<oological Journni
o/ /he
Llnnean SocieLy
, 1990), 99:
205-237. LVith 25 figures
The skull of Cteniogenys, a choristodere
(Reptilia: Archosauromorpha) from the Middle
Jurassic of Oxfordshire
SUSAN E. EVANS
Department o f Anatomy and Developmental Biology, Uniuersity College and Middlesex
School of Medicine, Rockefeller Building, University Street, London W C l E 633
Received
Jub
1989, nccrpted,for publitallon September 1989
Th e genus CteniopnyJ. was originally described as a lizard on the basis of isolated jaw fragments from
the Upper Jurassic deposits of Como Bluff, Wyoming. T h e discovery of nrw material from a Middle
Jurassic locality at Kirtlington, Oxfordshire, showed that Cteniogenys was not a lizard, hut an early
choristodere. T h e skull is represented by a collection of isolated bones, associated on the basis of fit
and dermal sculpture pattern. T he bones are here described and compared with those of thc Late
Cretaceous and I’alaeocenc choristoderes Champsosaurus and Simoedosaurus. .4llowing for thc much
smaller size of thc Middle Jurassic form, there is a close correspondenrr between thr isolatcd hones
of the three grnrra. Comparison with known choristoderes, based on an analysis of 53 dcrivcd
character states, suggests that Qeniogenjs is the most primitive of known genera. I n gencral, the
character statrs shared by all choristoderes support recent analyses which concludc that
choristoderes arc dcrivcd from archosauromorph diapsids, not lepidosauromorphs as once thought.
At Kirtlington, O’teniogenys forms part of a diverse miCr0VCrtcbrdtC assemblage including amiids,
sharks, frogs, salamandcrs, lizards, mammals, crocodiles, pterosaurs, turtlrs and small dinosaurs.
KEY LVORDS:
faunas.
Rcptilia
Choristodcra ~ ~ J u r a s s i c skull
~
~
Britain
ph?logrn)
~
mirrovertrhratr
~
CONT E NTS
Introduction .
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Material
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Description
Skull .
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Lowrr jaw
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Dentition .
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Discussion
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Ontogcnctic age of the material .
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Systematic position of Cteniogenjs .
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Phylogenetic position of the Choristodrra
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Palaeoenvironment
Acknowlrdgemrnts
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References.
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Abbreviations usrd in the text and figures .
Apprndix .
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205
206
207
207
222
224
22’
225
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432
233
234
234
236
237
I N 1RODUC 1 I O N
The Choristodera was a group of gavial-like aquatic reptiles known, until
recently, on11 from the Cretaceous and Palaeocene. By far the best known are
0024-4082/90/070205 t 33 $03 00/0
205
Q
1990 The Linneaii Societ\ ol London
206
S. E. EVANS
Champsosaurus (Cope, 1884; Brown, 1905; Parks, 1927; Erickson, 1972, 1985) and
Simoedosaurus (Gervais, 1877; Dollo, 1884; Russell-Sigogneau & Russell 1978;
Sigogneau-Russell, 1981a; Erickson, 1987) from the Upper Cretaceous and
Palaeocene of Europe and North America. T o these have been added Tchoiria
(Yefimov, 1975, 1979) and Khurendukhosaurus (Sigogneau-Russell & Efimov,
1984) from the Lower Cretaceous of Soviet Mongolia, and Zkechosaurus
(Sigogneau-Russell, 1981b) from the Middle Cretaceous of Inner Mongolia,
People’s Republic of China. The genus Pachystropheus from the Late Triassic of
Britain and Germany was originally classified as a choristodere (Huene, 1935)
but, as described, it lacks diagnostic character states and remains incertae sedis
pending re-examination.
Choristoderes are diapsids but their origins have remained obscure due, at
least in part, to their specialization and inadequate fossil record. The earliest
Cretaceous genus, Tchoiria, is already highly derived. The discovery, therefore, of
more primitive Jurassic choristoderes provides a valuable source of new data.
The genus Cteniogenys was named on the basis of a few isolated dentaries from
Upper Jurassic deposits at Como Bluff, Quarry 9, Wyoming (Gilmore, 1928)
(Fig. 1A-C). Gilmore identified the bones as lizard, an interpretation supported
by Estes (1983) and Seiffert (1973) who described new specimens from the
slightly older deposits at Guimarota in Portugal (Fig. lD, E). More recently,
however, further, more abundant, material of Cteniogenys has been recovered
from the Middle Jurassic of Britain. Many skull and postcranial elements have
been identified and it is now clear that Cteniogenys was a small early choristodere
and not a lizard (Evans, 1989).
MATERIAL
The bones were recovered from the Kirtlington Mammal Bed, a
microvertebrate locality near the base of the Forest Marble (Upper Bathonian,
Middle Jurassic), dating from about 170 million years B . P . (Freeman, 1976,
1979). Details of the collection and preparation techniques are given in Freeman
(1976, 1979) and Kermack et al. (1987). The deposit contains the dissociated
remains of a variety of fish and small tetrapods, including amphibians (frogs and
salamanders) (Evans, Milner & Mussett, 1988, 1990), mammals (Freeman,
1976, 1979; Kermack et al., 1987), ornithischian and saurischian dinosaurs,
pterosaurs, crocodiles, lizards and turtles. Most of these animals are represented
only by their more durable parts-teeth, scutes, jaws and vertebral fragments.
By contrast, a few genera (possibly those which have been least transported)
have most of their skeletal elements preserved. The majority of reptilian bones
are represented by two principal morphological types, one of which can be
assigned to a small lizard-like reptile (which will be described elsewhere), the
other to Cteniogenys (Evans, 1989). Reconstruction of individual genera from a
mixed assemblage of dissociated bones is always problematical. At Kirtlington,
identification of individual choristodere skull bones rests upon three principal
criteria: uniform pattern of skull ornamentation; careful matching of articular
facets; and the close similarity of individual bones to those of Cretaceous genera.
The postcranial material will be fully described elsewhere.
MIDDLE JURASSIC CHORISTODERE
207
A
C
Figure I . C t e n q e n p antzquus. A, B, Holotype lert dentary (USNM 6134) from Quarry 9, Como Blur,
Wyoming, in A, lateral and B, medial views. C, Paratype right dentary (USNM 16519, originally
Yale 1068) in medial view. D, E, Holotype right dentary of ‘C. reed? Seiffert (GUI.A.331 from
Guimarota, Portugal, in D, medial and E, lateral views (redrawn from Seiffert, 1973: 14, fig. 3.
DESCRIPTION
Skull
As reconstructed (Fig. 2), the skull is 3-4 cm long, and just over 1 cm wide (at
the rear). The dermal bones are ornamented with a pattern of anteroposterior
striae which become coarser and less regular with increasing size. In dorsal view
(Fig. 2A), the skull proportions are those of a primitive diapsid, with some
A
B
..__..
. .---.-
Figure 2. Ctenioge,ys, Kirtlington. Reconstruction of the skull in A, dorsal and B, palatal views
208
S. E. EVANS
elongation of the preorbital region due to the inclusion of the prefrontals into the
median series. The orbits are large and the external nares are terminal and
confluent. The temporal fenestrae appear to have been large, but without the
postparietal and squamosal processes, their shape is speculative (Fig. 2A). The
lower temporal bar was complete.
A palatal view (Fig. 2B) shows a combination of primitive and derived
characters. There is a complex palatal dentition, with retention of teeth on the
pterygoid flange but not on the parasphenoid. The choanae had been displaced
backward by the vomeromaxillary contact, and the suborbital fenestrae
remained large. Between the vomer, the palatine and the pterygoid, there was a
small additional palatal foramen. The interpterygoid vacuity was, at best,
narrow with no palatine involvement.
The skull of Cteniogenys was akinetic. Although the quadrate may have
retained a small dorsal joint cartilage, strong sutural attachments to
neighbouring bones would have held it firmly in place. Furthermore, firm
articulations between the parasphenoid and pterygoid and the opisthotic and
quadrate, coupled with reduction of the basipterygoid processes, render
metakinesis unlikely.
The elongation of the preorbital region, skull flattening and extension of the
primary palate are all indications that Cteniogenys was, like other choristoderes,
an aquatic animal. This conclusion is supported by the postcranial skeleton.
The nasals of Cteniogenys are not known but they can be reconstructed from
facets on neighbouring bones. The shape of the facets on the prefrontals suggests
that the nasals remained paired. More significantly, the absence of facets on the
premaxillae indicates that the nasals did not extend forward beyond the
maxillae. This contrasts with the condition in Cretaceous and Palaeocene genera
where a single nasal meets both premaxillae and excludes the maxillae from the
borders of the external nares.
The frontals of Cteniogenys (Fig. 3) were paired with a strong midline suture
(Fig. 3C, H ) , the bulk of the bone lying between the orbits. Anteriorly, the
frontals met the prefrontals, posteriorly, the parietals, and laterally, the
postfrontals. I n ventral view, the smooth orbital border was orientated so that
the cristae cranii met, or almost met, in the ventral midline to enclose a narrow
olfactory canal (Fig. 3G). Posteriorly, the cristae diverge slightly to expose the
cranial surface. At this level, the orbital border bears a large parietal facet
(Fig. 3B).
The frontals of Cteniogenys resemble those of later choristoderan genera in the
deep median suture, the enclosure of the olfactory tracts by the in-turned cristae,
and the broad orbital border. They differ in being shorter and in making little
contribution to the postorbital skull roof-unlike Champsosaurus and Simoedosaurus
where much of the frontal lies behind the orbit. The frontals of Cteniogenys and
Champsosaurus, but not Simoedosaurus, bear a separate postfrontal facet.
The parietals of Cteniogenys (Fig. 4) are paired and lack a parietal foramen. The
anterior region is broad and bears dorsal and ventral frontal facets. Posteriorly,
the bone narrows but only one specimen (BMNH R.11379) shows the base of the
postparietal process (Fig. 4H, I ) . The specimens vary in size. Juvenile bones
(Fig.4J,K) are unsculptured with a weak midline suture and a small lateral
wing bearing an elongate postorbital facet. In ventral view (Fig. 4K), a low
descending crest borders the arched cranial surface. In more mature specimens,
MIDDLE JURASSIC CHORIS'IODERE
Pr
I
cr c
G
cr c
Pf
Figure 3 . C / m q e r ~ y . \ , Kirtlington. Frontal. A-D, Posterior section of ii right I r m t a l
(BMNH R.1 1732' in A, dorsal, B. ventral, C, medial and D, lateral, virus. E-H. Anterior srction of
a right frontal (BMNH K . l 1737) in E, dorsal, F, ventral. C;, anterior cross-~ectional,and H. mrdial
virus. Scale liars = I mm.
P
Figure 4. C ' l e n z r p q u , Kirtlington. Parietal. A-D. left parietal ( B M S H K . I17341 i n 4, dorsal,
B, ventral, C. lateral and D, medial views. E, F, left parietal (BMNH R.11738) in E, dorsal a n d
F, ventral \.irwF. G I , Posterior part of a right parirtal (BMNH K . l 1739) in G, dorsal. H,
ventrolateral and I, latrral views. ,J-K, Left juvenile parietal iBMNH R.1 17401 i l l , ] , dorsal and K.
ventral views. Scale bars= I mm.
209
210
S. E. EVANS
A
V
\
Pr
F
\-. - .
\
Pm
I
\
Figure 5 Cteniogenzs, Kirtlington. Upper jaw. A-C, Right premaxilla (BMNH R.11730) in A,
ventral, B, dorsal and C, lateral views, D, E, Juvenile left prernaxilla (BMNH R.11741) in D, lateral
and E, medial views. F, Partial reconstruction of a left maxilla in medial view. Scale bars= 1 mm.
the sculpturing is elaborate and creates a ridge which separates the dorsal
surface from a well-developed lateral wing. The median suture is deeper.
Ventrally, the descending crest is strong and expands posteriorly as the cranial
surface levels out.
The parietals of Cteniogenys, Champsosaurus and Simoedosaurus are generally
similar. I n Cteniogenys and Champsosaurus, the bulk of the postorbital facet is borne
on an expanded lateral wing. I n Simoedosaurus, where the postorbitofrontal has its
major contact with the frontal, the lateral wing is reduced.
The premaxillae of Cteniogenys (Fig. 5A-E) were paired, meeting in a short,
straight suture to form a narrow anterior rostrum. A small dorsal process indents
the anterior margin of the external nares, but the nares remained confluent.
Their dorsal margins diverge from the anterior midline and are smooth. There
are no nasal facets, but each premaxilla has a strong posterior suture with the
maxilla. I n ventral view (Fig. 5A), the alveolar border bears four to six conical
teeth in round shallow pits; the second tooth is usually the largest. The palatal
shelves met anteriorly in the midline and bear facets for the vomers. I n lateral
view (Fig. 5C, D), the premaxilla is low with a slight anterior expansion and a
double line of sensory foramina.
The premaxillae of Cteniogenys resemble those of other choristoderes in shape,
tooth implantation, and the presence of confluent external nares. However, both
Champsosaurus and Simoedosaurus lack any trace of a dorsal process, and the dorsal
premaxillary margins carry nasal facets.
There are no complete maxillae in the Kirtlington collection but available
fragments (Fig. 6) are sufficiently numerous (80 specimens) to permit a partial
reconstruction (Fig. 5F). The maxilla is long and low, with 25-30 tooth positions
and an inrolled dorsal edge. The lateral surface is ornamented with a pattern of
fine anteroposterior striae and sensory foramina. Anteriorly, the bone had an
+
MIDDLE JURASSIC CH O RISTO D ERE
A
B
211
E
N
Pr
P
V
ch
Pl
nld
D
Figure 6 . C'teniugenys, Kirtlington. Maxilla. A, B, Anterior fragment of a left maxilla
(BMNH R. 1 1742) in A, lateral and B, anterior views. C, Partial right maxilla (BMNH R. 1 1743; in
medial view. D, Posterior process of a left maxilla (BMNH R.11744) in medial virw. E, Juvenile
right maxilla iBMNH R . 1 1745) in medial view F, G , Left maxilla (BMNH R. I 1 736) in E'. mcdial
and G, dorsolateral views. Scale bar= 1 mm.
interdigitating articulation with the premaxilla. From there, it expands into a
low facial plate, tapering posteriorly into the long orbital process. This process
bears a slender dorsolateral facet for the jugal, preceded by a shorter lacrimal
facet (Fig. 6 G ) . In medial view (Figs 5F, 6), the dorsal nasal facet is followed by
a broader prefrontal facet, while the alveolar border has facets for the
premaxilla, vomer, palatine and ectopterygoid respectively. A gap between the
vomerine and palatine facets marks the position of the internal nares. A large
foramen above the palatine facet opened into the superior alveolar canal and
carried the maxillary nerve and accompanying blood vessels.
The maxilla of Cteniogenys differs from that of other choristoderes in being
relatively shorter and less inrolled. O n the medial surface, the structure of the
choana is similar, but the maxilla does not normally enter the choanal margin in
Champsosaurus and Simoedosaurus. I n these genera, a nasolacrimal groove runs
anteriorly across the maxilla to join the choanal groove as it enters the choana; a
few specimens of Cteniogenys (e.g. R. 1 1736, Fig. 6F), show this feature.
Theprefontal (Fig. 7) is a triradiate bone with short frontal, nasal and palatine
processes. The dorsal surface is sculptured and limited laterally by a diverging
posteroanterior ridge. The straight medial edge bears a deep suture for the
opposite prefrontal-the
two bones thus separating the frontals from the nasals.
Laterally, there are facets for the lacrimal and, anteriorly, for the maxilla. The
medial surface ((Fig.7C), is excavated by paired concavities separated by a
curved ridge. Seen from behind (Fig. 7D), this ridge, the medial edge of the
palatine process, forms the margin of a channel leading into the nasal cavity.
Thus the posterior prefrontal concavity is the end of the olfactory canal
(continued forward from the frontal), while the anterior concavity houses part of
the nasal chamber.
212
S. E. EVANS
D
H
I
E
PI
I
F
Figure 7. Cteniogenys, Kirtlington. Prefrontal. A-D, Right prefrontal (BMNH R. 11733) in A, dorsal,
B, lateral, C, medial and D, anterior views. E, Posterior region of a right prefrontal
(BMNH R . 11746) in dorsal view. Scale b a r = 1 mm.
The prefrontal of Cteniogenys is shorter than that of other choristoderes, and has
a longer palatal process, reflecting the deeper skull.
A small lacrimal (Fig. 8) lies between the maxilla, prefrontal and palatine in
the anterior wall of the orbit. It is wedge-shaped, with a smooth dorsolateral
surface and a ventromedial surface which bears lateral facets for the maxilla and
palatine, and a more medial facet for the prefrontal-the
four bones being
closely interlocked. The orbital surface is perforated by one or two foramina
which lead into a short nasolacrimal canal.
Pr
nld
Mx
nI
C
-
Figure 8. Ctenzogeny, Kirtlington. Lacrimal. A-D, Left lacrimal (BMNH R. 1 1747) in A, lateral,
B, medial, C, ventral and D, posterior views. E, Left lacrimal (BMNH R. 11748) in posterior view.
Scale bar= 1 mm.
MIDDLE JURASSIC CHORISTODKRE
A
I3
P
213
C
Figure 9. C ' t e n q o y s , Kirtlington. A , B, partial rrconstruction of thc postorbital in A, larrral and
B, medial views; C, left postfrontal (BMNH R.11749) in dorsal view. 11-F. Right .juga1
(BMNH R . l 1735) in D, lateral, E. dorsal and F, mrdial views. Scale bars= I mnr.
The lacrimal of Cteniogenys resembles that of Simoedosaurus in enclosing the
nasolacrimal canal; in Champsosaurus, the duct ran through a canal between the
lacrimal, maxilla, palatine, prefrontal and, more posteriorly, the jugal.
In Cteniogenys and Champsosaurus, the postfrontal and postorbital bones are
separate, but in Simoedosaurus, there is a single, composite, postorbitofrontal. In
Champsosaurus, one of the two bones, the smaller, enters the orbital margin
between the frontal and jugal; the other is excluded from the orbital margin and
meets the parietal medially and the squamosal posteriorly, its long posterior
process contributing to the margins of both the upper and lower temporal
fenestrae. Brown (1905) and Russell (1956) term the smaller, anterior, bone, the
postorbital, and the more posterior one, the postfrontal. Romer (1956) and
Erickson (1972) reverse the nomenclature. With the exception of the jugal
contact, the relations of the two bones, by comparison with more generalized
diapsids-including
Cteniogenys, support the interpretation of Romer and
Erickson. Their terminology is used here.
The postfrontal of Cteniogenys (Fig. 9C) is a small, triradiate bone. Its straight
medial border is extended into anterior and posterior processes and bears a
narrow frontal facet. A small lateral process forms the apex of the bone. Between
it and the anterior process, there is the smooth orbital margin. The lateral
process fits into a recess in the postorbital while the posterior process overlies the
postorbital/parietal junction. The ventral postorbital facet is therefore extensive
and emerges onto the dorsolateral surface where the postfrontal slotted into the
postorbi tal.
214
S. E. EVANS
Only the more robust orbital parts of the postorbital have been recovered
(Fig. 9A,B). The lateral surface is convex and ornamented; the medial surface
concave and essentially smooth. Dorsally, there is a lateral postfrontal facet and
a medial parietal facet. Ventrally, the orbital border is drawn into a short
process bearing articular surfaces for the jugal and the ectopterygoid.
The postorbitofrontal complex of Cteniogenys differs from that of Simoedosaurus
in retaining two discrete elements, and from Champsosaurus in having both
elements enter the orbital rim. In all three genera, the postorbitofrontal complex
met the ectopterygoid behind the jugal, entered the borders of both upper and
lower temporal fenestrae and has a thick, orbital section of comparable shape.
The jugal of Cteniogenys (Fig. 9ITF) is triradiate. The anterior process bears a
narrow ventromedial facet for the maxilla. The posterior process is
mediolaterally flattened and meets the quadratojugal in a long medial facet. A
large, triangular ectopterygoid facet lies at the junction of the two processes
(Fig. 9F). The postorbital meets the dorsal process laterally, but extends
medially to overlap the ectopterygoid, another example of the close fit between
skull bones.
Choristodere jugals share a number of common features, notably the long
maxillary and quadratojugal processes, the contact between the postorbital and
ectopterygoid, and the absence of a squamosal facets. The jugals of Cteniogenys
and Champsosaurus retain a triradiate shape, although the dorsal process is
reduced in comparison to more generalized diapsids. In Simoedosaurus, the bone is
biradiate, with long anterior and posterior processes but virtually no dorsal
process. The postorbital facet in this genus is almost horizontal.
The squamosal of Cteniogenys is incompletely known. The main body, as
A
B
C
Figure 10. Cteniogetys, Kirtlington. Squamosal. A, B, Juvenile right squamosal (BMNH R.11750)in
A, lateral and B, medial views. C, Left squamosal (BMNH R.11751) in lateral view. D-F, Fragment
of left squarnosal (BMNH R . 11752) in D, lateral, E, dorsal and F, posteromedial views. Scale
bar= 1 mm.
215
MIDDLE JURASSIC CHORISTODERE
preserved (Fig. lo), is a triangular plate with anterolateral and anteromedial
postorbital facets. Although traces of emargination are seen in some specimens
(e.g. R.11751, Fig. lOC), the precise shape of the anteroventral border, the size
of the postorbital facet and the shape of the quadratojugal process cannot be
reconstructed. The dorsal border of the squamosal is strengthened medially by a
broad ridge which thickens as it curves to form the posterior border of the upper
temporal fenestra. In life, this border would have continued anteromedially as
the parietal process. The ventrolateral border of the squamosal slopes anteriorly
and is extended into a broad medial flange (Fig. 10F). This flange contributed to
the occipital surface of the skull, and bears a medial pit and groove for the
quadrate.
The squamosal of Cteniogenys resembles that of Champsosaurus and Simoedosaurus.
In all three genera, a curved posteromedial process extended the upper temporal
fenestra backward, beyond the level of the craniomandibular joint. However,
elongation of the the parietal, squamosal and postorbital processes results in a
greater expansion of the upper temporal fenestra in both Champsosaurus and
Simoedosaurus.
The quadratojugal has not been identified. Facets on the jugal and quadrate
suggest the bone was slender and that the quadratojugal/quadrate contact was a
simple overlap, in contrast to the condyloid joint seen in Champsosaurus and
Simoedosaurus.
The quadrate of Cteniogenys (Fig. 11) is triangular, with a narrow dorsal apex
capped by a small cephalic condyle. There is no lateral conch. T h e mandibular
condyle is wide, the articular surface forming a narrow continuous strip with
medial and lateral expansions, of which the latter is slightly larger. The cephalic
condyle has a pitted surface, suggesting that a remnant of joint cartilage
H
Figure 1 1 . Ctemogenjs, Kirtlington. Quadrate. A-D, Right quadrate lBMNH R. I 1753) in
A, posterolateral, B, medial, C, lateral and D, anterior views. E-G, Dorsal rcgion of a right
quadrate (BMNH R. 11754) in E, posteromedial, F, medial and G, anterior views. H, Dorsal ~-tgion
of a juvenile right quadrate (BMNH R.11755) in medial view. Scale bars= Imm.
216
S. E. EVANS
remained, although the surrounding facets make it unlikely that any movement
could have occurred.
The lateral border of the quadrate bears a small, flat quadratojugal facet and
a long, narrow squamosal facet which extends up to the apex and continues onto
both the medial and anterior surfaces of the bone. Since the quadratojugal facet
lies deeper than the squamosal facet, it is probable that the squamosal
overlapped the quadratojugal and approached the inferior margin of the cheek.
The medial border of the quadrate bears a series of interlinked facets
(Fig. 1 lB, E-H). A large, ventral pterygoid facet extends about half way up the
medial edge. Above it, separated by a ridge, there is a second facet, behind
which is a raised area bearing a third facet. These features are separated from
the dorsal squamosal facet by a groove. I n a generalized diapsid, the pterygoid
facet occupies the entire medial edge, but comparison with Charnpsosaurus and
Sirnoedosaurus suggests that the raised facet is for the tip of the paroccipital process
(Op.ft), while the more anterior facet is probably for the bone described as the
neomorph (Nm.ft, see below).
The quadrates of Cteniogenys, Charnpsosaurus and Sirnoedosaurus all bear a
reduced cephalic condyle with a pitted articular surface; a short, broad
mandibular condyle, lateral facets for the quadratojugal and squamosal, and
medial facets for the squamosal, opisthotic, pterygoid and neomorph. The
quadrates of both Cteniogenys and Charnpsosaurus have a narrow apex and a
relatively small contact between the opisthotic and quadrate. However, the
quadrate of Cteniogenys differs from that of both Cretaceous genera in the weak
development of the quadratojugal process (differentiated and convex in
Charnpsosaurus and Simoedosaurus j , the lack of development of the pterygoid
process (enlarged in the more recent genera, particularly Charnpsosaurus), the
larger pterygoid facet and the more dorsal position of the opisthotic facet. This
latter feature suggests that the paroccipital process was horizontal in Cteniogenys,
rather than downwardly deflected (as in both Charnpsosaurus and
Sirnoed0saurus)-an interpretation supported by the exoccipital (see below).
In Charnpsosaurus and Sirnoedosaurus, the neornorph lies between the parietal, the
squamosal, the quadrate and the opisthotic. Russell-Sigogneau & Russell ( 1978)
discuss various interpretations of this element, the most likely being that it is
derived from the pterygoid. I n many reptiles, the quadrate wing of the pterygoid
is extensive, meeting the full height of the medial wing of the quadrate. Although
the main body of the quadrate has an endochondral origin, the medial wing is
an ossification in the same membranes as the quadrate wing of the pterygoid,
despite the sutural separation. Reptilian groups differ in the extent of the
quadrate and pterygoid components of this plate and in the pattern of sutures
between them. From its position and relations, the choristoderan neomorph
appears to correspond to the upper part of this pterygoid/quadrate plate, now
separated from the remainder by a suture.
The neomorph of Cteniogenys has not been identified. Its presence is indicated
by the anterodorsal facet on the quadrate (see above and Fig. l l ) , clearly
separated from that of the pterygoid, but on the same vertical plane. I n the
absence of the relevant parts of the squamosal and parietal, little can be said
about the shape of the bone. The pattern of facets on the quadrate suggests that
the neomorph of Cteniogenys more closely resembled that of Charnpsosaurus. I n
Simoedosaurus, there is a n extensive opisthotic facet. In Charnpsosaurus and
-
MIDDLE JURASSIC CHORIS'I'ODERE
B
217
f rnx
f
93
L
92
sof
91
MX
Figure 12. C f r m p g u , Kirtlington. Palatinr. 4,Right palatiric ( B M N H R . l 1756 i n palatal \ic\v.
B, Right palatine ( B M N H R.11757) in palatal view. C-D, Left palatine (BMXH R.11758 in
C, dorsal and 1). lateral vitws. Sealt b a r = 1 mm.
Cteniogenys, however, the larger facet is for the neomorph which, at least in
Champsosaurus, bears most of the opisthotic facet-only the tip of the paroccipital
process meeting the quadrate at the raised facet.
In the absence of the quadrate process of the pterygoids and the squamosal/
parietal junction, no comment can be made on the presence or absence of
epiptevgoids, supratemporals or tabulars.
The uomer has not been recovered, but facets on the maxilla and palatine
permit a partial reconstruction (Fig. 2B). T h e bone was broader and shorter
than those of Cretaceous genera, but, like them, met the maxilla laterally,
extending the primary palate and pushing the choanal opening towards the back
of the mouth.
The palatine of Cteniogenys (Fig. 12) is triradiate, with maxillary, vomerine and
pterygoid processes. Between the vomerine and maxillary processes, the bone
entered the margin of the choana; between the maxillary and pterygoid
processes, it formed the anterior border of the suborbital fenestra; and medially,
at the junction of the vomerine and pterygoid processes, it entered the margin of
a small palatal foramen.
The palatal surface is smooth (Fig. 12A,B) except for a broad anterior
choanal groove and a shallower groove leading from the palatal foramen. There
is a single row of small teeth, a feature not affected by the size of the specimen. In
dorsal view (Fig. 12C) the bone is more complex. Centrally, it bears a large,
irregular prefrontal facet which is flanked posteromedially by the narrow
lacrimal facet. This bridges a deep groove (g.I:i which extends from the margin of
the suborbital foramen to the centre of the maxillary process. The canal so
formed corresponds in position to the canal for the maxillary nerve, and
accompanying vessels, in generalized diapsids. A smaller groove (g.2) crosses the
back of the palatine at an angle of almost 90" to the first, to enter the cavity
218
S. E. EVANS
below the lacrimal facet. This may have carried an anastomosing branch of the
facial nerve, as in modern reptiles. A third groove (g.3) begins a t the palatal
foramen and runs under the prefrontal facet to join the main cavity. I t clearly
carried a relatively large structure, or group of structures-probably vascular.
The lateral face of the maxillary process is L-shaped in cross-section and
carries a large maxillary facet. Anteriorly, a dorsally directed foramen provides
the exit for the maxillary nerve. Leading from the foramen anteriorly, there is a
faint groove (g.4, Fig. 12D) which probably carried a small neurovascular
bundle.
The palatine of Cteniogenys corresponds to that of Champsosaurus and
Simoedosaurus in the presence of both prefrontal and lacrimal facets, and of a
small palatal foramen, although this feature has been missed in many
reconstructions. It differs in having a single, rather than multiple, tooth row.
The width of the row varies with size in Champsosaurus, with a double row in a
juvenile specimen (PMA 79-8-67) about twice the size of Cteniogenys, but six or
more rows in the adult (e.g. the holotype of C. laramiensis, AMNH 982).
Only the palatal portion of the pterygoid (Fig. 13A-E) has been recovered. It is
roughly triangular, with a stout medial border and a lateral flange bearing facets
for the ectopterygoid. The anterior tip is not preserved, but facets on the palatine
show that the pterygoid met the vomer and entered the medial border of the
palatal foramen. I n small specimens, a single or double row of teeth runs along
the medial border, with a single tooth row on the pterygoid flange. I n larger
specimens, the medial row thickens to form a battery four to six teeth wide while
A
B
r
Mx
L
Mx
Figure 13. Ctenzogenys, Kirtlington. Pterygoids and ectopterygoids. A, Left pterygoid
(BMNH R.11759) in palatal view. B-D, Juvenile left pterygoid (BMNH R. 1 1 760) in B, palatal,
C, dorsal and D, medial views. E, Juvenile left pterygoid (BMNHR.11761) in palatal view.
F, G, Left ectopterygoid (BMNH R. 11762) in F, lateral and G, dorsal views. Scale bars= I mm.
219
MIDDLE JURASSIC CHORIS'I'ODERE
the pterygoid flange row doubles. Dorsally, the pterygoid bears a large palatine
facet (Fig. 12C).
The absence of facets along medial edge suggests that the pterygoids were not
fused in the midline. At the junction of the palatal plate and quadrate process,
the sphenoid facet is divided into two parts by a notch and groove. The notch
corresponds in position to the reduced basipterygoid process, but there can have
been no significant movement at the basipterygoid joint.
The pterygoids of Cteniogenys, Champsosaurus and Simoedosaurus are generally
similar in their shape and relations, and in the development of tooth batteries on
the palatal plate. In Champsosaurus, the tooth battery is relatively narrow
(although there is interspecific variation) and laterally placed. In Simoedosaurus,
by contrast, the pterygoid bears two batteries, each up to fifteen teeth wide,
separated by a narrow groove. Unlike Cteniogenys, Champsosaurus and Simoedosaurus
have a strong interpterygoid suture, while Champsosaurus is characterized by a
much longer pterygoid/sphenoid suture.
The ectopterygoid (Fig. 13F, G ) of Cteniogenys has a lateral head, a short neck
and a horizontally orientated pterygoid plate. The lateral head is covered by a
jugal facet, with a small apical postorbital facet. A narrow facet for the maxilla
runs along its ventral margin. The broad medial plate bears facets for the
pterygoid flange.
The ectopterygoid of Cteniogenys is most similar to that of Champsosaurus. I n
Simoedosaurus, there is no neck and the pterygoid plate is much shorter.
The sphenoid of Cteniogenys (Fig. 14, 15) is a complex bone, with dorsal, ventral,
anterior and lateral surfaces. I n dorsal view (Fig. 14B), the main body
corresponds to the basisphenoid ossification. I t is concave, with fine grooves for
dural blood vessels. O n each side, deeper grooves lead into the paired abducent
canals which pierce the dorsum sella. Posteriorly, the raised lateral borders bear
facets for the prootics. Anteriorly, the borders are smooth and entered the
ventral margin of the incisura prootica (exit of the trigeminal nerve). They end in
pitted surfaces which carried the unossified pilae antoticae. The main body ends
A
I
6
.
tc
/
Pro
I
Figure 14. Ctenzogenys, Kirtlington. Sphenoid. Reconstruction of the sphenoid, based on srveral
specimens, in A, ventral and B, dorsal views. Scale bar= I mm.
S. E. EVANS
220
A
re
n
n
t.c'
-
bt.pr
Figure 15. Cteniogenys, Kirtlington. Sphenoid. A-C, Sphenoid (BMNH R.11763) in A, left lateral,
B, anterior and C, ventral views. Scale bar= 1 mm.
abruptly posteriorly in a pitted transverse surface where an unossified remnant of
the basal plate filled the gap between the basisphenoid and basioccipital. The
region is underlain by part of the parasphenoid plate.
The hypophysial region lies in front of the dorsum sella, below the level of the
cranial surface. I n dorsal view (Fig. 14B), it is seen as a narrow fossa flanked by a
pair of rounded ridges-the ossified bases of the trabeculae cranii. Immediately
lateral to these are grooves for the lateral head veins. Seen in anterior view
(Fig. 14B), the hypophysial region may contain two recesses, one above the
other. The upper one, not always present, is a blind-ending pocket at the base of
the dorsum sella. Lateral to it, the walls of the dorsum sella contain the openings of
the abducent canals, the emerging nerves supplying eye muscles originating from
this region. The pituitary fossa lies at a lower level, between the trabeculae and
above the anterior part of the parasphenoid plate. Paired internal carotid canals
open into its rear.
A lateral view (Fig. 15A) shows steep sides, prootic facets and the smooth edge
of the incisura prootica. Each of the abducent foramina opens into a depression
delimited above by a shallow ridge and below by the venous groove. A ridge
separates the groove and, more anteriorly, the hypophysial region, from the
ventrolateral surface. Here the reduced basipterygoid process is flanked by
pterygoid facets. The tip of the process is pitted and may have retained a
remnant of articular cartilage. Towards the back of the sphenoid, there is a
lateral notch which opens ventrally into the Vidian groove. A similar structure
exists in Simoedosaurus (Russell-Sigogneau & Russell, 1978, Fig. 21B), but it lies
further forward and forms a closed channel.
Ventrally, (Figs 14A, 15C), the raised parasphenoid plate is flanked by a pair
of deep Vidian (or parabasal) grooves which carried the internal carotid arteries
and the palatine branches of the facial nerve. At the level of the basipterygoid
tubercles, paired canals carried the arterial trunks upwards to the h ypophysial
region, but, by comparison with living reptiles, palatine branches would have
MIDDLE .JURASSIC CHORISTODERE
221
continued forward with the nerve. The region is often damaged, but some
specimens (e.g. R.11763, Fig. 15C:1show the parasphenoid roofing the Vidian
canal.
The sphenoids of Cteniogenys, Champsosaurus and Simoedosaurus share many
features, notably: the entry of the sphenoid into the margin of the incisura prootica,
with thepilae antoticae, if ossified, part of the sphenoid rather than the prootic; the
reduction of the basipterygoid processes; the presence of pterygoid facets, with
loss of metakinesis; the abducent canals opening laterally into grooves, rather
than pockets, for eye muscle origin; the Vidian canals covered for at least part of
their length; and the variable presence of an additional fossa below or between
the pilae antoticae. The sphenoid of Cteniogenys differs, however, from that of
Simoedosaurus in that the pilae antoticae remain unossified and are laterally placed
(Ossified and medially placed in Simoedosaurus) . In Cteniogenys, the trabeculae cranii
remain separate and retain their primitive relationship to the hypophysial fossa.
By contrast, the trabeculae are fused in Simoedosaurus, and lie dorsal to the fossa.
Reference to Fox (1968: 102, fig. 4) suggests that the condition in Champsosaurus
lay closest to that of Cteniogenys. Ventrally, however, the sphenoid of
Champsosaurus differs markedly in its shape. The exposed parasphenoid is wide
posteriorly, narrowing sharply into a rostrum which bears a long pterygoid
facet. The basipterygoid tubercles are not visible in this view, and there is no
corresponding notch in the pterygoid.
The basioccipital (Fig. 16) of Cteniogenys is stout and triangular, the occipital
condyle forming a posterior apex. Posteroventrally (Fig. 16B), the broad base
bears a large parasphenoid facet and is extended laterally into basal tubera. The
anterodorsal surface shows age variation. In a mature bone (Fig. 16A), i t bears a
A
c0.c
B
D
SP
tb
I
i
Figure 16. CtenzogenyJ, Kirtlington. Occiput. A-C, Basiocciptal ( B M N H R. 11764) in A, dorsal,
B, ventral, and C, posterior views. D, Juvenile basioccipital (BMNH R. 1 1765) in dorsal view.
E-G, Right exoccipital (BMNH R. 11766) in E, posterolateral, F, anteromedial and G, anterolatcral
views. Scale bars= 1 mm.
222
S. E. EVANS
series of paired facets and concavities. The oblong exoccipital facets are large
and diverge from the posterior midline where they are separated only by a
narrow ridge. This ridge continues forwards between paired oval concavities
which floor the medullary region. Laterally, these cavities are flanked by
articular surfaces for the opisthotics and, beyond them, by a second pair of
concavities which are enclosed on three sides by the opisthotic sutures. By
comparison with living reptiles in which the otic region has been reduced (e.g.
chamaeleons), these lateral cavities floored the cochlear cavities. This last feature
is not seen in immature specimens (Fig. 16D) where the opisthotic facet appears
solid.
The basioccipital of Cteniogenys bears a general resemblance to that of
Champsosaurus and Simoedosaurus. No specimen of Simoedosaurus shows a lateral otic
cavity, but this can occur in Champsosaurus (e.g. T M P P180-16-1043). The
basioccipitals of Champsosaurus and Cteniogenys are shallower than that of
Simoedosaurus, with the basal tubera laterally, rather than ventrally, directed.
The exoccipital (Fig. 16E-G) of Cteniogenys has an expanded base and a slender
dorsal ramus. The base bears a ventral basioccipital facet and a rounded
occipital tubercle. The anterior opisthotic facet is divided into dorsal and ventral
parts by the vagus notch, while the body of the bone is pierced by hypoglossal
foramina-two or three medially and two laterally. The apex of the exoccipital is
slender and met the opisthotic (and perhaps also the supraoccipital) .
In Simoedosaurus and Champsosaurus, where the tip of the paroccipital process is
deflected downwards, the dorsal part of the exoccipital has a similar orientation.
In Cteniogenys, as inferred from the quadrate, the paroccipital process was more
horizontally oriented, and the exoccipital was upright.
Lower j a w
The lower jaw of generalized reptiles is made up of six bones: dentary,
splenial, coronoid, angular, surangular and articular. Of these, only t h r e e t h e
dentary, coronoid and surangular, have been identified in Cteniogenys.
The dentary of Cteniogenys (Figs 17, 18) is long and slender with about thirty
tooth positions. The symphysial surface is elongated by comparison with more
generalized reptiles. I n larger specimens, it lies both above and below the
Meckelian fossa (Fig. 17B), but in small specimens, it is weaker and confined to
the dorsal border (Fig. 17C). More posteriorly, facets for the splenial are clearly
visible, particularly on the thick subdental ridge (e.g. Fig. 17D). They show that
the splenial was excluded from the symphysis, leaving the anterior part of the
Meckelian fossa open. The splenial facets end towards the rear of the tooth row.
Here, the subdental ridge narrows abruptly into a shelf separating the dorsal
coronoid facet from a ventral prearticular facet. Posteriorly, a horizontal angular
facet lies in the floor of the Meckelian fossa while the entrance of the inferior
alveolar canal lies in its lateral wall.
The external surface of the dentary is either smooth or finely striated. Its most
consistent feature is a double line of sensory nerve foramina, each of which opens
into a short groove (Figs 17B, F, 18B).
The dentary of Cteniogenys resembles that of Champsosaurus and Simoedosaurus in
its shape; the presence of an elongated medial symphysis divided by the
MIDDLE JURASSIC C H O R I S T O D E R E
223
n
H
I
C
h
Figure 17. Ctenzogenys, Kirtlington. Dentary. A, B, Partial right dentary (BMNH R . 1 1728, in
A, medial and B, lateral views. C, Posterior fragment of a left dentary (BMNH R . I 1729) in medial
view. D-F, Symphysial region of a right dentary (BMNH R . 1 1725) in D , orclusal, E, medial and
F, lateral views. Srale b a r = 1 mm.
Meckelian fossa; and the double row of lateral nerve foramina (although these
are not always obvious in Champsosaurus) . However, Champsosaurus is
characterized by a much longer symphysis, which incorporates the splenial,
while the dentary of Simoedosaurus has an expanded anterior tip.
R
U
Figure 18. Ctenzogenys. Kirtlington. Dentary. A, Juvenile right dentary (BMNH R . I I731
[EF 76: 1 :3:6] in medial view. B, Symphysial region of a left dentary (BMNH R . I 1726) in rnrdial
view. C, S)mphysial region of a juvenile left dentary (BMNH R.11727) in medial view.
D, E, Juvenile left dentary (BMNH R11724) in D, medial and E lateral views. Scale bars= 1 mm.
8 ,
S. E. EVANS
224
C
L
Pa
D
G
Pa
Figure 19. Cteniogenys, Kirtlington. Postdentary bones. A, B, Left coronoid (BMNH R.11767) in
A, medial and B, lateral views. C, D, Right coronoid (BMNH R.11768) in C, medial and D, lateral
views. E, F, Left surangular (BMNH R. 1 1769) in E, medial and F, lateral views. G, H, Juvenile left
surangular in G, dorsal and H, medial views. Scale bar= 1 mm.
The coronoid of Cteniogeys (Fig. 19A-D) is a long, shallow bone. As preserved,
it consists of anterior and posterior processes (flattened and vertically oriented)
separated by a low coronoid prominence. Posteriorly, a small free margin marks
the anterior border of the adductor fossa. The medial surface is smooth, the
lateral surface bears anterior and posterior facets, for the dentary and surangular
respectively, and a small ventral facet which is probably for the prearticular.
The coronoid is similar in all choristoderan genera.
The surangular of Cteniogenys (Fig. 19E-H) is robust. The broad ventrolateral
surface (Fig. 19F) shows the pattern of grooved foramina seen on the dentary.
The ventral margin bears a large angular facet and, in at least one specimen
(R.1 1769, Fig. 19F), there is another, more anterior, facet which is probably for
the dentary. I n dorsomedial view (Fig. 19E), the surangular is divided into
anterior, adductor, and posterior, articular, regions by a sharp crest. The dorsal
border of the adductor compartment is thick and rounded; the ventral border
bears a facet for the prearticular. Two nerve foramina open into the back of the
adductor fossa. Posteriorly, a pitted concavity received the articular and,
laterally, contributed to the joint surface for the quadrate.
The surangular is similar in all choristoderes, although the bone is
proportionally longer in Champsosaurus.
Dentition
The teeth in the maxilla, premaxilla and dentary are similar, varying only
slightly in their proportions. Each tooth is conical, with a striated tip and smooth
MIDDLE ,JURASSIC CHORISTODERF,
225
Figure 20. Qtniopenys, Kirtlington. Scanning electron micrographs of the dentition. A, Dentar) tooth
(BMNH R.l I728), lingual view. B, Maxillary tooth (BMNH R.11778), lingual virw. Magnification
x 40.
base (Fig. 201. The striations are most obvious on the lingual side. Tooth
implantation is subthecodont. The teeth are actively replaced, the new tooth
eroding a pit in the lingual face of the old tooth (e.g. Figs 17A, 18D). The new
tooth is initially weakly attached (as shown by empty tooth positions), becoming
firmly implanted when mature (Edmund, 1960). Several specimens show tiny
unankylosed teeth lying in larger tooth shells.
Tooth implantation and replacement in Cteniogenys are identical to those of
Champsosaurus and Simoedosaurus, although the rounded, rather than tranversely
oval, sockets are closer to the condition in Champsosaurus. Tooth shape, however,
is strikingly different. The adult teeth of Champsosaurus and Simoedosaurus closely
resemble those of some crocodiles-attenuated
cones with sharp anterior and
posterior keels. The short broad striated cones of Cteniogenys, however, are similar
to the palatal teeth of the adult Champsosaurus and to the marginal teeth of
juveniles (Fig. 21). This may be the primitive choristoderan tooth form.
DISCUSSION
Ontogenetic age of the material
Cteniogenys, as represented at Kirtlington, is a very small animal compared to
described choristoderes from the Cretaceous and Palaeocene. This poses the
C
Figure 2 1. Champsosaurus sp., Juvenile dentition, Judith River Formation, Alberta, Canada.
A, B, Dentar); (TMP 75- I 1 -50) in A, medial and B, lateral views. C, Dentary tooth (PC 84-167-25)
in lateral virw. Scale bar = 1 mm.
226
S. E. EVANS
question as to whether the Kirtlington material might be juvenile. Age
determination in terrestrial animals relies on a combination of size and degree of
ossification (e.g. Brinkman, 1988), particularly with respect to the ends of the
long bones, the neurocentral sutures of the vertebrae and the development of the
tarsus and carpus. I n an aquatic animal, however, there is the added problem of
reduced ossification due to habitus. I n Champsosaurus, the presacral and anterior
caudal neurocentral sutures remain open into adult life, and the degree of
development of the limbs may be related to the amount of time spent on land
(Erickson, 1972), with more terrestrial species-such as C. ambulator, showing the
greatest development. However, Champsosaurus is represented by both juveniles
and adults and provides a basis for discussion of the Kirtlington material.
( 1 ) In the skull of Cteniogenys, most bones show age-related variation. Small
parietals (e.g. R. 1 1740, Fig. 4J, K ) are smooth with weakly developed lateral
flanges and no crests. Larger specimens (e.g. R. 1 1738, Fig. 4E, F) are heavily
sculptured, with crests and expanded flanges. Similar differences are found
between the juvenile and adult skull bones of Champsosaurus.
(2) Amongst the vertebrae assigned to Cteniogenys, there is also variation.
Figure 22 shows three cervical vertebrae. The smallest, R. 11773 (Fig. 22F-H),
retains a wide notochordal canal, has open sutures for the neural arch, and a
short dorsal ridge. I n the intermediate specimen, R.11772 (Fig. 22C-E), the
notochordal canal has closed, leaving small pits, and the dorsal ridge is longer
and better developed. The neurocentral sutures remain open, as they do in the
majority of specimens. R . 1 1771 (Fig. 22A, B) is the largest specimen. Its
notochordal canal is fully closed, the ventral keel is strong, the rib facets are welldeveloped and the neurocentral sutures are, unusually, closed.
A
I3
E
G
H
Figure 22. Cteniogenys, Kirtlington. Cervical vertebrae. A, B, BMNH R. 1 177 1 in A, left lateral and
B, posterior views. C-E, BMNH R.11772 in C, left lateral, D, posterior and E, dorsal views.
F-H, BMNH R.11773 in F, left lateral, G , posterior and H, dorsal views. Scale bar= 1 mm.
227
AMMIDDLEJURASSIC CHORIST'ODERE
(3) One of Erickson's (1972) illustrated examples of ontogenetic variation in
Champsosaurus involved the distal end of the humerus. The major differences
recorded by Erickson between a juvenile humerus and that of the adult, apart
from size, are: the length and depth of the ectepicondylar sulcus, the degree of
development of the radial and ulnar condyles, the degree of separation of the
supinator process from the distal humeral head (also mentioned by Brinkman,
1988), the development of the entepicondyle and its articular surface and the
development of the entotuberosity-a swelling between the radial and ulnar
condyles which tends to straighten out the distal head and reduce the waisting
when seen end on.
A range of humeri are available for Cteniogenys (Fig. 23), with the degree of
ossification, using the above criteria, generally correlating with size. In the
smallest specimen, R. 1 1774 (Fig. 23A, E ) , the ectepicondylar groove is short and
shallow, the surface of the supinator process is confluent with that of the distal
head and the radial and ulnar condyles, though convex, are only weakly
developed. In R.1 I775 (Fig. 23B, F), the ectepicondylar groove has deepened
and the condyles are larger. In R.11776 (Fig. 23C, G ) , the condyles are more
fully developed, the supinator process is separate from the distal articular surface
and there is some development of an entotuberosity between the radial and
ulnar condyles, such that the distal end of the bone is nearly straight. These
trends are taken further in R.11777 (Fig. 23D, H), with the extension of the
articular facet up the side of the entepicondyle. R . 1 1776 and R. 1 1777 seem to be
equivalent to Erickson's (1972) sub-adult and adult stages in Champsosaurus,even
though they are less than one-tenth of the size.
(4)There is a small size overlap between the largest available vertebrae of
.A.
I
L
Figurc 23. Ctenqenys, Kirtlington. Humerus. A-D, Distal humeral fragmcnts in dorsal view, A. left
humerus (BMNH R.11774). B, left humerus (BMNH R.11775), C, left humerus (BMNH R.117761,
and D, right humerus (BMNH R.11777). E-H, the same fragments in distal view. Scale
bars= 1 mm.
228
S. E. EVANS
A
B
C
D
E
Figure 24. Choristoderan skulls in dorsal view (not to scale). A, Clenzogevs. B, Tchozrza namsarai
(redrawn from Yefimov 1975). C, Champsosaurus gigas (redrawn from Erickson, 1972).
D, Simoedosaurus lemoine (redrawn from Russell-Sigogneau & Russell, 1978). E, Szmoedosaurus
dakotenszs (redrawn from Erickson, 1987).
Cteniogenys and the smallest available vertebrae of Champsosaurus ( T M P
collection), but those of Cteniogenys are much better ossified and, like the humeri,
are comparable in their development to Champsosaurus bones of far greater size.
I n conclusion, the available skull and postcranial material indicates that, at
Kirtlington, Cteniogenys is represented by animals of more than one age class. The
largest specimens are well-ossified, and are probably-but not certainly-adult.
Systematic position of Cteniogenys
Comparison of Cteniogenys with other known diapsids shows an overall
resemblance to choristoderes, currently represented by a small group of genera
from the Cretaceous and Palaeocene of Europe, North America and Eastern
Asia. Of these, two, Champsosaurus and Simoedosaurus, are known from a large
number of specimens. Direct observations have been supplemented by data from
the literature to construct a set of 5 3 derived character states, using the early
diapsid Araeoscelidia (Reisz, Berman & Scott, 1984) as an outgroup.
(1) External nares paired, separated by nasal process of premaxilla, 0; nasal
process reduced or absent, nares confluent, 1.
(2) Prefrontals lie in anterolateral orbital margins, separated by frontals, 0;
prefrontals meet in the midline, 1.
(3) Large pineal foramen present, 0; absent, 1.
(4)Preorbital skull moderately short and rounded, 0; elongated, but less than
50% of the total skull length, 1; more than 50% of the total skull length, 2.
(5) Dorsal process of maxilla vertical, 0; inrolled medially, 1.
( 6 ) Lacrimal meets maxilla ventrally, 0; meets maxilla and palatine, 1.
(7)Postorbital meets jugal ventrally, 0; extends medial to the jugal to meet a
process from the ectopterygoid, 1. N.B. characters 5, 6 and 7, are all related to
the overall flattening of the skull.
(8) Vomer meets premaxilla anteriorly and palatine posteriorly, laterally the
vomer enters the border of the choana, 0; vomer meets maxilla laterally,
extending the primary hard palate and displacing the choana posteriorly, 1.
(9) Vomer, palatine and pterygoid meet at a closed three point suture, 0; a
MIDDLE JURASSIC CHOKISTODEKE
229
small palatal foramen separates the three bones, 1. N.B. Published figures of
C‘hampsosaurus and Simoedosaurus do not show this feature, but examination of the
original material provides confirmation. The condition is not known in Tchoiria
and Ikechosaurus.
(10) Pterygoid and ectopterygoid meet to form a ventrally directed flange, 0;
have a horizontal overlap, 1.
(1 1) Basipterygoid processes are received into cotyles in the medial surface of
the pterygoids as part of a metakinetic joint, 0; process and cotyle are reduced
and the two bones are tightly sutured, 1.
(12) Broad pterygoid flange meets medial quadrate wing but remains free of
skull roof, 0; an additional ossification, neomorph, exists between pterygoid,
quadrate and skull roof, 1.
(13) Occipital condyle broad and kidney-shaped, 0; hemispherical, 1. N.B.
Yefimov’s (1978) figures of Tchoiria appear to show the primitive condition.
(14) Paroccipital process of opisthotic meets tip of quadrate, 0; is sutured to
the pterygoid and the pterygoid wing of the quadrate, 1 .
(15) Dentary symphysis small and terminal, 0; longer and medial, but
splenial not included, 1; very elongated, with inclusion of splenial, 2.
(16) Little or no development of anterior odontoid prominance, 0; atlas
pleurocentrum has an inverted ‘L’ shape in lateral view, 1. N.B. The retention of
a large free atlas pleurocentrum is a primitive feature of choristoderes.
( 1 7) Neurocentral sutures close in adult, 0; remain open in adult, 1. N.B. This
is a common condition in aquatic animals.
(18) Sacral and caudal ribs are fused to the vertebra in the adult, 0; remain
free, 1. N.B. Also an aquatic feature.
(19) Caudal zygapophyses lie at a small angle to the horizontal, 0; are nearly,
or completely vertical, 1. N.B. This condition is also found in the posterior
caudals of crocodiles.
(20) Caudal vertebrae bear shallow ventral groove for caudal blood vessels, 0;
groove is flanked by deep ventral flanges, 1.
(21) Small adult size, 0; large adult size, 1. N.B. The polarity of this character
within the Choristodera remains speculative. Cteniogenys could be primitive or
paedomorphic.
(22) Nasals paired, 0; fused, 1.
(23) Nasals do not meet premaxillae, 0; nasal meets premaxillae laterally to
exclude the maxillae from the margins of the external nares, 1.
(24) Premaxillae bear a small, dorsal process, 0; lack any trace of a dorsal
process, 1.
(25) Large, laterally directed orbits, 0; small, dorsally directed orbits, 1.
(26) Postfrontal and postorbital discrete, both enter orbital margin, 0;
discrete, but postorbital excluded from orbital margin, 1; postfrontal and
postorbital fused, 2.
(27) Squamosal with a short, broad anterior process, 0; long slender process,
1.
(28) Quadratojugal of similar height throughout, 0; widest posteriorly, 1,
(29) Quadratojugal has a simple overlapping facet for the quadrate, 0; bears
a cotyle which meets a rounded quadrate process, 1.
(30) Quadrate has a broad pterygoid wing and a large pterygoid facet, 0;
bears a slender pterygoid process with a reduced articular facet below that for
the neomorph, 1.
230
S. E. EVANS
( 3 1) Relatively small, oval upper temporal fenestra, 0; long, posteriorly flared
fenestra due to elongation of the postparietal processes, 1 .
(32) Single narrow pterygoid tooth battery, 0; broad, paired tooth batteries,
separated by a groove, 1; palatal detention reduced, 2.
(33) Pterygoids separate or just touching in the midline, 0; midline suture, 1.
N.B. I n primitive diapsids, the pterygoids are separated by a wide interpterygoid
vacuity. Reconstruction suggests that the pterygoids of Cteniogenys either met or,
at least, approached one another in the midline.
(34) Paroccipital processes horizontal, or slightly depressed, 0; deflected
ventrally, 1. N.B. The condition in Cteniogenys is reconstructed from the position
of the facets on the quadrate.
(35) Supraoccipital free, 0; sutured to parietal, 1 . This may also apply to
Cteniogenys. The Kirtlington material includes a set of 16 supraoccipitals bearing
parietal facets. In the absence of suitable parietal specimens, however,
association of the supraoccipitals remains speculative.
(36) Presacral vertebral centra longer than wide, 0; short and spool-like, 1 .
N.B. The centra of Chamjsosaurus are slightly longer than those of Simoedosaurus,
but the overall shape is the same. The presacrals of the enigmatic
Khurendukhosaurus are comparable with those of Cteniogenys, but somewhat longer.
Comparison with other diapsids suggests this is the primitive condition.
(37) Small interpterygoid foramen closed anteriorly and posteriorly by
pterygoids, 1; no posterior closure, 0.
(38) Small, but distinct, pterygoid notch remains for the basipterygoid
process, 0; absent, 1.
(39) Short sphenoid/pterygoid sutures, 0; long sutures, 1.
(40) Parasphenoid only moderately expanded posteriorly, with a short
anterior rostrum, 0; parasphenoid has a large posterior expansion and a long
narrow anterior spur, 1 .
(41) Internarial present (Erickson 1972), 1; absent, 0.
(42) Lacrimal perforated by one or two foramina, 0; none present, lacrimal
duct passes between lacrimal, prefrontal, maxilla, jugal and palatine, 1 .
(43) Tooth sockets circular, 0; transverse, 1.
(44) Anterior maxillary teeth large, 0; smaller than those following, 1.
(45) Maxillary shelf narrow, 0; expanded anteriorly, 1.
(46) Jugal bears a short dorsal process, 0; little or no process, 1.
(47) Quadrate tapers dorsally, 0; has broader dorsal surface, 1 .
(48) Supraoccipital lightly arched, sutural surfaces for parietal placed
anteriorly, 0; supraoccipital keeled, parietal facets extend to posterior margin, 1.
(49) Parietal has a broad contact with the postorbital/postfrontal complex, 0;
reduced contact, 1 .
(50) Pila antotica remain unossified, 0; ossify as part of the sphenoid, 1. N.B. In
terms of diapsids in general, incorporation of the jila antotica into the sphenoid
appears to be primitive. It occurs in captorhinomorphs and primitive diapsids.
In ‘higher’ diapsids it either forms part of the prootic or a discrete laterosphenoid
(Evans, 1986).
(51) Dentary tapers anteriorly, 0; has a swollen tip, 1.
(52) Anterior dorsal vertebrae are keeled like the cervicals, 0; no keels, 1.
(53) Small spinous processes below the presacral postzygapophyses bear
accessory facets, 1; processes absent, 0.
MIDDLE JURASSIC CHORISTODERE
I n
3
v,
3
L
v,
x
C
01
m
.-0
c
aJ
3
L
m
7
In
0
v,
n
E
m
+
1
U
U
231
-.
I/)
.-m
L
.-
0
1
U
I-
m
v,
0
f
U
aJ
Y
3
L
3
m
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73
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0
E
._
ul
Figure 25. Cladogram showing hypotheses of relationships amongst choristoderes, based on the
characters discussed in the text and the data matrix presented in the Appendix. Key to the
diagnostic information for the nodes and terminal taxa (all characters refer to state 1 unless
otherwise stated): Node 1 (Choristoderaj, 1--3, 4[1 or21, 5-14, 15[1 or21, 16-19; Node 2 (unnamed
group), 21-31. 32[1 or21, 33-36; Node 3 (Champsosauridae), 4[2], 15[2], 37-40; Nodc 4
(Sirnoedosauridae),43-45; Cteniogenys,20; Champsosaurus, 26, 4 1 4 2 ; Ikechosaurus, 32121; Simoedosaurus,
26[2], 46-53.
The appendix summarizes the distribution of these character states amongst the
six described choristoderan genera and Fig. 25 presents a cladogram constructed
from the results of the analysis.
Khurendukhosaurus has been excluded from Fig. 25 because its position remains
indeterminate.
Although detailed comparison showed many similarities between the bones of
Cteniogenys and Champsosaurus (see descriptive section), the majority of these
resemblances seemed to result from the retention of primitive character states
and a similar aquatic lifestyle (Sigogneau-Russell, 1981a ) . Champsosaurus,
Simoedosaurus and (for those characters preserved) Tchoiria share a suite of 16
derived character states (2 1-36) which are not developed in Cleniogenys.
However, it must be acknowledged that a number of these character states
(21,22,25,26,29,31-33) could be size/age related, and they are used on the
assumption, qualified above, that the larger specimens of Cteniogenys are adult.
Champsosaurus and Tchoiria share at least six derived character states
(4[2], 15[2],37--40) which can be used to diagnose the family Champsosauridae,
while Simoedosaurus possesses at least nine autapomorphies to distinguish it from
champsosaurids. The placing of Zkechosaurus with Simoedosaurus is provisional,
based on the few character states available (43-45).
Excluding Khurendukhosaurus, which remains incertae sedis, Cteniogenys is the most
232
S. E. EVANS
primitive of known choristoderes and provides a basal form for comparison with
other diapsids.
Phylogenetic position of the Choristodera
Romer (1956), in his classification of reptiles, placed the Choristodera within
the Lepidosauria as part of his paraphyletic, plesiomorphous Eosuchia.
Sigogneau-Russell & Russell (1978) left the question open, pending further
work, while Evans (1988) concluded that the group were either
archosauromorphs or descendants of an early diapsid offshoot. Erickson ( 1972,
1985) initially followed Romer, but in a more recent paper (1987), he placed the
Choristodera within the Archosauromorpha-a position also adopted by Currie
(1981), and by Gauthier, Kluge & Rowe (1988) in the latest analysis of amniote
relationships.
In recent years, several workers (Gaffney, 1980; Gauthier, 1984, 1986;
Gauthier, Kluge & Rowe, 1988; Gauthier, Estes & de Queiroz, 1988; Benton,
1985; Evans 1988) have applied cladistic methodology to the problem of diapsid
relationships. While small differences exist, there is an overall consensus, with
most authors recognizing two major subdivisions of the Diapsida-the
early
Araeoscelidia (Petrolacosaurus, Araeoscelis) and the Neodiapsida (Benton, 1985; the
Sauria of Gau thier, 1984, 1986).
The Choristodera share most of the diagnostic neodiapsid character states;
where they differ, e.g. in the retention of a complex palatal dentition or the
absence of a retroarticular process, the condition may be secondary (as a result of
feeding specializations or skull flattening).
As currently conceived (Gauthier, 1984; Gauthier et al. 1988a, b; Evans, 1984,
1988; Benton,
1985), Neodiapsida comprises two major clades:
Archosauromorpha and Lepidosauromorpha (Gauthier, 1984). Choristoderes
show no evidence of lepidosauromorph affinity (Evans, 1988; Gauthier, Estes &
de Queiroz, 1988), but the most recent analysis (Gauthier, Kluge & Rowe,
1988) strengthens the case for inclusion within the Archosauromorpha. The
group Archosauromorpha (Gauthier, 1984) encompasses Rhynchosauria,
Prolacertiformes, Archosauria and a number of isolated genera, such as
‘Trilophosaurus. They share the following suite of derived character states
(Gauthier, 1984, 1986; Gauthier et al., 1988; Benton, 1985).
Premaxilla with enlarged dorsolateral process; maxilla usually excluded
from border of external naris.
Nares elongate and close to midline.
Transverse processes of vertebrae project.
Tall quadrate.
quamosal/quadratojugal posterior margins bowed (otic notch).
Posterior process of jugal extends almost to the back of the skull.
Tall quadratojugal with reduced anterior process.
Metatarsal 4 elongated, forming 40% or more of digit.
Concave/convex joint between astragalus and calcaneum (Brinkman,
1981).
Entepicondylar foramen absent.
No cleithrum.
No foramen between ulnare and intermedium.
11IDDLE JURASSIC CHOKIS'I ODEKE
233
(m) Premaxilla large, forms most of tip of snout.
(n) Paroccipital processes directed posteriorly.
(0) Craniomandibular joint lies posterior to occiput.
(p) Vertebrae not notochordal.
(4) Pineal foramen reduced or absent.
(r) Loss of tabulars.
(s) Presence of lateral calcaneal tuber.
( t ) Metatarsal 5 hooked in one plane.
( u ) Loss/fusion of distal tarsal 5.
(v) Tibia/astragalar joint convex (tibia)/concave (astragalus).
If Cteniogenys represents the primitive choristoderan condition, then early forms
lacked character states a-j, even though character states a and b are found in the
Cretaceous genera. The absence of character states d-i might be secondary, as a
result of skull flattening, postorbital expansion and aquatic lifestyle. An
entepicondylar foramen (j)is reported in Tchoiria (Yefimov, 1979), but is not
found in any other genus. Character states k and 1 are not known in Cteniogenys,
although present in later genera, and states m-v are shared by choristoderes and
archosauromorphs. Within the Archosauromorpha, Gauthier et al. ( 1988) place
choristoderes as the sister group of Prolacertiformes (Protorosauria) and
Archosauria, but the evidence for this is less convincing. Of the five
synapomorphies used to support this hypothesis, one-squamosal
broadly
separated from the ventral margin of the skull, is removed by Cteniogenys and
another, the absence of the pineal foramen, is said to be variable in Prolacerta
(Gow, 1975). This leaves three characters of which one, nasals longer than
frontals, is a character state also found in the lepidosauromorph Youngina (Gow,
1975) and another, ploughshare-like cervical ribs running parallel to the
vertebrae, is not known in Cteniogenys. The third, pyriform recess extending to the
anterior palatine level, is problematical. Pyriform recess is a term used as an
alternative to interpterygoid vacuity (e.g. Estes et al., 1988), a feature virtually
non-existent in Cretaceous and Tertiary choristoderes due to the interpterygoid
suture.
However, prolacertiforms, rhynchosaurs and archosaurs share a number of
derived braincase character states not found in choristoderes (Gow, 1975; Evans,
1986) including: presence of a club-tipped ventral ramus of the opisthotic; tall
dorsum sella with abducens foramina lying between the prootic and sphenoid;
prootics extend medially and have a transverse suture with the dorsum sella;
exoccipitals do not meet the opisthotics ventrally (as they do in
captorhinomorphs and some younginiforms); pila antotica well-ossified as part of
the prootic (part of the sphenoid in captorhinomorphs). In addition,
rhynchosaurs share with prolacertiforms and archosaurs the archosauromorph
character states a-v. Current evidence seems therefore to suggest that
choristoderes branched early from the main archosauromorph stem and were an
independent lineage throughout the Mesozoic and at least part of the Permian.
Palaeoenvironment
The Kirtlington Mammal Bed (3p layer of McKerrow et al., 1969) overlies the
Coral Epithyris Limestone (30 of McKerrow et al.) which, according to Cope et al.
S. E. EVANS
234
(1980) forms the top of the White Limestone sequence. The Mammal Bed thus
lies at the base of the Forest Marble (Upper Bathonian).
At the time of deposition (approximately 170 myrs B . P . ) , Kirtlington lay off
the north-west coast of the Anglo-Belgian landmass (Sellwood, 1978; Cope et al.,
1980). Palmer & Jenkins (1975) and Palmer (1979) place Kirtlington on or near
the south-west shore of a small island barrier some 30 km from the coast of the
Anglo-Belgian landmass. Although the 3p layer is not specifically discussed, the
presence of lignite, charophytes and freshwater ostracods and gastropods in
marly sediments is said to characterize shallow swampy coastal regions with
creeks, lagoons and fresh water lakes (Palmer, 1979). According to Briden
(personal communication to McKerrow et al.), this region of Oxfordshire lay at a
subtropical latitude (30"N). Palmer & Jenkins (1975), Sellwood (1978) and
Palmer (1979) liken the palaeoenvironment to that of the Florida Everglades
which lie a t a similar latitude. The vertebrate fauna of the Kirtlington Mammal
Bed with its amphibians (Evans, Milner & Mussett, 1988, 1990) and aquatic
reptiles
(choristoderes, crocodiles
and
turtles)
supports such a
palaeoenvironmental interpretation. Terrestrial elements are rarer, and are
largely represented by jaw fragments and teeth. These components may have
been carried in from localities further inland.
ACKNOWLEDGEMENTS
Thanks are due to Dr Denise Sigogneau-Russell; Professor Max Hecht; Dr
Richard Estes; and staff of the Tyrrell Museum of Palaeontology, Drumheller
and American Museum of Natural History, New York, for access to specimens of
choristoderes. Dr 0. Rieppel, Dr Sigogneau-Russell and Professor Hecht have
read earlier versions of this text, and I have benefitted from their comments.
This work was partly supported by a grant from the Central Research Fund,
London University. Mrs Barbara Morrissey typed the manuscript and M r Peter
Rowles took the SEM photographs.
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~
S. E. EVANS
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SIGOGNEAU-RUSSELL, D., 1981b. Prtsence d'un nouveau Champsosauridi: dans le Crttace suptrieur de
Chine. Comptes Rendus Acadknie des Sciences, Paris, 292: 1-4.
SIGOGNEAU-RUSSELL, D. & EFIMOV, M., 1984. Un Choristodera (Eosuchia?) insolite du Crttact
Inftrieur de Mongolie. Palaontologisches Zeitschnft, 58: 279-294.
YEFIMOV, M. B., 1975. Khampsozaurid iz nizhnego mela mongolii. [Champsosaur from the Lower
Cretaceous of Mongolia.] In N. N. Kramarenko (Ed.), Zskopayemya fauna ;flora Mongolii [Fossil fauna and
flora of Mongolia], Soumestnaya Souetsko-Mongol'skaya paleontologicheskaya ekspeditsiya, 2: 84-93. [Transactions
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[ Tchoiria (Champsosauridae) from the Lower Cretaceous of Khamaril-Khural, Mongolia.] In R. Barsbold
(Ed.), Fauna Mezozoya i Kaynozoya Mongolii [Mezozoic and Cenozoic fauna of Mongolia], Sounestnaya
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ABBREVIATIONS USED IN THE TEXT AND FIGURES
Institutional abbreviations: AMNH, American Museum of Natural History, New York; BMNH, British
Museum (Natural History), London; GUI, Guimarota collections, Freie Universitat, Berlin; TMP, PC,
PCA, Tyrrell Museum of Palaeontology, Drumheller, Canada; USNM, Museum of Natural History,
Smithsonian Institution, Washington, D.C.; EF, Specimens donated by Mr E. Freeman and retaining his
specimen numbers.
A angular facet
ac - anterior cavity
a.pr - anterior process
bt.pr - basipterygoid process
b.tb - basal tubercle
C - coronoid facet
ch - choana
c o x - cochlear cavity
cr - crest
cr.c - crista cranii
D - dentary facet
E - ectopterygoid facet
E.pr ectopterygoid process
ed - intact edge
Eo - exoccipital facet
F frontal facet
f.mx - foramen for maxillary nerve
fs - fossa for articular
G -groove (see text)
iac - opening of inferior alveolar canal
ica - opening for internal carotid artery
J - jugal facet
L - lacrimal facet
Ihv - groove for lateral head vein
Mx - maxillary facet
n - notch
N - nasal facet
nld - groove for nasolacrimal duct
Nm - neomorph facet
~
~
~
oc - olfactory channel
O p - opisthotic facet
o.pr - orbital process
P - parietal facet
pa - base of pila antotica
Pa - prearticular facet
pc - posterior cavity
Pf- postfrontal facet
PI - palatine facet
P1.f- palatal fenestra
Pm - premaxillary facet
Po - postorbital facet
Pr - prefrontal facet
Pro - prootic facet
Pt - pterygoid facet
Q- quadrate facet
Q - quadratojugal facet
Q r . -quadrate recess
S - splenial facet
sac - opening of superior alveolar canal
sof- suborbital fenestra
Sp - sphenoid facet
Sq - squamosal facet
t.c. -ossified trabeculae cranii
V - vomerine facet
v.g. - vidian groove
v.pr - ventral process
6 - foramen for abducens nerve
MIDDLE JURASSIC CHOKIS’IODEKE
237
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