and medial views. c1, c2 AMF

137328, right lateral view (c1, larger white arrow indicates anterior).

d1, d2 AMF 137329, medial view (d1 larger white arrow indicates

anterior). d1, black arrows indicate circular growth lines of individ-

ualized neural arch base. Arrowheads in d2 indicate transition from

individualized neural arch bases to fusion of bases and deposition of

perichondral bone along this margin. Abbreviations: vert. 1–4vertebral elements 1–4. a, b From Johanson et al. (2010), reproduced

with permission from the Int J Dev Biol. Scale bars a, c, d = 1 cm

c

98 Zoomorphology (2013) 132:95–110

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Miles and Young, 1977 (Miles and Young 1977; Long

1997), Materpiscis attenboroughi (Long et al. 2008;

Trinajstic et al. 2012)], and the Rhenanida [Nefudina

qalibahensis (Lelievre and Carr 2009), Jagorina pandora

Jaekel, 1921 (Stensio 1963; Johanson et al. 2010)]. All

ptyctodont and most arthrodire specimens are from the

Gogo Formation (Devonian, Western Australia) and were

prepared in weak acetic acid to remove the surrounding

Zoomorphology (2013) 132:95–110 99

123

Fig. 5 Placoderm synarcuals.

a–c Mb.f 510.2, Jagorinapandora. a Dorsal view of

counterpart. White arrowindicates position of synarcual.

b, c View of part and

counterpart specimen,

respectively. c white arrowindicates synarcual. Largerwhite arrows indicate anterior

direction. Scale bars = 1 cm.

Abbreviations: pect.girdpectoral girdle. b, c From

Johanson et al. 2010,

reproduced with permission

from the Int J Dev Biol

100 Zoomorphology (2013) 132:95–110

123

limestone nodule, with three ptyctodont specimens

embedded in resin to preserve the association of skeletal

elements (see Long et al. 2006 for a more complete

methodology). Rubber latexes of the part and counterpart

moulds of Cowralepis mclachlani were taken. Recent

chondrichthyan embryos from the AMNH Zoology

Fig. 6 Recent chondrichthyan embryos, cleared and stained.

a AMNH 55040, Chimera monstrosa (Holocephali), lateral view.

Black arrows indicate neural arch elements being added to the

synarcual. b, c Closeup of synarcual and first free vertebrae, b AMNH

4128, Torpedo torpedo. c AMNH 8193, Rhinobatos lentiginosus

(Garman, 1880), ventral view. d AMNH 4128, Torpedo torpedo(Linnaeus, 1758) (Batoidea; Torpediformes). e AMNH 8193, Rhin-obatos lentiginosus (Batoidea; Rhinobatiformes). f AMNH 16350,

Raja texana Chandler, 1921 (Batoidea; Rajiformes), ventral view.

Black arrows indicate position of first free vertebral centrum

Zoomorphology (2013) 132:95–110 101

123

collections were cleared and stained using standard proto-

cols (e.g. Dingerkus and Uhler 1977). All specimens are

illustrated using macrophotography. Institutional abbrevi-

ations: AMF, Australian Museum, Sydney; AMNH,

American Museum of Natural History, New York; CMC,

Cincinnati Museum Center, Cincinnati; Mb.f, Museum fur

Naturkunde, Berlin. NHMUK PV P, Natural History

Museum, London; WAM, Western Australian Museum,

Perth. Abbreviations: art.con, articular condyles; gr.r,

growth rings; haem, haemal arch/spine; keel, keel (or

flange) of the dorsal synarcual; keel, dorsal synarcual keel;

kl, keel on internal surface of trunkshield; na/n.sp, neural

arch/neural spine; nc, neural canal; occ, occipital; pect.gird,

pectoral girdle; sp.n, foramina for spino-occipital nerves;

syn, synarcual; vert.1-4, vertebrae 1-4; v.syn, ventral syn-

arcual; zyg, zygapophysis.

Results

Eubrachythoracida; Arthrodira

Incisoscutum ritchiei (NHMUK PV P.50934;

WAM 86.9.668)

Acid-prepared specimens of Gogo placoderms such as

I. ritchiei (NHMUK PV P. 50934) preserve substantial

morphological detail, including the vertebral column.

Posterior to the bony trunkshield plates, the neural and

haemal arches are well-developed, positioned dorsal and

ventral to the notochord, with long spines (Fig. 2a1,2).

Fusion is absent, with contact limited to zygapophyses on

the neural arches (Fig. 2a1). Haemal arches are used here in

the sense of Miles and Westoll (1968) to refer to ventral

elements along the vertebral column, not necessarily

restricted to the caudal region. In other specimens where

the synarcual is present anteriorly (WAM 86.9.668), two

fused neural elements are visible immediately posterior to

the ventral keel of the trunkshield median dorsal plate. A

single neurapophysis (lamina of the vertebral arch lacking

zygapophyses) is located directly behind the fused ele-

ments. Its morphology suggests it is a third element, which

has broken away from the posterior margin of the synarc-

ual. These are displaced but appear identical to the anterior

elements of the synarcual in Compagopiscis, described

below.

Compagopiscis croucheri (WAM 94.12.2)

In Compagopiscis croucheri, at least six neural arches have

fused to form the incompletely preserved synarcual. The

right half of the synarcual preserves four fused neural

arches. The left half of the synarcual also preserves four

neural arches, however, the posterior two neural arches

have a dorsolaterally expanded neural spine (Fig. 2b2,

black asterisk and in inset, Fig. 2b1), although the height of

this is reduced when compared to the neural spines on more

posterior vertebrae. Directly posterior to this is a separated

element also with an extended spine, which may have been

fused to the other element in life. As in ptyctodonts (see

below) the synarcual rested on the notochord and did not

surround it. Although there is no expanded dorsal keel or

flange present in Compagopiscis, the two posterior neural

spines are incompletely fused, having an opening between

Fig. 7 Recent chondrichthyan embryos, cleared and stained. a,

b AMNH 30607, Dasyatis americana (Batoidea; Myliobatiformes).

a Lateral view. b Ventral view. Black arrow indicates distinct

segmentation posterior to the developing synarcual. White arrowhead

indicates loss of this segmentation anteriorly. White asterisk indicates

ongoing incorporation of elements into synarcual, with loss of

basiventral separation on one side of the free vertebra, but not the

other

102 Zoomorphology (2013) 132:95–110

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the two neural elements. Two small foramina are pre-

served, one at the base of the neural arch and a second on

the anterior margin of the expanded neural spine. There is a

lateral process along the dorsal margin of the neural spine

and a reduced prezygapophysis on the more posterior ele-

ment. There are no haemal elements preserved below the

synarcual. Posterior to the right half of the synarcual, there

are three isolated neural arches with neural spines pre-

served directly behind the fused elements. At the anterior

margin of the median dorsal trunkshield keel, there is a row

of separate neural elements with reduced spines. The

haemal elements are separate, and there is no haemal spine

development. Within the eubrachythoracid arthrodires

from the Gogo Formation, neural and haemal spines only

fully develop posterior to the median dorsal plate (Fig. 2a2,

b2, compare neural arches labelled ‘na/n.sp with those

more anterior). The neural and haemal elements do not

otherwise fuse at any point along the vertebral column.

Dunkleosteus sp. (CMC VP8544)

The synarcual of Dunkleosteus Lehman, 1956 shows the

highest degree of fusion among the placoderms described

here (Fig. 3), except for the rhenanid Nefudina. The dorsal

synarcual is broken, so it is uncertain whether the synarcual

had an expanded flange or keel as in other placoderm taxa.

Seven spino-occipital foramina are preserved, indicating

that at least eight neural elements have been fused into the

synarcual. Just behind the last foramen is a groove, which

may represent an incompletely fused neural element, or it

may be part of a crack, which extends dorsoventrally in this

region (Fig. 3b). The posteriormost part of the synarcual

also appears to be incomplete. Anteriorly, the synarcual is

modified into two articular condyles (art.con, Fig. 3a),

which would have articulated with the posterior occipital

region of the braincase. Openings for the neural arch and

canal are also visible in anterior view. In ventral view, a

faint seam can be seen extending anteroposteriorly

(Fig. 3c), comparable to that described in batoid chondricth-

yans and representing the fusion of the basiventrals (Fig. 7a,

b; Claeson 2011). Although synarcual fusion has occurred

before perichondral bone deposition, addition of haemal

elements to the rounded ventral portion of the synarcual in

Materpiscis (see below) suggests that haemal elements also

comprise the ventral synarcual in Dunkleosteus sp.

Placoderm synarcuals: Arthrodira

Cowralepis mclachlani (AMF129164, AMF137328,

AMF137329)

The synarcual of Cowralepis was described previously

(Ritchie 2005; Johanson et al. 2010) and is commonly

preserved in ventral view, showing significant changes

through ontogeny. In the earliest known stage, the syn-

arcual is rectangular and short (syn, Fig. 4a, b), becoming

elongate and spool-like with maturity, particularly anteri-

orly (Johanson et al. 2010: fig. 2F). This anterior edge

matches and articulates with a comparably rounded margin

of the occipital region of the braincase (occ; Fig. 4a, b). In

certain specimens, the two halves of the synarcual have

become separated, allowing a partial view of the internal

surface of the dorsal portion of the synarcual (Johanson

et al. 2010: fig. 2B, F). Here, several rounded elements can

be seen, arranged in an anteroposterior sequence, with

multiple circular growth rings visible in each (Fig. 4b1,

black arrows). These represent the periodical growth of the

cartilaginous neural arch bases and associated perichondral

bone deposition (Ritchie 2005: fig. 18C), showing no loss

of identity (via fusion) within the synarcual. Small tubes

can also be seen, representing the course of the spino-

occipital nerves between the neural arches (Fig. 4b2, white

arrows). The two halves of the synarcual were also sepa-

rated in a larger (and so presumed older) specimen,

showing the distinctive rounded neural arch bases (Johan-

son et al. 2010: fig. 2I). In these older specimens, the

anterior synarcual was modified into a thick, rounded

articular surface.

A new specimen of Cowralepis (AMF 137328, AMF

137329; Fig. 4c, d) preserves the synarcual in lateral view

for the first time, allowing for a more complete description

of synarcual development. The ventral portion of the syn-

arcual (v.syn) extends anteroposteriorly, and, as noted, is

thick and rounded anteriorly (cranio-vertebral articulation)

where it meets the occipital. However, the anteriormost

face of the synarcual is not visible, so the presence of

separate articular condyles cannot be determined. The

ventral synarcual is surmounted by a thin dorsal keel

comprising the neural arch bases (Fig. 4c1, keel), identified

by the multiple, circular growth rings as described above

(gr.r; Ritchie 2005). The keel is lower anteriorly, gradually

increasing in height posteriorly. The anterior keel is made

up of three to four neural arch bases, although this is dif-

ficult to distinguish anteriorly, particularly in external view

(Fig. 4c). What appear to be single foramina for the spino-

occipital nerves are present (sp.n), but not associated with

the first two or three vertebrae (Fig. 4c2, d1, area between

the arrowheads and arrows). Laterally, the growth rings

associated with these first three vertebrae appear less

rounded and more flattened when compared with more

posterior neural arch bases (Fig. 4c2). This suggests that at

this point in development, the cartilaginous neural arch

bases have begun to fuse and lose their identity, growing

more as a unit, with corresponding appositional deposition

of perichondral bone along the edges of this unit (black

arrowheads, Fig. 4c2). However, in medial (internal) view,

Zoomorphology (2013) 132:95–110 103

123

the neural arch bases have remained distinct, particularly

ventrally, where the first three to four bases can be dis-

tinguished (Fig. 4d1, region between the black arrows).

However, along with being less distinct externally, these

arches are partially covered anteriorly by the rounded

ventral cranio-vertebral articulation (Fig. 4c, d).

The synarcual keel increases in height posteriorly, and

the growth rings are more rounded than they are anteriorly

(more similar to their appearance in the more posterior

neural arch bases of the vertebral column), and they are

also stretched. This could be related to the growth of the

synarcual, but the Cowralepis fauna in the Merriganowry

Quarry has experienced postmortem deformation (Ritchie

2005: fig. 8), which could have resulted in this stretching.

Nevertheless, these posterior neural arch bases are more

distinct than those anteriorly, although they have fused

together to produce spino-occipital nerve foramina. As

well, the ventral part of the synarcual appears to bend or

kink just posterior to the rounded articular area (Fig. 4c, d).

Posterior to this bend, the neural arch bases appear to have

retained more individuality than anteriorly.

Development and composition of the ventral portion of

the Cowralepis synarcual is more difficult to determine, as

it is smooth and rounded, with no indication of the addition

of individual elements, including in medial view (Fig. 4d).

In living taxa such as the holocephalan Chimera monst-

rosa, the ventral elements are clearly fusing to the body of

the synarcual (Fig. 6a). By comparison, in the Batoidea, it

appears that the ventral parts of the synarcual develop from

ventral extensions of the more dorsally positioned basi-

ventrals growing around the vertebral centra, which remain

distinct from the surrounding synarcual (Figs. 6, 7; Claeson

2010, 2011). The ventral part of the Cowralepis synarcual

is distinct from the dorsal (neural arch-derived) part even in

early ontogenetic stages (e.g. Fig. 4a, b), where it com-

prises a smooth, thin flap of perichondral bone underlying

the more distinct dorsal neural arches, lacking the rounded

growth rings of the neural arch bases. However, rounded

growth rings also characterize the morphology of the

ossified haemal arch bases (Ritchie 2005), so if these were

contributing to the ventral portion of the synarcual, they

must have fused completely as cartilaginous units, prior to

ossification. Clearer evidence for the contribution of hae-

mal arches to the synarcual, and more specifically to the

rounded, spool-like ventral synarcual, is provided by the

ptyctodont placoderm Materpiscis attenboroughi, descri-

bed below.

In early ontogenetic stages of Cowralepis (Fig. 4a, b),

the neural arch bases are rounded and maintain an identity

that can also be identified in larger, older specimens

(Fig. 4c, d). The four neural arch bases seen in the smallest

specimen (Fig. 4a, b) can be compared to the first four

neural arches in the larger specimen (Fig. 4d1, between the

black arrows). In both, these are individual arches that

show growth rings of perichondral bone. As the synarcual

develops in the larger specimen, these arches merge or are

fused, with further layers of cartilage and perichondral

bone deposited along the edge of the fused cartilage

(Fig. 4c, d2, arrowheads). This fusion, which does not

characterize the more posterior arches of the synarcual

keel, may be related to the function of the synarcual, with

increased fusion anteriorly increasing rigidity and strength

closer to the articulation with the braincase.

Although the Cowralepis neural arch keel may have

been affected by postmortem distortion, it is a vertically

oriented structure compared to the ventral, rounded syn-

arcual. Of interest is the morphology of the dorsal surface

of the occipital. Most of the Cowralepis occipitals are

visible in ventral view, and in the earliest ontogenetic stage

preserved, have the beginnings of the spool-like morphol-

ogy, while the synarcual is rectangular in shape (Fig. 4a).

One specimen preserved the dorsal surface of the occipital,

with opposing, but broken ridges running anteroposteriorly

(Johanson et al. 2010: fig. 2H, H’). The presence of these

vertical ridges was confusing, given the flattened head-

shield that was thought to characterize Cowralepis (Ritchie

2005: fig. 20A). However, these can now be reinterpreted

in light of the morphology of the synarcual, as representing

a vertical keel on the occipital, derived from articulating

neural arch bases. This corresponds to observations

reviewed above, of growth rings (representing the dorsal

occipital surface) when the occipital was preserved in

ventral view. These were originally interpreted as vertebral

elements added to the rear of the occipital region (Johanson

et al. 2010), to which can be added the presence of a

vertical keel on the occipital, formed by the neural arches/

spines, and the maintenance of the identity of these arches

(i.e. growth circles not lost, as in the synarcual keel).

Comparable occipital and synarcual regions were already

noted by Johanson et al. (2010), who suggested that simi-

larity of these structures in Cowralepis may have been due

to Hox gene misexpression.

Placoderm synarcuals: Ptyctodontida

Campbellodus decipiens (WAM 11.9.1)

Although broken dorsally and ventrally, the synarcual in

Campbellodus decipiens (Fig. 2c, d) is complete anteriorly

and posteriorly, comprising three fused neural elements.

Incomplete fusion creates open spaces between the neural

spines for the spino-occipital nerves. However, the spaces

between these elements are approximately equal in size,

indicating that fusion was regular through the synarcual.

The most anterior neural spine is the widest. Dorsally, the

neural spines have expanded and fused into a hollow flange

104 Zoomorphology (2013) 132:95–110

123

or keel (Fig. 2d, smaller white arrow). Four neural ele-

ments from the main vertebral column are preserved pos-

terior to the synarcual (Fig. 2c, d) and show the rounded

neural arch base (Fig. 2c, smaller white arrow) that also

characterizes Cowralepis (Arthrodira). There is no indica-

tion that these posterior elements are being fused and

incorporated into the synarcual. A slight protrusion on the

neural elements directly behind the synarcual may repre-

sent the zygapophysis (zyg, Fig. 2d), but there is no evi-

dence of zygapophyses on the more posterior vertebral

elements (Fig. 2c), or fusion medially between the left and

right neurapophysis or spines.

In a second specimen of Campbellodus (WAM

86.9.672), the synarcual shows better preservation. It

comprises three to four fused neural arches, with incom-

plete fusion between three of the neural spines resulting in

two open spaces between them for the spino-occipital

nerves, allowing the original number of neural spines to be

determined. The most anterior neural spine is the widest,

although the posterior one is broken and the synarcual is

also broken ventrally. Dorsally, the neural spines have

expanded and fused into a hollow flange.

Austroptyctodus gardineri (NHMUK PV P.57665)

The synarcual is short, comprising four neural elements

(Miles and Young 1977; fig. 2E). In the ventral portion of

the synarcual, distinct spool-shaped regions can be seen

(Fig. 2e, small white arrows), believed to represent ventral

haemal elements that have become incorporated into the

synarcual (see Materpiscis attenboroughi, below). Anteri-

orly, the ventral synarcual is modified into the rounded,

concave articulation with the occipital. As in Campbello-

dus and Materpiscis, the dorsal flange of the synarcual is

large and expanded. The most posterior arch is less com-

pletely fused, although this is seen in the flange (neural

spine, n.sp), rather than the spool-shaped part of the syn-

arcual. The incomplete fusion results in an elongate

opening between the last neural element, and the rest of the

synarcual. Zygapophyses are absent.

Materpiscis attenboroughi (WAM 07.12.1)

A synarcual is present, but incomplete, with only the right

side being preserved (Fig. 2f, g). In Materpiscis, the syn-

arcual includes five neural arches/spines, which differ

morphologically from more posterior elements of the col-

umn in that zygapophyses appear to be absent. In lateral

view (Fig. 2f), the last three arches are discrete and less

completely fused into the synarcual, with a more complete

fusion of the anterior two arches. The anterior margin of

the synarcual is expanded into a rounded anterior contact

with the occipital. A large dorsal flange extends

posteriorly, possibly resulting from an expansion and

fusion of the neural spines of the first two arches. More

posteriorly, neural spines are distinct, although the spine

associated with the third arch is noticeably shorter and

smaller than the more posterior two arches. The spine of

the fourth arch runs along the posterolateral margin of the

flange, while the spine of the fifth is positioned along the

posterior flange (Fig. 2f). The fifth neural arch is the most

distinct arch of the synarcual, but is associated with the

spool-like morphology of the base, and is considered part

of the synarcual. Internally, the relative contributions of

neural and haemal elements to the basal part of the syn-

arcual can be determined. The two most posterior elements

added to the synarcual (Figs. 2f, g, 4, 5) were described

above as being distinct in lateral view. The neural arch base

of the last element added to the synarcual is also discrete

and visible in internal or medial view (na, Fig. 2g), pos-

sessing a squared base, but more importantly, a rounded

depression in this base, comparable to the base of more

posterior neural arches in the axial skeleton (e.g. Fig. 2c,

smaller white arrow). In the last element, a second square

base with a rounded depression can be seen ventral to the

neural arch base. This is identified as a haemal arch base

(Fig. 2g, haem, compare to Fig. 2c, haem). More anteriorly

in the synarcual, the haemal arch base of the fourth element

is also present, with the more dorsal neural arch base less

clearly visible (4, Fig. 2g). A haemal arch element may

also be present associated with the third element of the

synarcual (3, Fig. 2g), as indicated by its position in

the synarcual relative to the haemal arches associated with

the fourth and fifth elements, and the presence of a

depression in the haemal base. The bases of the first two

arches are clearly fused, although their dorsal margins

remain somewhat distinct (Fig. 2g, smaller white arrow).

Also internally, the large posterodorsal flange has been

broken and is hollow. The posterior part of the flange

extends ventromedially to form a canal for the spinal cord.

Foramina for the spinal nerves appear to be absent.

Materpiscis is similar to other ptyctodonts in that the

synarcual rests upon the notochord, rather than enclosing it,

as occurs in the most other placoderms and chondrichth-

yans (Miles and Young 1977). Like Campbellodus and

Austroptyctodus, Materpiscis has a large posterodorsally

oriented flange, but one that appears to result from the

fusion and expansion of only the first two neural spines,

while the other remain distinct. In Campbellodus, all neural

spines appear to be involved in forming the flange, while in

Austroptyctodus, the last, and most distinct neural arch

appears to only contribute to the posterior margin of the

dorsal flange. As well, the dorsal flange in Materpiscis

shows a distinct ventromedial extension internally. All

synarcuals lack the zygapophyses that can be present on the

neural elements of more posterior vertebrae. Materpiscis

Zoomorphology (2013) 132:95–110 105

123

also provides important information, along with Cowral-

epis mclachlani, as to the vertebral elements contributing

to the development of the synarcual. The last vertebral

element added to the Materpiscis synarcual shows a more

dorsal neural arch base (preserved throughout the Cow-

ralepis synarcual) and a ventral haemal arch base. Haemal

arch bases can also be recognized more anteriorly, and

externally, these preserve the rounded morphology that

characterizes the ventral synarcual of most of the placo-

derms described here. For example, haemal arch bases

were not visible in the Cowralepis synarcual, but Mater-

piscis shows how these can be fully incorporated into the

ventral synarcual. This is relevant to the development of

the recent chondrichthyan synarcual, which shows differ-

ences in the composition of the ventral part of the synarcual

between holocephalans and the batoids.

Placoderm synarcuals: Rhenanida

Nefudina qalibahensis

The synarcual of Nefudina (Lelievre and Carr 2009: fig. 3)

is well fused with the original vertebral elements indicated

only by the foramina for the spino-occipital nerves. Ante-

roventrally, the synarcual forms paired articulations with

the occipital region that consist of thick and solid lateral

expansions (forming a basal element). A reduced noto-

chordal space separates the two lateral articular surfaces.

However, the axis for the lateral thickenings is oblique to

the anterior–posterior axis of the neural canal, extending

ventrally and tapering posteriorly. The basal element may

represent cranial zygapophyses that are structurally rein-

forced for the functional craniovertebral joint. The pres-

ence of five grooves on the dorsal surface of the basal

element suggests the fusion of six vertebrae in its forma-

tion. A low neural arch extends anteriorly over the basal

element. It increases in depth as the basal element angles

ventrally. The arch consists of internal and external peri-

chondral laminae. The external lamina of the arch is fully

fused. Posterior to the basal element, only the neural arch is

present. The ventral edge of the arch is at the level of, or

just above, the spino-occipital foramina so that no estimate

can be made for the number of fused vertebrae. However,

based on the spacing of foramina over the basal element,

the fused neural arches in this posterior region may rep-

resent an additional six vertebral elements.

Jagorina pandora (Mb.f 510.2)

The synarcual of Jagorina is short and was reconstructed

as being formed from three vertebral elements (Stensio

1963: fig. 7). The holotype of Jagorina pandora is pre-

served as part and counterpart, with the synarcual being

more complete in the counterpart (Fig. 5c, smaller white

arrow), although only preserved as an impression. In

Fig. 5b, the most anterior spino-occipital foramen is bro-

ken, while in Fig. 5c, the foramen is complete (as part of an

elongate opening). The two spino-occipital foramina

present indicate the presence of three neural spines in the

dorsal synarcual keel. A concave indentation is present

dorsally along the posterior margin of the synarcual, rep-

resenting one half of the spino-occipital foramen, which

would have been matched by a comparable indentation on

the next, independent vertebral element. The ventral part of

the synarcual, below the dorsal keel, is similar to the

ventral synarcuals described above in Cowralepis, as well

as Dunkleosteus sp. and the ptyctodonts. This region

appears rounded, with a well-developed anterior portion

articulating to the occipital region of the braincase (cranio-

vertebral articulation). The concave anteriormost margin of

the articulation surface is preserved, followed by a bend or

kink in the ventral synarcual, with the dorsal keel of the

synarcual increasing in height posterior to this point. Pos-

terior to the synarcual, the neural arches/spines are distinct

(Fig. 5b), with elongate spines and rounded, spool-shaped

elements ventrally (Fig. 5c, possibly arcocentra sensu Ar-

ratia et al. 2001). There is no indication that additional

neural arch elements are in the process of being fused to the

rear of the synarcual, nor of the elongate haemal arch/

spines that characterize other placoderms (e.g. Fig. 2a2),

although the Jagorina specimen is incomplete posteriorly.

In Cowralepis, the concentric growth lines of the individual

neural arch bases were clearly visible, particularly poste-

riorly. This suggested that the keel was distinct from the

ventral part of the synarcual, and that the latter developed

from haemal elements. Direct addition of haemal arches

was described in Materpiscis. However, in Jagorina, the

neural arch bases are less clearly individualized, placo-

derm-type haemal arches are not preserved, and vertebral

centra or arcocentra may be present just posteriorly in the

vertebral column. It is possible that the rounded ventral

part of the synarcual developed from expansion of the

neural, rather than haemal, arch bases with some incorpo-

ration of vertebral centra (as in skates and rays, described

below).

Chondrichthyan synarcuals

Holocephali

In Chimera monstrosa, separate dorsal elements are added

to the posterior margin of the developing synarcual, with

the elongate space between these developing into the dorsal

and ventral spino-occipital nerve foramina that are present

more anteriorly in the mineralized synarcual (Fig. 6a, black

arrows; Garman 1913; Didier 1995; Johanson et al. 2010).

106 Zoomorphology (2013) 132:95–110

123

Ventral to the notochord, individual rectangular elements

are also added, and incorporated into, the posterior margin

of the synarcual. Didier (1995) identified these dorsal and

ventral elements as homologous to the basidorsals and

basiventrals comprising the chondrichthyan vertebral col-

umn, which is followed here (and see above). However, in

Chimera, the ventral elements are located along the ventral

margin of the notochord, while most batoid basidorsals and

basiventrals are dorsal and dorsolateral to the notochord,

respectively (e.g. Fig. 7b; Miyake 1988; Claeson 2010). As

well, in elasmobranchs such as Dasyatis americana Hil-

debrand and Schroeder, 1928 (Fig. 7a, Myliobatiformes),

spinal nerve foramina pierce the basidorsals and basiven-

trals, but as noted above, only the neural arch elements of

Chimera contribute to the spinal nerve foramina, not the

more ventral elements. The posterior margin of the Chi-

mera synarcual has a scalloped appearance, as the devel-

opment of this margin lags behind the addition of the dorsal

and ventral elements. The dorsal synarcual has become

highly modified in shape and supports the first dorsal fin

spine.

Elasmobranchii; Batoidea

The Batoidea includes the Torpediniformes, Pristiformes,

Myliobatiformes, Rhinobatiformes and Rajiformes. The

batoid synarcual is complex, with the morphology of the

synarcual differing among these groups (Garman 1913;

Miyake 1988; Figs. 6b–e, 7). Development of the batoid

synarcual was recently described by Claeson (2011; also

Miyake 1988), who noted that the synarcual was formed

from the fusion of vertebral centres, and in early ontoge-

netic stages of Raja asterias Delaroche, 1809, a synarcual

composed of uncalcified cartilage and a surficial layer of

tesselated cartilage surrounded the first free vertebrae. The

free vertebrae comprise the vertebral centra, which in

chondrichthyans are composed of areolar cartilage. The

presence of free vertebrae characterizes the batoid axial

skeleton (Figs. 6b–f, 7). In Raja asterias, the first free

vertebrae is positioned near the midregion of the gill

arches, but this position shifts posteriorly in older speci-

mens (Claeson 2011; Figs. 6b–e, 7). Neural elements are

incorporated into the synarcual from anterior to posterior

(Claeson 2011). In the myliobatid Dasyatis americana

(Fig. 7), the vertebral elements being added to the posterior

margin of the synarcual include the neural arch (basidorsal,

neural spine) and basiventral, both carrying the spinal

foramina; this also is the case in the other batoid groups

(Garman 1913; Miyake 1988). An usual feature of the

vertebral column (except the Torpediniformes) is that

the basiventrals are positioned dorsolaterally relative to the

notochord, rather than ventrally (Fig. 7b, Miyake 1988;

Claeson 2010). In Dasyatis, as in other batoids (Claeson

2011), the basiventrals grow ventrally and meet medially,

creating a seam along the ventral surface of the synarcual

(Figs. 6c–e, 7b). It appears that the basiventrals fail to meet

around the free centra, with the first visible free centra in

Figs. 6, 7 located approximately halfway along the syn-

arcual, near the lateral pectoral fin articulations. Claeson

(2011) noted that the free vertebrae were anterior in early

ontogenetic stages and shifted posteriorly to this position

near the pectoral fin. As an alternative explanation to this

shift, the first free vertebrae visible in Fig. 6b, c are small

and/or poorly mineralized. These vertebrae also lack the

distinctive spool-shape of more posterior free vertebrae,

indicating their reduced development. Comparable verte-

brae may have been present even more anteriorly, but re-

sorbed and incorporated into the synarcual during

development, rather than shifted posteriorly.

In Dasyatis, the basiventrals develop ventrally towards

the free vertebra, and in ventral view, the basiventral

appears to be associated with the vertebrae, with spaces

anteriorly and posteriorly between basiventrals (Fig. 7b).

Just posterior to the developing synarcual, a more overt

segmentation occurs, associated with the basiventral and

free vertebra (Fig. 7b, black arrow). In ventral view, this

segmentation is unequal, occurring on the right side (to the

bottom of the Figure), but not yet on the left. Segmentation

is lost anteriorly, presumably as the basiventral of the

neural arch is incorporated into the synarcual (Fig. 7b,

white arrowhead). Incorporation into the synarcual is also

indicated by the loss of one of the spaces between the

basiventrals ventrally, but the retention, for the moment, of

the space on the other side of the free vertebra (Fig. 7b,

white asterisk). Segmentation is absent posteriorly, repre-

senting the formation of the second synarcual characteristic

of Dasyatis and the Family Myliobatidae (Compagno

1977).

Discussion

The anterior vertebrae form a synarcual in placoderms

(Rhenanida, Ptyctodontida, Arthrodira) and some chon-

drichthyans (Holocephali, Batoidea). The synarcuals in

these groups are not homologous, suggesting that devel-

opment of the synarcual in these groups may differ. Syn-

arcual morphology varies among these groups, and

although comparable vertebral elements (neural, haemal,

centra) are added, differing numbers of these elements are

incorporated into the synarcual, and the way these are

added to the developing synarcual differs substantially in

the batoids. The synarcuals of placoderms and holoceph-

alans are the simplest, while those of the batoids are more

complex. For example, the batoid synarcual possesses

protrusions called lateral stays, and posterior to this, lateral

Zoomorphology (2013) 132:95–110 107

123

pectoral processes with a varying number of pectoral

condyles (Garman 1913; Gonzalez-Isais and Domınguez

2004; Claeson 2011). The pectoral process is associated

with a suprascapular element in Torpediformes, Mylio-

batiformes and Rajiformes that can be broad in taxa such as

Raja (Fig. 6e). Among the Batoidea, the Myliobatiformes

are also characterized by a second, more posterior thora-

columbar synarcual (Garman 1913; Compagno 1973; de

Carvahlo et al. 2004). As well, the batoid synarcuals sur-

round free vertebrae posteriorly (de Carvahlo et al. 2004;

Claeson 2011; Figs. 6, 7).

In the holocephans and the placoderms described above,

the synarcual is composed of the neural/basidorsal and

haemal/basiventral elements, but lateral stays and pectoral

processes are absent. There is no involvement of inde-

pendent or free vertebral centra that characterize the ba-

toids (sensu Claeson 2011; centra generally absent in

placoderms, or represented by arcocentra, and represented

by notochordal rings in holocephalans; Patterson 1965;

Didier 1995). An exception to this may be the rhenanid

Jagorina, with putative arcocentra just posterior to the

rounded ventral part of the synarcual, suggesting the ven-

tral synarcual may have surrounded the centra as in ba-

toids. In placoderms and holocephalans, fusion of dorsal

vertebral elements creates spino-occipital foramina

between the elements. In the holocephalans (Chimera and

Callorhinchus Lacepede, 1798), both neural and haemal

elements contribute to the synarcual, with mineralization

occurring shortly thereafter (e.g. Chimera, Fig. 6a, indi-

cated by alizarin red staining). The relative contribution of

neural and haemal elements to the placoderm synarcual can

also be determined because the synarcual is surrounded by

perichondral bone which effectively preserves early stages

of ontogenetic development. For example, in the arthrodire

Cowralepis mclachlani, the anterior synarcual preserves

the first vertebral elements incorporated into the synarcual,

including rounded neural arch bases and more ventral

haemal elements. The anterior synarcual also preserves the

change from these discrete, growing bases to a fusion of

these bases and a more continuous bone deposition

(Fig. 4c–f). In the ptyctodont Materpiscis attenboroughi,

vertebral elements added to the posterior synarcual were

not completely incorporated and preserve the distinctive

neural arch bases dorsally within the synarcual, and the

haemal arch bases ventrally (Fig. 2f). These observations

indicate that the placoderm synarcual resembles that of the

Holocephali in being simpler, generally lacking batoid-like

centra and incorporating ventral vertebral elements directly

into the synarcual with no ventral expansion (the synarcual

of the arthrodire Dunkleosteus sp. may also be an exception

in this regard). It is noteworthy that such ontogenetic detail

can be obtained from fossil synarcuals. Growth of the

synarcual in the Holocephali effectively obscures earlier

ontogenetic stages (e.g. Fig. 6a), while in placoderms,

perichondral bone deposition around discrete cartilaginous

elements may have inhibited further incorporation. In the

synarcuals of the arthrodire Dunkleosteus sp. (Fig. 6) and

the rhenanid Nefudina (Lelievre and Carr 2009), individual

vertebral elements are more difficult to recognize, such that

these may have largely fused or been incorporated into the

synarcual prior to perichondral bone deposition. Observa-

tions in the placoderm synarcual, and developmental sim-

ilarity with respect to the Holocephali supports previous

observations that in chondrichthyans, the synarcual first

forms from separate vertebral centres (Miyake 1988;

Claeson 2011), rather than from a failure of proper verte-

bral segmentation. The latter can affect the vertebrate

vertebral column, being a condition recognized in many

human skeletal pathologies where the anterior cervical

vertebrae are also fused (e.g. Klippel-Feil syndrome;

Schaffer et al. 2005).

In the batoids, Claeson (2011) noted that in early

ontogenetic stages, the synarcual consists of uncalcified

cartilage, formed from coalescing vertebral chondrification

centres, covered by a layer of prismatic cartilage. This thin

mineralized layer allows the synarcual to grow and

develop. At later stages, neural arches are added or incor-

porated into the synarcual, and the synarcual continues to

develop posteriorly, with the more dorsally positioned

basiventrals growing ventrally, on either side of the free

vertebral centra (Fig. 7b; Miyake 1988). By comparison, in

placoderms and holocephalans, haemal/basiventrals are

more ventral in position and do not expand, but are added

to the synarcual directly.

Claeson (2011) observed that the first free vertebra was

located under the gill arches in a younger specimen of

Raja, which then shifted posteriorly in older specimens.

This would require the addition of neural arch elements

anterior to the first free vertebrae, and rostrocaudal growth

of the synarcual relative to the free vertebrae. Alterna-

tively, it was noted above that anterior free vertebrae were

smaller and less mineralized (Fig. 6b, c) and could have

been resorbed as the synarcual developed, rather than

shifted posteriorly. However, Dean et al. (2009) noted that

chondrichthyan cartilage is incapable of remodelling itself

and growth is only possible via cartilage deposition. This

may be true as a general condition of the avascular carti-

laginous skeleton (Dean et al. 2009), but more localized

resorption, in conjunction with the most anterior free ver-

tebrae in the synarcual, may have been possible.

The presence of a synarcual in early vertebrates is

important because it represents the first indication of the

differentiation of the vertebral column into a distinct

anterior region. In other vertebrates, the transition between

the anterior cervical and more posterior thoracic region in

the zebrafish, as well as birds, mice and Xenopus was

108 Zoomorphology (2013) 132:95–110

123

correlated with the anterior expression boundary of the

gene Hoxc6 (Burke et al. 1995; Morin-Kensicki et al.

2002). Holocephalans possess all four paralogous Hox

genes, including HoxC, although the latter is absent in

sharks such as Scyliorhinus canicula (Linnaeus, 1758) and

Heterodontus francisci (Girard, 1854) and the ray Leuco-

raja erinacea (Mitchill, 1825)(Ravi et al. 2009; King et al.

2011; Oulion et al. 2011). It would be interesting to test

whether the chondrichthyan synarcual is associated with

any Hox gene expression boundaries, comparable to the

cervical-thoracic transition in other groups. However, the

synarcual appears to continue to develop through early

ontogeny (Figs. 6, 7), despite being associated with the

position of the pectoral fin in the batoid groups Myliobatidae,

Rajidae and Rhinobatidae. In holocephalans, the pectoral fin

is not supported by the synarcual, but Didier (1995) noted

that the synarcual is composed of 10 vertebral elements.

However, the Chimera specimen illustrated in Fig. 6a

already has ten sets of nerve foramina, with neural arch

elements still being added posteriorly (numbers of spino-

occipital foramina can vary within taxa; Claeson 2011).

Placoderm synarcuals are shorter in most groups, although

longer in Nefudina and Dunkleosteus, where individual ver-

tebral elements are difficult to distinguish. As noted, this

suggests that fusion of cartilage elements occurred before

perichondral bone deposition. In taxa with shorter synarcuals,

such as Materpiscis and Cowralepis, perichondral bone had

been deposited around individual elements, particularly pos-

teriorly, potentially hindering ongoing synarcual develop-

ment. Synarcual development appears more variable in

placoderms, associated with onset of bone deposition, rather

than a fixed boundary. Establishing whether gene expression

boundaries exist in extant chondrichthyans, particularly in

holocephalans (showing more development similarity to

placoderms), will provide more information in this regard.

Acknowledgments KT would like to acknowledge the receipt of a

QEII Fellowship and KT and ZJ acknowledge DP110101127 awarded

by the Australian Research Council. We also thank Mikael Siversson

Western Australian Museum for access to the collections. AR would

like to thank Mr. Alex McLachlan, owner of the quarry from which

Cowralepis mclachlani has been collected. Mr. McLachlan has pro-

vided unlimited access to the quarry and substantial financial support.

We would also like to thank Mr. Bruce Loomes, Canowindra, for

discovering the specimen of C. mclachlani described in this paper,

and for recognizing its importance. RC thanks Scott Schaefer for

access to AMNH Ichthyology Collections. Finally, we would like to

thank an anonymous reviewer and Kerin Claeson for their comments,

which were very helpful in improving this paper.

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