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PEABODY MUSEUM OF NATURAL HISTORY

YALE UNIVERSITY
NEW HAVEN, CONNECTICUT, U.S.A.

_

Number 87 et December 10, 1964

THE BRAINCASE OF THE ADVANCED
MAMMAL-LIKE REPTILE BIENOTHERIUM

By

JAMES A. HOPSON

PEABODY MUSEUM OF NATURAL History, YALE UNIVERSITY

INTRODUCTION

Recent discoveries of well-preserved cranial material of Meso-
zoic mammals, previously known almost exclusively from jaws
and teeth, have yielded extremely interesting and unexpected infor-
mation on the early evolution of the mammalian braincase. K. A.
Kermack and his co-workers (D. M. Kermack et al., 1956; K. A.
Kermack and F. Mussett, 1958) have described the periotic of
the probable docodont Morganucodon which possesses an “ante-
rior lamina,” i. e. a prominent forward extension of the periotic
on the side wall of the braincase, resembling the anterior periotic
process of living monotremes. This structure forms a large part
of the wall of the monotreme braincase in place of the ascending
lamina of the alisphenoid which is characteristic of the braincase
of marsupial and placental mammals. The presence of an anterior
lamina in the periotic of Morganucodon led D. M. Kermack et al.
(1956) to postulate a relationship between it and monotremes.

Z Postilla Yale Peabody Museum No. 87

More recently, however, with the discovery of an anterior
lamina in the periotic of the Upper Jurassic triconodont Triora-
codon, K. A. Kermack (1962, 1963) has concluded that this
structure may have occurred in several probably unrelated groups
of Mesozoic mammals and therefore does not necessarily indicate
a special relationship among any forms possessing it. He suggests
that the expansion of the brain in these early mammals led to a
concomitant expansion of the anterior margin of the periotic to
form a protective cover for the brain. He believes that an anterior
lamina was present in the late therapsids Diarthrognathus and
Oligokyphus, which would indicate to him that the expansion of
the periotic had begun at a pre-mammalian stage and had pro-
gressed independently in a number of lineages which crossed the
reptile-mammal grade boundary.

The periotic has been described in only two other genera of
Triassic and Jurassic mammals. A very fragmentary periotic of
Triconodon was described by Simpson (1928) and redescribed
by Kermack (1963), but it is essentially identical with the better
preserved specimen of Trioracodon. Patterson and Olson (1961)
have referred a fairly complete but badly crushed braincase to
Sinoconodon, a triconodont-like mammal from the latest Triassic
of Yunnan, China, but the structure of the anterior part of the
periotic in this specimen cannot be determined with certainty.
Rigney (1963) has recently given a short preliminary notice of a
complete skull of Morganucodon from near the type locality of
Sinoconodon, but nothing on the braincase has yet been published.
Rigney believes that the braincase referred by Patterson and Olson
to Sinoconodon actually belongs to Morganucodon.

The discoveries of these cranial remains are extremely signifi-
cant in the amount of light they have shed on the “Dark Ages”
of mammalian history. However, their great rarity and usually
fragmentary nature introduces major difficulties in interpreting
them correctly in the absence of structural series linking them with
more completely known and better understood forms. For example,
Kermack (1962, 1963) has drawn inferences as to the morpho-
logical relations of the alisphenoid with respect to the periotic in
Mesozoic mammals, yet in only three genera of mammal-like
reptiles from above the Middle Triassic has the side wall of the
braincase been described, and in only one, the ictidosaur Diar-

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MUS. COMP ZOOL

Dec. 10, 1964 The Braincase of Bienotherium bintac
[IBRARY

thrognathus, (Crompton, 1958), is the contact of the alisphenoid 1965
with the periotic preserved. In the other two, the tritylodonts ~~~
Oligokyphus (Kihne, 1956; Crompton, 1964) and Likhoeli@Rp
(Ginsburg, 1962), both elements are incompletely presérvedy ards | Ty
their mutual contacts are unknown. I am presently studying the
Upper Triassic tritylodont Bienotherium in which the braincase

is almost completely preserved. The epipterygoid and prootic
(reptilian homologues of the alisphenoid and anterior part of the
periotic of mammals) in this genus indicate that in previous
descriptions of tritylodonts and in Kermack’s descriptions of Mor-
ganucodon and Trioracodon the mutual relations of these elements

have been incorrectly interpreted. Consequently, Kermack’s inter-
pretation of the evolutionary history of the mammalian braincase
during the Mesozoic requires some modification.

The prootic and epipterygoid of Bienotherium are described
below. Comparisons are made with the homologous elements in
cynodonts in order to indicate the ways in which the tritylodont
braincase has been modified from the well-understood Early and
Mid-Triassic therapsid pattern. New interpretations of the side
wall of the braincase in other genera of tritylodonts and in the
Mesozoic mammals described by Kermack and his co-workers are
advanced, and a revision is offered of Kermack’s (1962, 1963)
theory of the evolutionary history of the orbito-temporal region
of the skull in the earliest mammals.

ACKNOWLEDGMENTS

This paper is part of a work to be submitted to the Committee
on Paleozoology, the University of Chicago, in partial fulfillment
of the requirements for the degree of Doctor of Philosophy. I
gratefully acknowledge the guidance of Dr. E. C. Olson during the
course of this study. Thanks are due Dr. A. S. Romer of the
Museum of Comparative Zoology, Harvard University, for permis-
sion to borrow comparative material, and to Dr. A. W. Crompton
for permission to examine undescribed cynodont skulls in his pos-
session. Dr. Olson and Drs. A. W. Crompton, J. H. Ostrom.
D. A. Russell, and E. L. Simons read the manuscript; their sugges-
tions and criticisms are appreciatively acknowledged.

+ Postilla Yale Peabody Museum No. 87

MATERIALS AND METHODS

The description of the braincase of Bienotherium yunnanense
Young (1940) is based primarily upon the posterior half of an
adult skull which was sectioned serially using the method described
by Olson (1944). In addition, several fragmentary specimens of
the ear region and part of the braincase were utilized for this
description. These specimens are part of the Yunnan, China,
Fossil Collection of the Fu Jen Catholic University of Peking,
China, and are designated by the abbreviation CUP. This collec-
tion is presently on loan to the Department of the Geophysical
Sciences of the University of Chicago.

The collection of fossils of which these specimens of Bieno-
therium comprise a small part was made under the direction of
Father Edgar Oehler in 1948 near the village of Lufeng in Yunnan
Province. China. The Bienotherium material is from the lower
Lufeng Series of latest Triassic age. .

The braincase was sectioned at intervals of 1.5 mm (sections
i-14) and 0.5 mm (sections 14-71), nitrocellulose peels being
made of each section. Enlarged graphic reconstructions were made
of the specimen from several views, two of which are illustrated
here (Figs. 2 and 3). Proportions were checked against a plaster
cast of the specimen made prior to sectioning.

Comparative material includes the sections of Thrinaxodon
liorhinus described by Olson (1944) as ““Cynodont B,” a complete
skull of this species (No. 4282), which served as the basis for
Fig. 1, from the South African Museum, Cape Town, and a skull
of Belesodon neidermeyeri (No. 1533) from the Museum of
Comparative Zoology, Harvard University.

THE EPIPTERYGOID AND PROOTIC OF Thrinaxodon

Before describing the braincase of Bienotherium, it is perhaps
useful to review the morphology of the epipterygoid and prootic
of a cynodont such as the well-known Lower Triassic species
Thrinaxodon liorhinus (Figs. 1, 5A, 6A). The skull of Thri-
naxodon has been well described by Parrington (1946), and
Olson (1944) has given a detailed account of the braincase and
ear region.

The epipterygoid is greatly expanded dorsally, being in broad
contact with the parietal and just meeting the frontal. Its postero-

Dec. 10, 1964 The Braincase of Bienotherium 5

Pt-Par For

Fig. 1. Thrinaxodon liorhinus Seeley. Lateral view of braincase. Squa-
mosal cut across depression which housed the quadrate. The outline of the
anterior border of the prootic medial to the epipterygoid is indicated by the
dotted line. Abbreviations on p. 29-30 X 2.

dorsal margin is in sutural contact with the prootic (Olson, 1944).
Anteriorly, it has a broadly emarginated border which forms the
edge of the large orbital fissure (Orb Fiss, Fig. 1), the opening
through which cranial nerves II, HI, 1V, Vi, and VI left the brain-
case. Ventrally, the epipterygoid contacts the pterygoid which sends
a slender quadrate ramus back to contact the quadrate (which is
missing in Fig. 1). A posteroventral process of the epipterygoid
also extends back toward the quadrate but does not reach it (Par-
rington, 1946). Between the posterior margin of the epipterygoid
and the front of the prootic is a large foramen through which
passed the maxillary and mandibular branches of the trigeminus
nerve and a vein, probably homologous with the vena cerebralis
media of sauropsid reptiles (see Cox, 1959).

The prootic forms the wall of the braincase in front of the ear
region, contacting the squamosal and parietal dorsally and the rear
margin of the epipterygoid anteriorly. Ventrally, the prootic bears
a thin lateral sheet which supports the quadrate ramus of the
epipterygoid and which is referred to here as the “lateral flange”
(Lat Fl, Fig. 1, 6A). The anterior end of the lateral flange lies
lateral to the trigeminal foramen. Its posterior end extends back
beyond the hind margin of the epipterygoid to contact the paroc-

6 Postilla Yale Peabody Museum No. 87

cipital process so as to enclose the pterygo-paroccipital foramen
(Pt-Par For, Fig. 1).

The prootic component of the foramen for the trigeminus
nerve is somewhat complex and requires a detailed description,
based in part on the sections of “Cynodont B.” The notch in the
anterior border of the prootic is the prootic incisure. From the
anteroventral corner of this notch a process, the pila antotica (P A,
Fig. 1), extends anterodorsally. It lies medial to the epipterygoid
and is barely visible in side view. Between the pila antotica and the
medial rim of the prootic incisure on the inside and the epiptery-
goid and the lateral flange on the outside is a narrow space, the
cavum epiptericum (Cav Ep, Fig. 6A). In living reptiles this space
contains the semilunar ganglion of the trigeminus nerve (see De
Beer, 1937, p. 430). In Thrinaxodon that part of the prootic
which forms the posterior border of the prootic incisure is very
slightly hollowed, presumably for the posterior part of the semi-
lunar ganglion. This hollow, though extremely shallow, has signifi-
cance in the interpretation of Bienotherium. The paths of the three
branches of the trigeminus nerve in Thrinaxodon were undoubtedly
as in modern reptiles: the ophthalmic branch (V,) passed forward
within the cavum epiptericum and left the skull in front of the
epipterygoid; the maxillary (V2) and mandibular (V;) branches
passed out of the skull behind the epipterygoid, directly lateral
to the prootic incisure. This is illustrated in Fig. 5A.

Along the dorsal margin of the prootic and epipterygoid is a
gutter for the protection of a vein. This is the so-called “sinus
canal” (S C, Fig. 1). A prominent foramen enters the braincase
from the sinus canal above the prootic-epipterygoid contact. The
posterior end of the gutter lies at the anterior opening of the post-
temporal fossa (P-T Foss, Fig. 1) which transmitted a vein for-
ward from the occipital region (see Cox, 1959). The prootic
forms a short protective flange in front of this opening. A slight
channel leads forward from the pterygo-paroccipital foramen to
the trigeminal foramen which Watson (1920) has interpreted in
other cynodonts as being for the vena capitus lateralis (V C L,
Fig. 1);

THE EPIPTERYGOID AND PROOTIC OF Bienotherium

The only first-hand studies of the skull of Bienotherium are

Dec. 10, 1964 The Braincase of Bienotherium 7)

those of Young (1940, 1947), though Watson (1942) has con-
tributed a very important interpretation of this material based on
Young’s earlier publication. The side wall of the braincase in
Young’s two skulls is largely unpreserved, and those parts which
are present have proved difficult to interpret.

The lateral wall of the braincase (Fig. 2) is formed by the
ascending lamina of the epipterygoid and the anterior part of the
prootic, although ventrolateral wings of the frontal and parietal
form a small part of the wall dorsally. In the cerebral region the
frontal wing extends far down, medial to the epipterygoid, to a
point below the floor of the cranial cavity (which, in front of the
pituitary fossa, lies well above the base of the skull). In this region
the floor and wall of the braincase proper are formed by a well-
developed orbitosphenoid which largely excludes both the epiptery-
goid and the frontal from participating in the formation of the
cranial wall (Fig. 3).

The epipterygoid has a high, broad ascending lamina extending
well above the level of the cranial cavity to overlap the frontal and
the anteroventral edge of the parietal. Its anterior margin is broadly
incised by the orbital fissure. A slender process of the epipterygoid
passes below the orbital fissure medial to the transverse flange of
the pterygoid. The posterior border of the ascending lamina has a
continuous overlapping contact with the prootic above the trige-
minal foramen. The nature of this contact is described in greater
detail below.

The inferolateral margin of the epipterygoid anterior to its
quadrate ramus is overlapped by the pterygoid. The epipterygoid
has a well-developed sutural contact with the basipterygoid process
of the basisphenoid (Bpt, Fig. 4B). The quadrate rami diverge
behind this point at an angle of about 45 degrees from the midline.
Their posteroinferior border is bluntly rounded and forms a thick-
ened out-turned rim. The quadrate ramus is more vertically ori-
ented and much deeper than that of any cynodont. Furthermore,
it lies entirely below the level of the cranial cavity and also below
and largely medial to the paroccipital process (Fig. 4A). It does
not contact the paroccipital process nor could it have reached the
quadrate (the probable position of which is discussed below).
Its suture with the prootic is seen in section to be deeply inter-
digitating (Fig. 4A, B).

8 Postilla Yale Peabody Museum No. 87

The prootic of Bienotherium has a relatively greater exposure
on the lateral surface of the braincase than does that of cynodonts,
a fact due in large part to the presence of a thin lamina which
extends well up on the lateral surface of the parietal to partially
cover a deeply incised channel in the latter (S C, Fig. 2, 4A).
This channel, the homologue of the sinus canal of cynodonts
(Watson, 1911), is covered more extensively by a short lappet of
the parietal. In cynodonts the sinus canal usually lies on or slightly
above the prootic-parietal suture and, in advanced forms, is often

A BC D E

II)

Par

Par Pr
VCL

a —
10 MM

Fig. 2. Bienotherium yunnanense Young. Lateral view of braincase
reconstructed from serial sections. Pattern of horizontal lines indicates cuts
across squamosal and distal part of posterolateral flange. Sagittal crest
partially restored. A-E, positions of sections A-E in Fig. 4. Abbreviations
on p. 29-30.

Dec. 10, 1964 The Braincase of Bienotherium a

covered by thin laminae from these elements. It marks the course
of a vein (cf. Watson, 1920) possibly homologous with the
parietal vein of living lepidosaurs (Cox, 1959). Between the pos-
terodorsal margin of the prootic and the cranial process of the
squamosal is a foramen in the parietal (For S C, Fig. 2) which
marks the posterior terminus of the “sinus canal.”

Anteriorly, the prootic forms a thin lamina overlapping exter-
nally the entire posterior margin of the ascending process of the
epipterygoid, except where both elements are emarginated by the

fj x Saris
UAPit.- .

PG)

_ Fig. 3. Bienotherium yunnanense Young. Internal aspect of braincase
In sagittal section. Basicranial process of prootic indicated by heavy ver-
tical lines. The foramen for the vena cerebralis media lies lateral to the
basicranial process and is thus not visible in this figure. A-E, positions of
sections A-E in Fig. 4. Abbreviations on p. 29-30.

10 Postilla Yale Peabody Museum No. 87

trigeminal foramen (V2,3, Fig. 2-4). This is the reverse of the
usual reptilian condition in which the prootic lies medial to the
epipterygoid; its significance will be dealt with at greater length
below.

Below the trigeminal foramen the prootic extends ventrally as
a vertical flange in sutural contact with the quadrate ramus of the
epipterygoid. This flange is homologous with at least part of the
thin lateral sheet of the prootic of cynodonts which supports the
quadrate wing of the epipterygoid, and which has been variously
termed the “lateral lamina” (Kiihne, 1956; Crompton, 1958) or
“lateral flange” (Kermack, 1963). I shall use the latter term in
this paper in order to avoid confusion with the “anterior lamina.”
In Bienotherium this vertical flange is homologous with only the
anterior part of the lateral flange of cynodonts and so will be dis-
tinguished as the “ventrolateral flange” (V-Lat Fl) in subsequent
discussion. It is vertically rather than ventrolaterally inclined as it
is in cynodonts, and so is not clearly distinguishable from the side
wall of the braincase proper. In cross-section (Fig. 4A-C), how-
ever, it is seen to lie below the floor of the braincase. It extends
back to a point level with and medial to the anterodistal extremity
of the paroccipital process.

In tritylodonts the anterior half of the distal end of the paroc-
cipital process is turned downward to form a prominent boss
(Q Pr, Fig. 2, 4A). In Bienotherium this boss may be seen to lack
a cover of periosteal bone on its lateral surface (Fig. 4A). Young
(1947, p. 549) identified this surface as “the medial contact with
the quadrate,” but did not specify which element of the skull
forms it. Kiihne (1956) believes this process supports the hyoid,
the quadrate lying entirely posterior to it, but Crompton (1964)
has convincingly argued that it indeed does support the quadrate.
A single specimen (CUP 2268) of Bienotherium, in which this
process is extremely well preserved, suggests that both Kiihne and
Crompton are correct, for the downturned process has both a
broad antero-lateral surface which I believe certainly supported the
quadrate and a medially-directed posterior process which I believe
provided attachment for the hyoid.

Immediately in front of the paroccipital process the prootic is
drawn out into a prominent laterally-directed process which is
pierced by a single large foramen (P-Lat Fl, Fig 2, 4A). This

Dec. 10, 1964 The Braincase of Bienotherium 11

process is not preserved in its entirety in any of the available speci-
mens of Bienotherium, and some details of its structure cannot be
determined. In the sectioned skull, its lower wing, that part below

Fig. 4 Bienotherium yunnanense Young. Transverse sections across
braincase in positions indicated in Figs. 2 and 3. A, through posterior part
of depression for semilunar ganglion; B, through pila antotica and basiptery-
goid joint; C, through trigeminal notch in ventrolateral flange; D, through
posterior end of ascending lamina of epipterygoid which lies medial to
prootic; E, through anterior end of prootic. Note that the sections are
obliquely oriented to vertical axis of skull. Prootic and pterygoid indicated
in black. Abbreviations on p. 29-30.

12 Postilla Yale Peabody Museum No. 87

the foramen, has its origin on the posterior end of the ventrolateral
flange just above the suture with the quadrate ramus of the epip-
terygoid. Distally, it extends to within 2 mm of the ‘“quadrate
process” of the paroccipital process (Fig. 4A) and in undamaged
specimens may have contacted it. The upper wing of the process
is best preserved in a partial skull (CUP 2241), in which it does
contact the anterior face of the “quadrate process,” thereby com-
pletely enclosing the pterygo-paroccipital foramen.

The structure of the laterally-directed process of the prootic,
as seen in the sections, strongly suggests that it may really be a
compound structure formed by distinct dorsal and ventral moieties
contacting one another distally to enclose the large foramen. In
Oligokyphus, this process, called the “lateral lamina” by Kiuhne
(1956) and Crompton (1964), appears in the published figures
and descriptions to be a single deep lamina pierced by one large
foramen and several smaller ones (see Kiihne, 1956, p. 47, Fig.
13, and Pl. 7, Fig. 2). In an undescribed skull of Tritylodon
(K405) from the South African Museum, Cape Town, the flange
is too poorly preserved to allow a determination of its possible
compound nature to be made, but it is clearly seen to be penetrated
by two large foramina. In the available specimens of Bienotherium
only a single foramen can be distinguished with certainty, but
because of the incompleteness of the material, the definite absence
of a second large foramen cannot be demonstrated. Despite this
uncertainly, I believe that the homologies suggested below (and
considered in greater detail on p. 17) between these structures in
cynodonts and tritylodonts are essentially correct.

The laterally-directed process of the prootic in Oligokyphus
has been homologized by Kiihne (1956) with the “lateral lamina”
(i. e. lateral flange) of cynodonts. However, as noted earlier, the
ventrolateral flange of Bienotherium, which contacts the quadrate
ramus of the epipterygoid, is homologous with at least the anterior
part of the lateral flange of cynodonts. Therefore, the laterally-
directed process of tritylodonts might best be called the “postero-
lateral flange.’” I propose to use this name for both the upper and
lower portions of this process although I shall try to show later
that only the lower moiety appears to be homologous with part of
the lateral flange of cynodonts. However, until this region of the
skull of tritylodonts is better understood, I believe it preferable

Dec. 10, 1964 The Braincase of Bienotherium 13

to refrain from further complicating the terminology used to
describe it, at least for the present.

The trigeminal foramen, preserved only in the sectioned skull,
lies about 10 mm anterior to the paroccipital process and about 5
mm in front of the posterolateral flange. It is about 5 mm long by
2.5 mm high and is slightly constricted at midlength so as to have
a roughly “dumbbell’’-shaped outline. Presumably this constriction
represents the incipient subdivision of the single trigeminal foramen
of reptiles into the separate foramina ovale and rotundum of
mammals.

Some 2.5 mm behind and slightly below the trigeminal fora-
men is a second, much smaller, foramen (V Cer Med) lying com-
pletely within the prootic. It is immediately anterodorsal to the
foramen in the posterolateral flange. Comparison with cynodonts
suggests that the vena capitus lateralis (V C L) passed back through
the latter foramen to the pterygo-paroccipital foramen, and thence
to the middle ear cavity. In Sphenodon and some lizards a branch
of the lateral head vein, the vena cerebralis media, leaves the
skull through the trigeminal notch (O’Donoghue, 1920). In Bi-
enotherium this vein probably passed out of the skull through the
separate small foramen.

THE INTERNAL ASPECT OF THE BRAINCASE

Kihne (1956) and more recently Crompton (1964) have
described the internal structure of the braincase of Oligokyphus.
The cranial cavity of Bienotherium (Fig. 3) is nearly identical to
that of Oligokyphus, so only the region anterior to the internal
auditory meatus, which is largely unpreserved in the known mate-
rial of the latter genus, will be described.

About 3 mm in front of the internal auditory meatus and
slightly behind the level of the pituitary fossa is a deep hollow in
the medial surface of the prootic (S G, Fig. 3, 4A). Kiihne (1956)
has interpreted this depression in Oligokyphus as the housing for
the semilunar ganglion of the trigeminus nerve; in Bienotherium
it surely served this function. In section (Fig. 4A), it may be seen
that this hollow lies quite low in the wall of the cranial cavity and
that it is floored by a very thin lamina of prootic which extends
medially to join the deep basisphenoid-parasphenoid complex.
Below this hollow is the great extracranial space enclosed laterally

14 Postilla Yale Peabody Museum No. 87

by the ventrolateral flange of the prootic and the quadrate ramus
of the epipterygoid. Directly lateral to the semilunar depression is
the posterolateral flange of the prootic.

Anteromedial to the depression for the semilunar ganglion is
a short dorsoventrally flattened process of the prootic (P A, Fig. 3,
4B-C) which extends forward and upward. It has the same rela-
tions to the semilunar depression and the dorsum sellae as does the
pila antotica of cynodonts (Olson, 1944; Brink, 1955) though it
differs somewhat in appearance from that of Thrinaxodon (Fig.
6A). The lamina of the prootic which forms the outer wall of the
braincase in this region extends forward approximately 8 mm
beyond the level of the pila antotica, and thus conceals it from
lateral view.

The braincase anterior to the pila antotica is completely floored
by an extensive chondrocranial ossification here termed an orbito-
sphenoid (Os) though its relations are those of a compound later-
osphenoid-orbitosphenoid. Posteriorly it contacts the pila antotica
on either side of the pituitary fossa (Fig. 4B, C). A peculiarity of
this skull is the pair of processes of the prootic which meet at the
midline to form a portion of the dorsum sellae (Bcr Pr, Fig. 4B,
C). “Basicranial processes” of the prootic have been described in
gorgonopsians (Olson, 1944) but never in cynodonts. I believe
they are not indicative of a close relationship with gorgonopsians
but may merely represent a convergent structural modification
correlated with the deepening of the braincase.

Lateral to the pila antotica the depression for the semilunar
ganglion opens anteroventrally into the large subcranial space
described above (Fig. 4B). At this point, the ventrolateral flange
is pierced by the small foramen for the vena cerebralis media.
This vein in Sphenodon passes down from the transverse sinus in
the roof of the cranial cavity to leave the braincase through the
prootic incisure (O’Donoghue, 1920). In Thrinaxodon it appears
to have had a similar course, being enclosed in a special channel
between the epipterygoid and the body of the prootic laterally and
a special dorsally-directed lappet of the prootic medially (see Fig.
6A). In Bienotherium it would have left the braincase by way of
the depression for the semilunar ganglion and the foramen in the
ventrolateral flange.

The trigeminal foramen pierces the side wall of the skull on

Dec. 10, 1964 The Braincase of Bienotherium iB)

the contact of the epipterygoid and the ventrolateral flange about 8
mm anterior to the depression for the semilunar ganglion. Thus,
the maxillary and mandibular rami of nerve V traversed about 8
mm of extracranial space before reaching the lateral surface of
the braincase.

COMPARISON WITH CYNODONTS

The epipterygoid and prootic of an early cynodont, Thri-
naxodon liorhinus, have already been described. Some variation
in this region of the skull exists in more advanced cynodonts, but
the basic pattern is as in Thrinaxodon.

The lateral flange of the prootic, which occurs in cynodonts
alone among pre-Upper Triassic therapsids, is a relatively simple
structure in the well-known members of this group. In Bieno-
therium it is much more complicated and, as pointed out earlier,
can be subdivided into two distinct parts which have been given
the names “posterolateral flange” and “ventrolateral flange.” Com-
parison with Thrinaxodon suggests that the lower process of the
posterolateral flange (the upper process is discussed later) may be
homologous with the hind portion which contacts the paroccipital
process behind the quadrate wing of the epipterygoid. With the
ventromedial migration of the anterior part of the lateral flange in
tritylodonts, i. e. that part in contact with the quadrate ramus of
the epipterygoid, the more posterolateral portion of the flange
would have become progressively isolated as a conspicuous later-
ally-directed process. Its retention as a distinct entity, set well off
from the ventrolateral flange (which serves the clear function of
buttressing the epipterygoid), may have been related to the need
for a protective cover for the anterior part of the middle ear
cavity. As such it would be functionally equivalent to the partial
bullae formed by the alisphenoid in didelphid marsupials or by
the basisphenoid in some insectivores.

The progressive ventromedial migration of the anterior part
of the lateral flange can be traced from Thrinaxodon (Fig. 1, 6A),
in which it extends ventrolaterally at an angle of 45 degrees,
through an advanced cynodont such as Belesodon, in which it is
oriented at approximately 70 degrees to the horizontal, to Bieno-
therium (Fig. 2, 6B) in which it is essentially vertical. In this

16 Postilla Yale Peabody Museum No. 87

sequence the flange progressively deepens, as does the quadrate
ramus of the epipterygoid.

The ventrolateral flange and, indeed, the entire anterior margin
of the prootic of Bienotherium differ from these structures in
cynodonts not only in their greater depth, but also in their much
greater anterior extent. In cynodonts the lateral flange generally
ends at the level of the prootic incisure (this is also true of Diar-
thrognathus; Crompton, 1958), and the inner surface of the prootic
is Only slightly hollowed for the semilunar ganglion so that the
greater part of the prootic incisure is open laterally. In Bieno-
therium both the ventrolateral flange and the anterodorsal border
of the prootic extend well forward of the prootic incisure, so that
the incisure and the pila antotica are hidden from lateral view.
Therefore, the prootic component of the trigeminal foramen in
Bienotherium is not strictly homologous with the prootic incisure
of cynodonts, for the former is merely a notch in the anterior
border of the ventrolateral flange (compare Fig. 6A with Fig.
4D). What has happened in the tritylodont is that the lateral part
of the prootic, mainly the ventrolateral flange, but also that part
which in Thrinaxodon forms the outer and posterior border of
the prootic incisure, has grown forward to close off the posterior
part of the cavum epiptericum in which the semilunar ganglion
lay. The cavum epiptericum in Bienotherium lies medial to both
the epipterygoid and the anterior part of the prootic. This point
is particularly significant in the interpretation of the braincase
of Mesozoic mammals and will be returned to in the final section
of this paper.

The posterolateral flange of Bienotherium differs from that of
Lower and Middle Triassic cynodonts in that it is pierced by a
large foramen which I have interpreted as having transmitted the
vena capitus lateralis forward from the pterygo-paroccipital fora-
men. In Karroo cynodonts the side of the prootic above the lateral
flange may bear a groove which extends between the pterygo-
paroccipital foramen and the prootic incisure, and which has been
interpreted by Watson (1916, 1920) as marking the course of the
vena capitus lateralis. In Diademodon this groove may be over-
hung by a thin flange of prootic from the hinder and outer end of
which “a special process is given off which runs outwards, lying
parallel to and in front of the paroccipital process, to meet a

Dec. 10, 1964 The Braincase of Bienotherium 7

similar special process of the squamosal” (Watson, 1916, p. 343).
A similar structure is also seen in Belesodon. Were this special
process of the prootic to move forward, away from the front of the
paroccipital process, and downward to contact the distal end of
the lateral flange lateral to the channel for the head vein, its rela-
tions would be the same as those of the upper half of the postero-
lateral flange of tritylodonts. As already pointed out, this flange is
damaged in all available specimens of Bienotherium, so it is not
possible to determine whether or not it is a compound structure.
In Oligokyphus it appears not to be. However, the late Middle
Triassic cynodont Exaeretodon from the Ischigualasto Formation
of Argentina, recently described by Bonaparte (1962), has two
well-developed and apparently distinct processes above and below
the groove for the head vein. Bonaparte interprets the inferior
process as being part of the epipterygoid (“‘aliesfenoide”), but
inasmuch as sutures are not visible in this region of his specimen,
it is perhaps better to interpret it as part of the prootic; its rela-
tions are those of the lateral flange of earlier cynodonts in which
this structure invariably lies below the groove for the head vein.
If this interpretation is correct, then conditions in Exaeretodon
support the hypothesis that the posterolateral flange of tritylodonts
is a composite structure formed by: (1) the posterior part of the
lateral flange of cynodonts; and (2) the special process, developed
in late cynodonts, which lies directly in front of the paroccipital
process and above the groove for the lateral head vein.

Bienotherium differs from most cynodonts but, again, resembles
Exaeretodon in having a distinct foramen in the lateral wall of the
prootic for the middle cerebral vein. With the forward extension of
the anterior part of the prootic lateral to the prootic incisure a
separate foramen was developed in Bienotherium, probably so
that the vein might retain its direct route from the braincase.
Bonaparte has also interpreted the similar foramen in the prootic
of Exaeretodon as being for the vena capitus lateralis (i.e. its
middle cerebral branch). Possibly, then, the trigeminal notch in
the prootic of this advanced cynodont may not represent the
prootic incisure as would naturally be supposed from comparing
it with Karroo cynodonts but may instead be a notch in the ante-
rior border of the lateral flange. If such is indeed the case, this
cynodont has progressed well along the way toward a tritylodont

18 Postilla Yale Peabody Museum No. 87

braincase structure. This, then, adds another piece of evidence to
the by now well-documented theory of Watson (1942) that the
tritylodonts arose from gomphodont cynodonts. However, because
of the apparently great degree of parallelism seen in late cynodonts,
[ do not believe that this resemblance necessarily indicates a partic-
ularly close relationship between Exaeretodon and _ tritylodonts.

COMPARISON WITH OTHER TRITYLODONTS

The braincase has been described in only two other genera of
tritylodonts: Oligokyphus from a Liassic fissure fill in Somerset,
England, described by Kiihne (1956) and Crompton (1964); and
Likhoelia from the Red Beds of Basutoland, described by Ginsburg
(1961, 1962). The latter is close to and may be generically
inseparable from Tritylodon. In all specimens of both genera the
prootic is incompletely preserved and the epipterygoid is known,.
albeit imperfectly, only in Oligokyphus.

Kihne (1956, Fig. 13) and Crompton (1964, Fig. 4-6) have
illustrated the “lateral lamina” of Oligokyphus as a laterally-
directed, obliquely-oriented sheet with the more anteroventral part
of the sheet turned more horizontally and extending forward below
the trigeminal notch. Crompton (1964, Fig. 14) has restored the
epipterygoid as contacting only this horizontally-oriented ventral
part of the “lateral lamina.”” Comparison with Bienotherium indi-
cates that Crompton’s restoration is correct and that the horizontal
part of the “lateral lamina” corresponds to the ventrolateral flange
and the oblique upper part to the posterolateral flange in the
Chinese form. The ventrolateral flange of Oligokyphus thus differs
considerably from that of Bienotherium in that it extends well out-
ward from the braincase, as it also does in cynodonts, rather than
being a vertical sheet in continuity with the lower part of the brain-
case wall.

Both Kiihne and Crompton appear to have correctly identified
the pila antotica in Oligokyphus. Crompton (ibid., p. 76) has
speculated on the possibility of a trigeminal foramen completely
surrounded by the prootic although he notes that there is no con-
clusive evidence for such a reconstruction because of the incom-
plete preservation of the anterior border of the prootic in the
available specimens. He suggests, however, that the anteroventral
corner of the prootic above the trigeminal nerve may have extended

Dec. 10, 1964 The Braincase of Bienotherium 19

downwards and forwards in life towards the pila antotica to thus
enclose the nerve. In Bienotherium (Fig. 3, 4B, C) the pila
antotica lies medial to the outer wall of the braincase which is
formed by the anterodorsal part of the prootic and the ventro-
lateral flange. In order for a closed trigeminal foramen to be formed
in Oligokyphus, the anteroventral corner of the prootic above the
trigeminus nerve would have to extend downwards, lateral to the
pila antotica and the semilunar ganglion, to contact the anterome-
dial border of the “lateral lamina.” Oligokyphus may have had
such a closed trigeminal foramen, as suggested by Crompton, but
the deep notch or damaged foramen in the prootic of all known
specimens of this genus may equally well represent a venous fora-
men as occurs in Bienotherium, with the trigeminal notch having
lain further forward in that part of the prootic which is not pre-
served.

In the beautifully preserved braincase of Likhoelia, Ginsburg
(1962) has identified as part of the epipterygoid (“‘alisphénoide’”’)
a portion of the posterolateral flange which in Bienotherium is
formed entirely by the prootic. The groove or foramen which
Ginsburg has identified as the “orifice du nerf trijumeau” appears
to be the depression for the semilunar ganglion with its outer wall
broken away. On this interpretation, the ventrolateral flange is
almost entirely missing in this specimen.

THE CAVUM EPIPTERICUM OF TRITYLODONTS
AND MESOZOIC MAMMALS

The cavum epiptericum, as described by De Beer (1937, p.
430), is an extracranial space situated laterally to the side wall of
the orbitotemporal region of the skull and medially to the proces-
sus ascendens of the palatoquadrate cartilage. It lodges the ganglia
of the trigeminal and facial nerves and is traversed by their
branches. In reptiles, including cynodonts (Figs. 5A, 6A), the
cavum epiptericum is bounded medially by the pila antotica and
laterally by the epipterygoid. In marsupial and placental mammals
(Figs. 5D, 6D), according to De Beer (1937), the expanded
alisphenoid (mammalian homologue of the epipterygoid) has
incorporated the cavum into the bony skull. The pila antotica is
no longer present and the boundary between the cavum and the
cranial cavity is indicated only by the dura mater, in which, how-

20 Postilla Yale Peabody Museum No. 87

ever, may be embedded isolated nodules of cartilage, possible
remnants of the pila.

The mammalian alisphenoid has expanded back even further
than has that of cynodonts so that it surrounds the mandibular
ramus of the trigeminus, which pierces it via the foramen ovale.
The maxillary branch may pass forward medial to the alisphenoid
or it may pierce it.

In monotremes (Fig. 5C, 6C), the cavum epiptericum is
especially well defined, for its medial wall is clearly indicated by
the persisting pila antotica (taenia clino-orbitalis) (Goodrich,
1930, p. 269). The alisphenoid is a small ossification fused to
the basicranium, its place in the side wall of the skull having been
taken by a large anterior process of the periotic (processus ante-
rior perioticus; Watson, 1916). This anterior process ossifies in
the membrana spheno-obturatoria which forms the outer wall of

‘Z

Fig. 5 Orbitotemporal regions of: A, Thrinaxodon; B, Bienotherium;
C, Ornithorhynchus; D, Didelphys. Periotic indicated by oblique lines,
epipterygoid-alisphenoid by heavy outline. Exits of branches 1 to 3 of the
trigeminus nerve are indicated. (A after Parrington, 1946; C and D after
Jollie, 1962.) Not to scale.

Dec. 10, 1964 The Braincase of Bienotherium pA

the cavum. It is notched ventrally to form with the basisphenoid a
foramen pseudovale for the mandibular branch of the trigeminus
nerve. The maxillary branch leaves the skull in front of the
periotic process.

In the periotics of the Mesozoic mammals Morganucodon (see
Fig. 8) and Trioracodon, described and figured by Kermack and
Mussett (1958) and Kermack (1963), there is an anterior process
pierced by a foramen pseudovale. This process is called the ‘“‘ante-
rior lamina” by these authors. On its medial side it bears a depres-
sion which Kermack interprets, undoubtedly correctly, as having
housed the semilunar ganglion. Its ventral margin he interprets as
forming a lateral flange of the sort seen in cynodonts.

1s
as

‘iv Cer Med

Cav Ep ry CH) Y
<Sx=) =.

Ss

Fig. 6. Transverse sections across orbitotemporal regions of: A, Thri-
naxodon; B, Bienotherium; C, embryo Ornithorhynchus; D, generalized
therian mammal. Periotic indicated in black. In A and B the brain and
other soft structures are restored in broken lines. Abbreviations on p. 29.
(C modified from Watson, 1916; D modified from Goodrich, 1930.) Not
to scale.

pH Postilla Yale Peabody Museum No. 87

Kermack (1963) has given an extended discussion of the
probable evolution of the braincase in Mesozoic mammals, based
on the above-mentioned periotics and a sphenoid of Triconodon
(earlier described by Simpson, 1928), with additional evidence
provided by the braincases of late therapsids, notably Oligokyphus.

The anterior lamina of the periotic, according to Kermack’s
interpretation, is “an ossification within the wall of the neuro-
cranium itself” (p. 97) which has grown forward internal to the
alisphenoid. His explanation for this expansion is that the ali-
sphenoid in advanced therapsids “played no part in the formation
of the wall of the braincase proper, from which it is separated by
the cavum epiptericum. In this condition, should an expansion and
an extension forward of the brain occur in evolution, the corre-
sponding ossification of the braincase to give it protection could
only have been an ossification within the wall of the neurocranium
itself: in other words a forward extension of the petrosal” (p. 97).
As the semilunar ganglion comes to lie medial to the anterior
lamina, as it does in monotremes, Kermack supposes that part of
the cavum epiptericum must have been incorporated within the
expanding braincase. Thus he interprets the cavum as having been
subdivided by the formation of the anterior lamina, with part of it
enclosed within the neurocranium and part of it left outside of the
braincase proper but still medial to the alisphenoid. Kermack’s
conception, as understood here, of the braincase of a Mesozoic
mammal such as Morganucodon is illustrated in Fig. 7A.

As Oligokyphus has a depression for the semilunar ganglion on
the inner surface of its prootic, Kermack believes that the anterior
lamina began to form at the therapsid structural level. Oligokyphus
has a much wider “lateral flange” than have Morganucodon and
Trioracodon; therefore, Kermack concludes that the space between
the anterior lamina and the alisphenoid has become progressively
narrower, undoubtedly as a result of brain expansion in the mam-
mals. At some time above the Upper Jurassic the anterior lamina
would come into contact with the alisphenoid, and “the cavum
epiptericum would finally vanish” — squeezed out of existence by
the expanding brain. At this stage one or the other of the two ele-
ments participating in the skull wall would be suppressed: in the
monotreme line it would be the alisphenoid, in the therian line the
anterior lamina. This basic differentiation of the braincase in the

Dec. 10, 1964 The Braincase of Bienotherium 23

Fig. 7. Hypothetical transverse sections through the braincase of a
Mesozoic mammal. A, Kermack’s (1962, 1963) interpretation, as it is
understood by the writer, in which the cavum epiptericum lies primarily
lateral to the anterior lamina; B, the writer’s interpretation, in which the
cavum epiptericum lies medial to the anterior lamina and lateral to a
persisting pila antotica. Periotic indicated in black. Abbreviations cn p. 29.

two main higher categories of living mammals, the Prototheria and
the Theria, need not have been accomplished until after the Late
Jurassic.

Despite Kermack’s belief that the anterior lamina is an ossifi-
cation of the neurocranium, the origin of the processus anterior
perioticus of monotremes as an intramembranous ossification
strongly suggests a similar mode of origin for the anterior lamina
of both tritylodonts and the Mesozoic mammals. With this in mind,
a comparison of the periotics of Morganucodon and Trioracodon
with the prootic of Bienotherium suggests the following interpreta-

24 Postilla Yale Peabody Museum No. 87

tions of braincase structure in the Mesozoic mammals: (1) the
lateral flange (probably the “anterior part” only) supported only
a persisting quadrate ramus of the alisphenoid; (2) the ascending
lamina of the alisphenoid lay primarily rostral to the anterior
lamina, the entire anterior margin of which it probably contacted
except perhaps at the foramen for the maxillary branch of the
Vth nerve which may have lain between the two elements; and
(3) the cavum epiptericum lay entirely medial to both the anterior
lamina of the periotic (posteriorly) and the alisphenoid (ante-
riorly). Further, because of the primitive structure of the described
braincases of Triassic aand Jurassic mammals, I believe they quite
probably retained a pila antotica, as still persists in monotremes.
In Fig. 7B, is illustrated my conception of the braincase of a
Mesozoic mammal such as Morganucodon.

In Fig. 8, the periotic of Morganucodon (drawn from the
stereophotograph in Kermack, 1963) is figured with the periotic
(prootic + opisthotic) of Bienotherium, and a number of features
in the former are reinterpreted in the light of information obtained
from the latter. For a more complete discussion of the morganu-
codont periotic, see Kermack (1963).

In both genera the depression for the semilunar ganglion has a
well-developed floor formed by a thin shelf of the periotic. In

Can Pro

Ant Lam

For Pseud

4 ee

Floor

Lat Fl

Fig. 8. Left: Morganucodon. (After Kermack, 1963.) Right periotic
viewed medially and probably somewhat anterodorsally. Right: Bieno-
therium. Right periotic viewed medially and somewhat anterodorsally;
details of internal auditory meatus added from Oligokyphus; stippling
indicates areas of contact with epipterygoid. Not to scale. Abbreviations
on p. 29.

Dec. 10, 1964 The Braincase of Bienotherium po)

cynodonts the space in which the semilunar ganglion lay is open
ventrally, as is usual in reptiles. But, as noted above on p. 6,
there is in Thrinaxodon a very slight hollowing of the posterior
border of the prootic incisure, and it seems reasonable to assume
that this hollow has been deepened in Bienotherium and Morganu-
codon by a relative forward movement of the neurocranial portion
of the periotic both lateral to and below that part of the cavum
epiptericum which housed the semilunar ganglion. This would
account for the well-developed floor of this hollow and the fact
that its lateral wall in Bienotherium clearly has the appearance of
having been ossified in cartilage (see Fig. 4A), though further
forward this wall seems to be an intramembranous ossification.
The hollow has a semicircular medial border (Rim Pro Inc, Fig.
8) which corresponds to the medial border of the prootic incisure
of Thrinaxodon.

As in a cynodont, the medial border of the prootic incisure of
Bienotherium is terminated anteriorly by the pila antotica. In
Morganucodon there is a short process in the corresponding loca-
tion which may represent an ossified pila antotica (P A?., Fig. 8).

The anterior lamina and ventrolateral flange are much better
developed in Bienotherium than in Morganucodon. In the trityl-
odont they broadly overlap the outer surface of the epipterygoid,
which is a strong piece of evidence for their being intramembranous
rather than neurocranial ossifications. The thin anterior part of the
anterior lamina in Morganucodon which meets in front of the
foramen pseudovale is also most likely to be an intramembranous
ossification.

The differences in the foramina for the trigeminus nerve and
the vena cerebralis media are such as one might expect in two
forms at their respective evolutionary levels, with Morganucodon
the more advanced in a mammalian direction.

As pointed out above, the “anterior lamina” of the periotic of
monotremes is an intramembranous ossification within the mem-
brana spheno-obturatoria which lies completely lateral to the
cavum epiptericum. Goodrich (1930, p. 269) points out that the
alisphenoid in therian mammals may also be partly ossified within
the membrana spheno-obturatoria. In Didelphys, for example, the
entire ascending lamina of the alisphenoid is ossified in this mem-
brane (De Beer, 1937, p. 439). The ascending lamina of the

26 Postilla Yale Peabody Museum No. 87

epipterygoid of Bienotherium is a thin, solidly-ossified sheet of
bone which, like the anterior lamina of the prootic, may have
ossified at least partly intramembranously. If this is so, it might
have been a matter of relatively slight functional and, perhaps,
developmental significance if one element were to expand at the
expense of the other.

The history of the monotreme orbito-temporal region, as inter-
preted here, has mainly involved the progressive extension forward
of the periotic into the membrana spheno-obturatoria and the
concomitant regression in front of it of the ascending lamina of
the alisphenoid. Morganucodon and Trioracodon represent an
intermediate structural stage in which there was still an ascend-
ing lamina of the alisphenoid, but in which the mandibular ramus
of the trigeminus nerve was surrounded by the periotic. Sino-
conodon (Patterson and Olson, 1961) possesses an alisphenoid,
but the presence of an anterior lamina of the periotic cannot be
determined in the single braincase referred to this genus.

Morganucodon, if it is indeed a docodont, and the Tricono-
donta and Monotremata are all generally acknowledged to be non-
therian mammals (see Simpson, 1961), and the Tritylodontia are
reptiles which almost certainly left no mammalian descendants.
Therefore, the anterior lamina of the periotic as presently known
occurs only in lines which did not lead to the living Theria. Inas-
much as nothing is known of the cranial structure of symmetro-
donts or pantotheres, nothing can be said about the possible
presence of an anterior lamina in the ancestry of the marsupials
and placentals. I do not believe, as Kermack (1962, 1963) has
argued, that the early ancestors of the Theria more likely than
not did possess an anterior periotic lamina.

Concerning the therapsid ancestry of the different lines of early
mammals, many of which appear to have evolved independently
from a reptilian structural grade, the Cynodontia are the best can-
didates for the ancestry of those which possess an anterior periotic
lamina. The presence of a lateral flange and various protective
laminae (probably of membranous origin) overlying venous chan-
nels suggests that the prootic of cynodonts might easily have formed
an expanded intramembranously-ossified anterior lamina. Indeed,
in one descendant group, the Tritylodontia, it has actually done so.
The common possession of cheek teeth with three main longitu-

Dec. 10, 1964 The Braincase of Bienotherium pH |

dinally-oriented cusps by carnivorous cynodonts, triconodonts, and
Morganucodon lends support to this hypothesis. Ictidosaurs, which
possess a lateral flange but not an anterior lamina, and bauria-
morphs, which possess neither, would appear to be less likely
ancestors for these mammals. Further speculation, however, must
await the discovery of cranial material of symmetrodonts and
pantotheres, and also of a greater variety of Upper Triassic therap-
sids than is presently known.

SUMMARY

The braincase of the Upper Triassic tritylodontid Bienotherium
yunnanense (Therapsida; Reptilia) is characterized by: (1) the
anterior lamina, a forward extension of the anterior portion of the
prootic which overlaps externally the posterior margin of the epi-
pterygoid; (2) the ventrolateral flange, a deep vertical sheet of the
prootic which extends below the level of the braincase to contact
the quadrate ramus of the epipterygoid and which is notched
anteriorly for the exit of the second and third branches of the Vth
cranial nerve and is penetrated by a small foramen for the vena
cerebralis media; and (3) the posterolateral flange, a laterally
directed process of the prootic which closes the pterygo-paroc-
cipital foramen and is penetrated by a foramen for the vena
capitus lateralis.

The ventrolateral flange is homologous with only the anterior
part of the lateral flange of cynodonts. The posterolateral flange
appears to be a composite structure, formed by: (a) the posterior
part of the cynodont lateral flange; and (b) a special process devel-
oped in late cynodonts which lay above the channel for the vena
capitus lateralis.

The anterior lamina of the prootic of Bienotherium lies lateral
to the cavum epiptericum and is probably an intramembranous
ossification rather than an anterior extension of the neurocranium.
A similar condition is seen in the periotic of living monotremes.
The notch in the anterior margin of the prootic is, therefore, not
homologous with the prootic incisure of typical reptiles, which is a
notch in the neurocranial part of the prootic.

Kermack’s (1962, 1963) theories on the evolution of the
mammalian braincase during the Mesozoic do not conform with
what is known of the developmental anatomy of living mammals,

28 Postilla Yale Peabody Museum No. 87

and are contradicted by the structure of the braincase in Bieno-
therium. It is probable that the cavum epiptericum in Morganu-
codon and Trioracodon lay entirely medial to both the alisphenoid
and the anterior lamina of the periotic, and that both of these
elements were largely ossified intramembranously. It is also likely
that these Mesozoic mammals retained a pila antotica, as do
monotremes.

The cynodonts appear to be the best candidates for the ancestry
of those mammals which possess an anterior lamina of the periotic,
i.e. morganucodonts, triconodonts and monotremes. No evidence
exists as to the possible ancestry of the Theria.

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ABBREVIATIONS
Al Alisphenoid
Ant Lam Anterior Lamina
Ber Pr Basicranial Process of Prootic
Bo Basioccipital
Bpt Basipterygoid joint between Basisphenoid and
Epipterygoid
Bs Basisphenoid
Can Pro Prootic Canal for Vena Cerebralis Media
Cav Ep Cavum Epiptericum
Ch Chondrocranial Wall
DE Endolymphatic Duct
Eo Exoccipital
Ept Epipterygoid
Floc Floccular Fossa

Floor Floor of Depression for Semilunar Ganglion

30

For Pseud
For SC
Fr

P-T Foss
Pt-Par For
Q Pr

Rim Pro Inc

VII
VI[[ Coch
VIl[Vest
XII

Postilla Yale Peabody Museum No.

Foramen Pseudovale

Foramen of Sinus Canal

Frontal

Hypophysis

Internal Auditory Meatus

Internal Carotid Artery

Jugular Foramen

Lateral Flange

Membranous Side Wall of Braincase
Membrana Spheno-Obturatoria
Opisthotic

Orbital Fissure

Orbitosphenoid

Pila Antotica

Parietal

Paroccipital Process

Periotic

Pituitary Fossa

Posterolateral Flange

Prootic

Parasphenoid

Pterygoid

Post-Temporal Fossa
Pterygo-Paroccipital Foramen
Quadrate Process of the Paroccipital Process
Medial Rim of Prootic Incisure
Sinus Canal

Semilunar Ganglion of Trigeminus Nerve
Supraoccipital

Squamosal

Vena Cerebralis Media

Vena Capitus Lateralis

Venous Foramen

Ventrolateral Flange

Branches 1-3 of Trigeminus Nerve
Facial Nerve

Cochlear Branch of Auditory Nerve
Vestibular Branch of Auditory Nerve
Hypoglossal Nerve

87

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