Peabody Museum

of Natural History

Yale University

New Haven, CT 06511

Postilla umber ise

15 February 1982

An Early Triassic Hybodont Shark
from Northern Madagascar

Keith Stewart Thomson

ita, a
Sea

(Received 2 April 1981)
Abstract

Material of the upper and lower jaws,
together with teeth and other remains, of a
Triassic hybodont shark from Madagascar
is tentatively referred to the genus Acrodus.
The material offers new evidence concern-
ing the jaw suspension in hybodont sharks
and its significance in the evolution of
Elasmobranchii.

Key Words

Elasmobranchii, Hybodonti, jaw suspen-
sion, Acrodus, Madagascar, Triassic.

Introduction

The evolutionary history and relationships
of the cartilaginous fishes comprise one of
the less known and more intractable areas
of study in vertebrate paleontology. The
Problems stem inmost part from the nature
of the skeleton which does not (except for
the dentition and spines) lend itself to pre-
Servation in fossil form. Thus, whereas
Many taxa of fossil chondrichthyan fishes
have been described on the basis of minute
differences in dental structure, the number
Of taxa that are known from other cranial or
Postcranial skeletal remains is frustratingly

© Copyright 1982 by the Peabody Museum of Natu-
ral History, Yale University. All rights reserved. No
Part of this publication, except brief quotations for
Scholarly purposes, may be reproduced without the
written permission of the Director, Peabody
Museum of Natural History.

small. The present contribution.offers a de-
scription of new cranial.material of a Trias-
sic hybodont shark and a discussion of the
evolution of certain features of head anato-
my in the Elasmobranchii (sharks, skates
and rays) and their immediate fossil rela-
tives within the cartilaginous fishes.

Two major models have been proposed
for elasmobranch relationships. In a seminal
work, Schaeffer (1967) synthesized availa-
ble information on living and fossil forms
into a three-part horizontal classification,
recognizing three grades—“cladodont" (es-
sentially Paleozoic), “hybodont” (essentially
Mesozoic, and “modern level” elasmo-
branchs. This rational organization was fol-
lowed by a new surge of interest in the
group, with descriptions of new taxa and
new analyses of relationships eventually
leading to the second model. Maisey (1975,
see also Campagno, 1977) proposed a
more cladal classification, realigning the
“hybodont” sharks into two vertical assem-
blages—“hybodontiform” (for example,
Tristychius, Hybodus, Acrodus, Astera-
canthus, Lissodus and Lonchidion) and
“ctenacanthiform” (including Ctena-
canthus, Spenacanthus, Goodrichthys,
Nemacanthus). The ctenacanths were
then linked formally with the modern level
sharks (“euselachiforms”) while the hybo-
donts, as thereby restricted, were removed
from any relationship with modern sharks.
Compagno (1977) further has reorganized
schemes of relationships among the
modern sharks and the three apparently
primitive groups— Heterodontus, Chlamy-
doselache and the hexanchoids—which
previously had been thought to be indepen-
dent relics of hybodont radiations, are now
more securely incorporated into the radia-
tion of euselachians.

i Early Triassic Hybodont Shark
from Northern Madagascar

Postilla 186

A nomenclatural note must be added
here. Maisey (1975) uses the term “eusela-
chian” for the ctenacanths plus modern
sharks, skates and rays, whereas Compagno
prefers the original use of Regan (1906) in
which the ctenacanths plus modern forms
are termed the “neoselachians” and the
term “euselachian” is restricted to the
modern level radiations which are consid-
ered to be monophyletic. This latter use will
be followed in the present paper.

These two models of elasmobranch rela-
tionships have had a great heuristic value
in focusing attention on the important
issues. Compagno’s work (1973, 1977) has
concentrated upon the living groups and
their immediate fossil relatives. Zangerl
(1973), Zangerl and Williams (1975),
Schaeffer and Williams (1977) and Schaef-
fer (in preparation), inter alia, have brought
important new information concerning the
complex radiations of Paleozoic elasmo-
branchs. Maisey (1975, 1976, 1977) and
Dick (1978) have restudied some of the
Mesozoic hybodont and ctenacanth mate-
rials. Dick (1978) has also questioned the
ctenacanth/hybodont separation, leaving
this question still to be resolved. Much
work remains to be done. Not only is there
little solid information that helps assign
relationships within and among the various
groupings, the validity of current groupings
still remains to be tested. In the present
work, new material is described of the Trias-
sic hybodont Acrodus and features of the
evolution of the elasmobranch palate are
discussed.

Description

In 1961, Professor Bernard Kummel of the
Museum of Comparative Zoology (MCZ),
Harvard University, made extensive collec-
tions from the famous nodule-bearing beds
of the Early Triassic of Northern Madagas-
car (see, for example, Piveteau, 1934, and
Lehman, 1952). Among the material he col-
lected was a single largish nodule (MCZ
13432, Ambilobe Bay Locality) that, on

preparation, revealed the presence of the
first shark material (except for scraps of
denticles) recorded from Madagascar. The
specimen Is preserved, as are all such
nodules, in part and counterpart with the
calcified material almost totally removed
by solution, leaving a natural cast of the re-
mains (Figs. 1 and 2). The upper and lower
jaws and dental barriers of the left side, part
of the right mandible, two (?) ceratohyals, a
fragment of a possible hyomandibular and
an indistinct indication of the posterior por-
tion of the braincase are preserved and
have been developed by very careful prepa-
ration, further revealing the natural cast, fol-
lowed by casting in various plastics.

As the teeth in this specimen are com-
parable with other teeth from around the
world usually ascribed to the genus
Acrodus Agassiz, the dentition of the new
Madagascar specimen will be described
first. The various described taxa of Acrodus
differ from each other in rather minor fash-
ion among the sizes and patterns of ridges
on the dental plates and the shape and cur-
vature of the crown. Typically each tooth is
lozenge-shaped or rhomboidal with a
single low crown.The maximum height of
the tooth is less than half of the maximum
length of the tooth. Each tooth is ornament-
ed with a series of fine ridges which more
or less radiate from the center of the crown
(Figs. 3 and 4).

There seem to be four rows of teeth in
each dental battery, although the possibility
of an extra row of small teeth at the anterior
margin of the battery cannot be excluded.
The teeth of the first row are distinctly smal-
ler than the remaining three which are all

Fig. 1 >

Half nodule completely negatively prepared to pro-
duce natural mold and then cast in Smooth-on
molding compound to show head structures in
mesial view. Scale in mm.

Fig. 2 >

Half nodule completely negatively prepared to pro-
duce natural mold and then cast in Smooth-on
molding compound to show head structures in later
al view. Scale in mm.

3 Early Triassic Hybodont Shark Postilla 186
from Northern Madagascar

5 Early Triassic Hybodont Shark
from Northern Madagascar

Postilla 186

longer and roughly of equal size to each
other. The teeth of the first and second
rows have a slightly more strongly curved
crown than those of the last two rows.
There seems to be no basic change in the
outline of the base of the crown among the
four rows. These characteristics of the
dental battery seem to exclude the material
from the genus Paleobates Von Meyer,
1849, which is described by Stensié (1921)
as having more tooth rows with the third
and fourth rows made up of teeth signifi-
cantly longer, flatter, and more rectangular
in shape than the other rows. Similarly, al-
though the mandible of Pa/eobates
polaris as described by Stensid (1921) is
short and deep like that described here, the
detailed shape is different and in the face of
So little comparative material taxonomic
comparisons are tenuous at best.

The dimensions of the largest teeth in
the Madagascar material are as follows:
Average length 8 mm: average breadth 2.3
mm; average crown height 2 mm. The
ridges on the teeth are relatively fine com-
pared with those of described Acrodus
Material and they show a pattern of bifurca-
tion as they proceed from the center of the
Crown. The general appearance is shown in
Figures 1 and 2.

A survey of described materials fails to
Show any Triassic shark dental material
with a pattern exactly comparable to that
of the new material from Madagascar. It
might be reasonable, therefore, to conclude
that the taxon represented in Madagascar
'S distinct and that a new species should be
Named for it. Here is a classic paleontolo-
gist’s dilemma, for it is certain that not all
the species of Acrodus or other genera dis-
tinguished by their authors on the basis of
dental ornamentation are true species
(however that might be defined). It may be
worthless to add another to this disreputa-

Fig. 3
’Acrodus sp. Tooth in side view.
Fig. 4

? ; crete
’Acrodus sp. Incomplete view of two teeth in side
and occlusal views.

ble list. Furthermore, although | have other
morphological data upon which to base a
description of the Madagascar specimen, |
have had no opportunity for comparison of
skull data with any other Acrodus material,
let alone the type material. For the moment
| will merely recognize the new material
from Madagascar as ?Acrodus sp. with
the note that, if | were willing to accept the

* dental evidence as prima facie evidence (as

lam not), it would be possible to distinguish
the material as belonging to a “species” dis-
tinct from other described materials.

Palatoquadrate

The two halves of the nodule show the pala-
toquadrate from the medial (Fig. 5) and la-
teral view (Fig. 6). The medial exposure of
the palate is virtually perfect on one half-
nodule; the whole mesial surface is ex-
posed apparently undistorted. The other
half principally shows the posterior portion
of the lateral surface, with some details of
the anterior tip of the palate. All the articular
surfaces of the palate are clearly visible.

The palatoquadrate (overall length = 9.0
cm; maximum depth = 2.4 cm) is elongate
with a relatively small postorbital expansion
and it lacks any significant deepening at
the otic process. The anterior three-quarters
of the palatoquadrate is formed as a
straight, stout bar with a pronounced down-
ward and mesial curvature of the tip, and
there is a broad ventromesial flange bearing
the dental battery.

The most prominent features of the
mesial surface of the palatoquadrate are
three articular surfaces (Figs. 5 and 7). The
largest of these is formed on the anterodor-
sal extreme of the otic process and forms
the articulation with the postorbital process
of the braincase. This articular surface is a
massive groove oriented not transversely
but directed anterolaterally at an angle of
about 17°. In the vertical transverse plane it
is directed ventromesially at an angle of
about 12° below horizontal. The whole ar-
ticular facet is set off from the surface of
the palatoquadrate by modest ridges.

Early Triassic Hybodont Shark Postilla 186
from Northern Madagascar

f Early Triassic Hybodont Shark Postilla 186
from Northern Madagascar

bas
post
eth
20mm c
Outline drawing of principal features of Figure 1, Restoration of the right palatoquadrate in lateral
half nodule prepared to show mesial view of head view (above) and mesial view (below). For abbrevia-
Structures. Abbreviations for this and following fig- tions, see Figure 5.

ures: bas = basal articulation; c = condyle; cer =
ceratohyal; eth = ethmoid articulation; ?hy =
?hyomandibular; m = mandible; post = postorbi-
tal articulation; pq = palatoquadrate.

4Fig.6
Outline drawing of the principal features of Figure
2, half-nodule prepared to show lateral view of head
Structures. For abbreviations, see Figure 5.

8 Early Triassic Hybodont Shark
from Northern Madagascar

Postilla 186

Anteriorly there are two other major
facets. A ventrally directed facet is formed
as a broad groove located about one-third
of the distance from the anterior tip of the
suborbital ramus. This articulation faces
ventrolaterally at an angle of about 40°
below horizontal and thus, when seen from
a directly anterior view, forms an angle of
some 52° with the groove of the otic pro-
cess. This facet is supported on a well-
developed process formed as a flange on
the mesial surface of the suborbital ramus
and therefore, properly speaking, is as
much a mesial as a ventral articulation. We
may term this articulation the basal artic-

ulation: it is Supported by the basal process.

The third major articulation may be called
the ethmoid articulation. This is a shallow,
concave surface, formed as an oval, borne
distinctly clear of the upper and slightly
mesial surface of the anterior end of the
palatoquadrate bar. This facet is oriented
forwards, upwards and slightly mesially
and evidently articulated with some sort of
ectethmoid process of the postnasal wall.

In addition to these three major facets,
the mesial surface of the ventrally curved
tip of the palatoquadrate is formed into a
flange that apparently was ligamentously
connected to the opposing structure of the
other palatoquadrate. Between this flange
and the ethmoid process the upper surface
of the palatoquadrate is marked by shallow
ridges and grooves, suggestive of a sliding
connection with the underside of the
postnasal wall. The upper dental battery
was borne upon a deep thick flange of the
palatoquadrate extending over the whole
length of the suborbital ramus.

The lateral surface of the palatoquadrate
(Figs. 4 and 7) is relatively uncomplicated.
The postorbital portion is deeply concave
and massively thickened. In lateral view the
upper margin of the palatoquadrate bar and
the lower surface of the flange bearing the
dental battery are parallel and horizontal.

The palatomandibular articulation is
typically double. The two parts of the joint
lie along the posterior rim of the posterior
process and make an angle of about 70°

from the sagittal plane. The lateral portion
of the joint, at the posterior tip of the palato-
quadrate, is a narrow, convex, somewhat
triangular process. The inner part of the
joint is a larger, deep glenoid facet formed
as an opposite triangle. Mesially, the inner-
most part of the flange forming the posteri-
or margin of the inner half of the joint is pro-
duced into a slight ventral process contin-
ued anterodorsally as a ridge on the mesial
surface of the postorbital process.

There is no obvious groove in the poste-
rior rim of the palatoquadrate of the sort
that would have marked the close apposi-
tion of the hyomandibular. However, the la-
teral and mesial angles marking the extent
of the bicondylar jaw joint are both devel-
oped significantly behind the curve of the
posterior surface of the postorbital ramus
and it is possible that the tip of the hyoman-
dibular could have fitted alongside either of
these.

Mandible

The mandible (overall length 8.3 cm; maxi-
mum depth 3.4 cm) is well exposed in both
mesial and lateral views. The main anterior
part of the ramus is essentially flat, with no
marked convexity. The mandible is relative-
ly deep, the maximum depth being con-
tained approximately 2.2 times in the over-
all length. In lateral view the mandible
shows a concavity in the posteroventral
region but no other major features. The
mesial surface is marked by a deep, long
horizontal groove which evidently was the
site for attachment of the lower dental bat-
tery. Almost in the center of the mesial sur-
face of the mandible there is a large scar,
probably for muscle attachment. The
bicondylar jaw joint is set at an oblique
angle to the main mandibular ramus which
is otherwise relatively straight. The more la-
teral and anterior of the two portions of the
joint are borne on a pronounced process
which is continued as a ridge forming the
angle in the posterior part of the mandible.
This ridge and the ridge on the palatoqua-
drate that leads to the upper articular facet

9 Early Triassic Hybodont Shark
from Northern Madagascar

Postilla 186

form an essentially single line and both
were evidently the site of a major ligamen-
tous connection between the upper and
lower jaws. The orientation of the two artic-
ular facets in the mandible shows that the
plane of the mandibular ramus was not
vertical when the gap was closed but was
inclined mesially at some 10°.

Branchial skeleton

Lying diagonally across the mesial surface
of the mandible (Fig. 5) is a large element
that is tentatively identified as the ceratohy-
al. Its anterior margin, particularly the anter-
Oventral part, is incomplete, but the posteri-
Or portion is intact. The total length and
Shape of the elements cannot be guessed.
Another fragment lying above the palato-
quadrate may possibly represent part of an
€pibranchial. This fragment again only
Shows the posterior portion. It is exposed in
lateral view and shows a massive lingual
Shelflike flange.

Slightly inside the posterior rim of the
Palatoquadrate (Fig. 6) is a rod-shaped sec-
tion of an element that is preserved in the
€xpected position of a hyomandibular. This
rod does not extend as far as the mandibu-
lar articulation and it is difficult to tell, if this
's the hyomandibular, what part it might
have played in the jaw suspension. The fact
that the element is circular in cross section,
rather than being flattened so as to be
Pressed to the palatoquadrate, is a small
tem of evidence suggesting a minor role at
best in the suspensorium for this element.

Relationship of the Braincase to the
Jaws

No part of the braincase is well preserved,
but the strongly developed articular facets
On the palatoquadrate allow us to make
Some tentative reconstructions, at least of
the overall proportions of the braincase and
Of its relationship to the jaws. First, we can
Note that the distinctly posterior placement
Of the otic articulation with the postorbital

process and the oblique orientation of the
“hyomandibular” strongly suggest that the
otic region of the braincase was short. Fur-
ther, the postorbital processes were well
developed not only in the lateral extent but
also were deep ventrally. The basal articula-
tion between palate and braincase is inter-
esting because it is relatively far forward
and must be in an antorbital rather than
suborbital position. There must have been
paired rodlike basal processes on the antor-
bital/suborbital shelf of the braincase, pro-
jecting directly laterally. In addition, there
must have been well-developed, paired ec-
tethmoid or antorbital processes of the
posterior nasal region for the articulaton of -
the ethmoidal articular facets of the palato-
quadrate. This must have been developed
immediately behind and/or below the nasal
capsule with a sliding articulation of

the capsule. However, the palatoquadrate
probably did not extend forward beneath
the whole of the capsule, but only to the
back of the capsule.

Having delineated the relationship be-
tween braincase and palatoquadrate, we
can also ask what the mobility of the jaws
was. It was clearly impossible for the jaws
to move anteroposteriorly relative to the
braincase. The postorbital and nasal artic-
ulations are arranged to allow only lateral
excursions of the palatoquadrate relative to
the braincase, whereas the ethmoid artic-
ulation suggests a rolling hinge. But it is dif-
ficult to see what sort of lateral movement
of the palatoquadrates occurred. The ob-
lique orientation of the transverse basal
and postorbital articulations is such that ex-
cursion with close connection of palate and
braincase at one joint would cause a sepa-
ration of the two structures at the other
joint. This result is heightened by the slight-
ly anterior orientation of the groove of the
postorbital articulation. If the joint were
somewhat loose, with ligamentous bind-
ings, it is possible that the palate was flared
laterally from the braincase, with the eth-
moid articulation forming the fulcrum and
the posterior part of the palate making the
greatest excursion, rolling outwards, and

10 Early Triassic Hybodont Shark
from Northern Madagascar

Postilla 186

slightly forwards as the basal articulation
slid outwards on the basal process. This
was accompanied by depression of the
mandible, the bicondylar jaw joint being ar-
ranged so that as the mandible was de-
pressed it rotated slightly, bringing the
mandibular ramus into a more vertical
plane.

The complex articulations between pala-
toquadrate and braincase and the specific
nature of the mechanical connection of the
two, with the palate very firmly braced
against the braincase by the two major
transverse articulations, make it unlikely
that the hyomandibular had a prominent
role in movements of the jaws.

Discussion

The palate of 7Acrodus shows many im-
portant differences from that of other
sharks, and these lead naturally to a discus-
sion of the plate and neurocranium in
sharks in general.

It is widely agreed that there have been
important changes in the nature of the jaw
suspension articulation in the evolution of
sharklike fishes, particularly in a general de-
velopment of a hyostylic jaw suspension
from an (ancestral) amphistylic condition.
The data are well summarized by Schaeffer
(1967) and Maisey (1980). Here, unfortu-
nately, little progress has been made in
refining this useful but broad generalization,
the reason being that scant new informa-
tion has come available concerning the
nature of the jaws in fossil elasmobranchs.
This being the case, it is frustrating in the
extreme to discover that the structure of
the palatroquadrate in ?Acrodus, so beauti-
fully demonstrated in the material de-
scribed here, is totally unlike that of any
other shark.

By drawing together the recent descrip-
tions of Cobe/odus by Zanger|l and Wil-
liams (1975), Denaea by Schaeffer and
Williams(1977), and the older work on C/a-
dodus by Gross (1937, 1938), we can
begin to define the nature of the palatoqua-

drate in Paleozoic “cladodont” elasmo-
branchs (see also the morphotype defined
by Zangerl, 1973). The palate seeems to
have been basically quite simple. The pos-
torbital ramus is very large having the typi-
cal primitive “cleaver” shape described by
Schaeffer (1975) and Schaeffer and Wil-
liams (1977). The postorbital articulation is
well developed in these sharks and this is a
primitive characteristic for all gnatho-
stomes (Schaeffer and Williams, 1977). The
nature of the actual articulation which is
borne on the ventral and posterior portion
of the postorbital process and a massive
otic process of the palatoquadrate is not
completely clear. The articulation was es-
sentially in a vertical sagittal plane and al-
lowed no fore-and-aft movement of the
palate except possibly through a rotatory
movement in the plane of the palate.

The suborbital ramus is relatively slender
and has a well-developed basal articulation
with the subocular shelf of the braincase.
The subocular shelf shows a lozenge-
shaped process which extended clear of
the subocular shelf and the articular shelf.
The articular surface between palate and
braincase is somewhat elongate anteropos-
teriorly. The basal articulation is developed
rather anteriorly in the orbit and it is not
necessarily homologous with the “basipte!-
ygoid articulation” developed between
palate and braincase in teleostome fishes
and tetrapods, which typically is formed at
the transverse level of the foramen for the
hypophysial opening (see discussion in
Jarvik, 1977, inter alia). There is no develop-
ment of ethmoidal processes between the
tip of the palatoquadrate and the nasal
capsule. The two halves of the palates
possibly met in the midline, posterior to the
nasal capsule, except in Cladoselache
where the mouth was terminal (Zangerl,
1973).

It has been claimed that Cladodus had
an orbital process and articulation, and als°
a “basal angle” in the floor of the braincas®
as in some modern sharks (see Jarvik,
1977). However, the material described by
Gross (see photograph in Gross, 1938, pl. |.

11 Early Triassic Hybodont Shark
from Northern Madagascar

Postilla 186

20 mm

fig. 2A) shows merely a slight thickening of Fig. 8

the tip of the suborbital ramus of the palato-
quadrate. In Gross’s reconstruction (1938,
fig. 2) this expansion has been slightly exag-
gerated (see also Jarvik, 1977, fig. 4D).
There seems to be a fundamental difference
between this sort of thickening of the
Suborbital ramus and a true orbital process
(see below). Further, the structure identified
by Jarvik (1977) as the articular surface on
the orbital wall for the reception of this pro-
Cess does not seem to fit the process and is
Probably no more than the angle produced
behind the postnasal wall. An orbital pro-
C€Ss is definitely absent in Cobe/odus and
Denaea.

Four sharks that would fall into the hybo-
dontiform assemblage of Maisey’'s (1975)
Classification have been described: Hy-
Lodus (see Woodward, 1916; and Maisey,
'N preparation), Asteracanthus (Peyer,
1946), Tristychius (Woodward, 1924, and
Dick, 1978) and Onchoselache (Dick and
a 1980). All four forms agree with
*Acrodus in that the postorbital ramus of
the palatoquadrate is relatively reduced

Asteracanthus. Sketch of the palatoquadrate in la-
teral view from British Museum (Natural History)
specimen 12614.

compared with the Paleozoic forms. It does
not have the “cleaver” shape and massive
otic process, being on the contrary low and
elongate. Similarly, in all four forms the
suborbital ramus is rather broader than in
the Paleozoic forms. Asteracanthus has
been described in some detail by Peyer, but
unfortunately his interpretations are diffi-
cult to follow and, after study of material in
the British Museum (Natural History), par-
ticularly specimen P. 12614, | believe that
he had worked with an incorrect orientation
of the materials. As shown in Figure 8, the
overall proportions of the palatoquadrates
of Asteracanthus are very similar to those
in ?Acrodus. However, the points of artic-
ulation with the braincase are completely
different. Specifically, the prominent trans-
verse otic-postorbital articulation of ?Ac-
rodus are completely lacking and no spe-
cial articular surfaces are developed in

12 Early Triassic Hybodont Shark
from Northern Madagascar

Postilla 186

either position. In this respect, Astera-
canthus agrees far more with Hyboadus :
in these two genera the articulations be-
tween braincase and palate were arranged

to produce a fore-and-aft sliding movement.

The articular surfaces of the palatoquadrate
are therefore merely the upper rim of the
postorbital ramus which fitted into an
anteroposterior groove on the under side .
of the massive postorbital process (Maisey,
personal communication) and a similar slid-
ing contact between the dorsomesial rim of
the suborbital ramus and the side of the
subocular shell of the braincase — probably
continuing directly into a similar ethmoidal
articulation with the undersurface of the
nasal capsule. The palatoquadrate of Hy-
bodus is far more massively developed es-
pecially much deeper in proportion com-
pared with that of ?Acrodus. All three
agree, however, in the absence of any basal
angle in the braincase and in the absence
of an orbital process.

As recently redescribed by Dick (1978)
and Dick and Maisey (1980), the overall pro-
portions of the palatoquadrate in /risty-
chius and Onchoselache are again quite
similar to those of ?Acrodus and Astera-
canthus, particularly in the low nature of
the postorbital ramus. The nature of the
postorbital otic articulation and basal artic-
ulation are not clear, but probably allowed
transverse movements of the palate as in
?Acrodus. An interesting feature is that
both have been restored with the anterior
part of the suborbital ramus showing a
small dorsal development that is identified
as an orbital process. However, at least in
Onchoselache, this is probably misinter-
preted and represents the relatively deep
anterior end of the palate which has
become flattened out.

Three other Meszoic sharks have been
described from material showing the skull:
Paleospinax (Woodward, 1889; Maisey,
1975); Synechodus (Woodward, 1886);
and Squalogaleus (Maisey, 1976), all of
which Maisey includes in the Paleospinaci-
dae as relatively primitive euselachians. All
three show features that allow the palato-

quadrates to be restored essentially as in

the apparently primitive living sharks
Chlamydoselachus and Heptranchias.

The postorbital ramus of the palatoquadrate
is relativley reduced and in all these forms
there is a well-developed orbital process of
the palatoquadrate. This is a dorsal projec-
tion from the upper and mesial! surfaces of
the suborbital ramus of the palate; it rises
in the orbit in front of the level marked by
the optic foramen in the orbital wall and
there is a sliding articular contact between
this orbital process and the anterior orbital
wall. The condition of the basal articulation
in these early fossil euselachians is not
available and therefore it is not possible to
tell to what extent the development of an
orbital process is correlated with the basal
articulation. In the modern Ch/amydose-
lachus, the orbital and basal articulations
are quite separate from each other, the
former being far forward in the orbit and
the latter far back in the posterior part of
the orbit. In modern Heptranchias, on the
other hand, the two articulations are essen-
tially confluent.

An orbital process is found in many lines
of modern sharks (for example, hexan-
choids, squaloids, lamnoids, carcharinoids,
and squatinoids) according to Compagno
(1977), but in several of these groups the
orbital process has become considerably
specialized, forming a major articulation
with the back of the nasal capsule rather
than a vertical flange within the anterior
orbit. Apart from the three paleospinacids
mentioned above, the orbital process is not
described with complete certainty in fossil
forms. On the basis of the limited amount of
evidence available, two possibilities exist.
First, the orbital process may be a primitive
feature for the elasmobranch fishes, presen!
in Paleozoic cladodonts, Tristychius and
Onchoselache plus modern sharks, and
present also in acanthodians (see Jarvik,
1977). In this case, the absence of the orbi- |
tal process in Hybodus, ?Acrodus, and AS Y
teracanthus is asecondary feature and
perhaps a specialization linking these threé
within the hybodonts. In this case also, the

Is Early Triassic Hybodont Shark

from Northern Madagascar

Postilla 186

absence of an orbital process in xenacanth
Sharks would be a second and independent
instance of secondary loss of this feature.
The second possibility is that the orbital
Process is incorrectly identified in Paleozoic
Cladonts (where the evidence is extremely
limited) and possibly in 7ristychius and
Onchoselache where the evidence is yet
more slight. In this case, the orbital process
Should be considered a specialization of
Certain modern sharks (Maisey, 1980) and
Its absence in the forms just mentioned,
together with xenacanths, ?Acrodus, Hy-
bodus, and Asteracanthus, would all re-
flect a primitive condition. The matter re-
quires considerable further research for cla-
rification and not least among such studies
Must be a careful examination of the rela-
tlonship between orbital and basal pro-
cesses and articulation.

Finally the structure of the palate in
?Acrodus (and to a lesser extent, Hybodus
and Asteracanthus) shows certain general
resemblances to that of modern heterodont
Sharks, particularly in the low postorbital
ramus, absence of an orbital process, well-
developed ethmoidal articulations and ab-
sence of a basal angle. In the past, this
would have been enough to allow one to
Suggest a close relationship between hybo-
donts and heterodonts. However, Com-
Pagno (1977) has recently attempted to
Show that hybodonts belong to a more de-
"Ved position within the euselachians, spe-
Cifically being allied with the galeoid oryc-
toloboids and lamnoids. If this is the case,
the absence of the orbital process in hetero-
donts might be considered a highly derived
Condition and the overall close similarity of
the palates of the two groups a conver-
Jence due perhaps to a common pattern of
fore-and-aft jaw movements. The present
inadequate state of knowledge of detailed
Nybodont anatomy prevents us from resolv-
'Ng this problem.

The result of the present study, therefore,

; to characterize part of the head in
‘Acrodus from the Early Triassic of Mada-
Yascar and to demonstrate the diversity of
Structure in early sharks. This diversity

serves to confuse rather than clarify the
phylogenetic relationships among Meso-
zoic sharks and among hybodonts, ctena-
canths, and euselachians.

An orbital process is found in many lines
of modern sharks (for example, hexan-
choids, squaloids, lamnoids, carcharinoids,
and squatinoids) according to Compagno
(1977), but in several of these groups the
orbital process has become considerably
specialized, forming a major articulation
with the back of the nasal capsule rather
than a vertical flange within the anterior
orbit. Apart from the three paleospinacids
mentioned above, the orbital process is not
described with complete certainty in fossil
forms. On the basis of the limited amount of
evidence available, two possibilities exist.
First, the orbital process may be a primitive
feature for the elasmobranch fishes, present
in Paleozoic cladodonts, 7ristychius and
Onchoselache plus modern sharks, and
present also in acanthodians (see Jarvik,
1977). In this case, the absence of the orbi-
tal process in Hybodus, ?Acrodus, and As-
teracanthus isasecondary feature and
perhaps a specialization linking these three
within the hybodonts. In this case also, the
absence of an orbital process in xenacanth
sharks would be a second and independent
instance of secondary loss of this feature.
The second possibility is that the orbital
process is incorrectly identified in Paleozoic
cladonts (where the evidence is extremely
limited) and possibly in 7ristychius and
Onchoselache where the evidence is yet
more slight. In this case, the orbital process
should be considered a specialization of
certain modern sharks (Maisey, 1980) and
its absence in the forms just mentioned,
together with xenacanths, ?Acrodus, Hy-
bodus, and Asteracanthus, would all re-
flect a primitive condition. The matter re-
quires considerable further research for cla-
rification and not least among such studies
must be a careful examination of the rela-
tionship between orbital and basal pro-
cesses and articulations.

Finally the structure of the palate in
?Acrodus (and toa lesser extent, Hybodus

14 Early Triassic Hybodont Shark Postilla
from Northern Madagascar

1

86

and Asteracanthus) shows certain general
resemblances to that of modern heterodont
sharks, particularly in the low postorbital
ramus, absence of an orbital process, well-
developed ethmoidal articulations and ab-
sence of a basal angle. In the past, this
would have been enough to allow one to
suggest a close relationship between hybo-
donts and heterodonts. However, Com-
pagno (1977) has recently attempted to
show that hybodonts belong to a more de-
rived position within the euselachians, spe-
cifically being allied with the galeoid oryc-
toloboids and lamnoids. If this is the case,
the absence of the orbital process in hetero-
donts might be considered a highly derived
condition and the overall close similarity of
the palates of the two groups a conver-
gence due perhaps to a common pattern of
fore-and-aft jaw movements. The present
inadequate state of knowledge of detailed
hybodont anatomy prevents us from resolv-
ing this problem.

The result of the present study, therefore,
is to characterize part of the head in
?Acrodus from the Early Triassic of
Madagascar and to demonstrate the di-
versity of structure in early sharks. This di-
versity serves to confuse rather than clarify
the phylogenetic relationships among
Mesozoic sharks and among hybodonts,
ctenacanths, and euselachians.

Acknowledgments

| am extremely grateful to Paul E. Olsen for
spotting this specimen in the Harvard col-
lections and for preparation of the delicate
material.| am grateful to Mr. Olsen, Amy R.
McCune and William Kohlberger for valua-
ble comments on the manuscript. This
work is part of a project on Triassic fishes
supported by the National Science Founda-
tion (grants DEB 77-08412 and DEB
79-21746). The drawings were prepared by
Linda Price Thomson and the photographs
by William K. Sacco.

15 Early Triassic Hybodont Shark Postilla 186
from Northern Madagascar

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Early Triassic Hybodont Shark
from Northern Madagascar

Postilla 186

The Author

Keith Stewart Thomson. Department of
Biology and Peabody Museum of Natural
History, Yale University, 170 Whitney
Avenue, PO Box 6666, New Haven, CT
06511.