BREVIORA

Museum of Comparative Zoology

US ISSN 0006-9698

CAMBRIDGE, MAss. 19 OctToBER 2009 NumeBer 518

AN ARCHOSAUR-LIKE LATEROSPHENOID IN EARLY TURTLES
(REPTILIA: PANTESTUDINES)

BHART-ANJAN S. BHULLAR! AND GABE S. BEVER2

ApstRACT. Turtles are placed with increasing consistency by molecular phylogenetic studies within Diapsida as
sister to Archosauria, but published gross morphology—based phylogenetic analyses do not recover this position.
Here, we present a previously unrecognized unique morphological character offering support for this hypothesis: the
presence in stem turtles of a laterosphenoid ossification identical to that in Archosauriformes. The laterosphenoid is a
tripartite chondrocranial ossification, consisting of an ossified pila antotica, pila metoptica, and taenia medialis +
planum supraseptale. It forms the anterior border of the exit for the trigeminal nerve (V) and partially encloses the
exits for cranial nerves III, IV, and II. This ossification is unique to turtles and Archosauriformes within Vertebrata.
It has been mistakenly dismissed as anatomically dissimilar in these two groups in the past, so we provide a complete
description and detailed analysis of correspondence between turtles and Archosauriformes in each of its
embryologically distinct components. A preliminary phylogenetic analysis suggests other potential synapomorphies
of turtles and archosaurs, including a row or rows of mid-dorsal dermal ossifications.

Key worps: Archosauria; Archosauriformes; Diapsida; turtle origins; chondrocranium; Proganochelys;
Kayentachelys; fossil; braincase; interorbital ossification; Testudines

Turtles (Pantestudines; Joyce et al., 2004)
have traditionally been classified as ‘“‘ana-
psid” reptiles owing to their lack of the
lateral and dorsal fenestration of the skull

‘Department of Organismic and Evolutionary Biology,
Harvard University, Biolabs room 4110, 16 Divinity
Avenue, Cambridge, Massachusetts 02138, U.S.A.; e-
mail: bhartanjan.bhullar@gmail.com

*Division of Paleontology, American Museum of
Natural History, Central Park West at 79th Street,
New York, New York 10024, U.S.A.

that is ancestral for diapsid reptiles, includ-
ing tuatara, lizards, crocodiles, and birds
(Gauthier et al., 1988, and references there-
in). Early gross morphology—based phyloge-
netic analyses suggested that turtles are the
sister taxon to Diapsida and thus one of the
two branches of the initial reptilian diver-
gence (Gauthier et al., 1988). Most subse-
quent morphological analyses have either
supported this position (Brochu, 2001;
Laurin and Reisz, 1995; Lee, 1997) or have

© The President and Fellows of Harvard College 2009.

to

placed turtles close to the marine Euryapsida
along the stem of the lizard/tuatara clade
Lepidosauria (Li et al., 2008; Rieppel and
Reisz, 1999), thus suggesting that they are
highly modified diapsids. One analysis
(Merck, 1997) similarly indicated affinities
to the Euryapsida but recovered a novel
result because the included characters of
non-turtle euryapsids placed the entire turtle
+ euryapsid clade as sister to the archosaur
lineage (Brochu, 2001).

In contrast, a growing body of molecular
phylogenetic work strongly supports a posi-
tion of turtles within Diapsida as sister to the
crocodile/bird clade Archosauria (Cao et al.,
2000; Iwabe et al., 2005; Kumazawa and
Nishida, 1999; Organ et al., 2008). Until
now, no unique gross morphological support
has been reported for archosaur affinities of
turtles (Rieppel, 2000), and in particular, no
morphological evidence has been forthcom-
ing that would help place turtles along the
archosaur stem. However, such evidence has
existed, largely overlooked, since the further
preparation and monographic description by
Gaffney of the best-preserved stem turtle,
Proganochelys quenstedti, from the Late
Triassic (Norian) of Germany (Gaffney,
1990).

A single specimen of P. quenstedti (SMNS
15759) preserves the region anterior to the
braincase. In this region, which would in life
have been occupied by the membranous
anterior braincase, a pair of dorsoventrally
tall, flat ossifications articulate with the
prootic and basisphenoid on each _ side
(Fig. 1A). The initial description of this
region by Gaffney (1990) documented the
form of these bones but did not treat the
detailed morphology of each of their pro-
cesses. It was noted that they are similar to a
pair of ossifications synapomorphic for the
clade Archosauriformes, the pleurosphe-
noids (Fig. 1B), which are now usually called
laterosphenoids (Clark et al., 1993). Howev-

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No. 518

er, the general consensus at the time,
including the hypothesis presented by Gaff-
ney (1990), was that turtles were sister to all
other extant reptiles. These elements were
thus termed “‘pleurosphenoids,” with the
quotation marks indicating probable non-
homology with those of Archosauriformes.
We posit, in contrast, that they are in fact
homologous to the laterosphenoids of Arch-
osauriformes.

MATERIALS AND METHODS

All specimens examined are from the
collections of the Museum of Comparative
Zoology, Harvard University. The following
specimens from the Herpetology collection
were examined: Alligator mississippiensis
MCZ 17711, 34323; Caiman crocodilus
MCZ 5031; Crocodylus cataphractus MCZ
13985, 175004; C. niloticus MCZ 4372; C.
porosus MCZ_ 72937; Gavialis gangeticus
MCZ 33950; Osteolaemus tetraspis MCZ
22913; Paleosuchus palpebrosus MCZ
84030; Tomistoma schlegeli MCZ 12459.
From the Ornithology collection: Tinamus
major MCZ 342723, 342774. From the
Vertebrate Paleontology collection: Eothyris
parkeyi MCZ 1161.

Phylogenetic analyses, as described below,
used a modified version of the matrix from
Dilkes (1998). Both used parsimony searches
in PAUP* v4.0b10 (Swofford, 2001) with the
branch-and-bound search option (1,000 rep-
licates), specifying Petrolacosaurus as the
outgroup as in Dilkes (1998). The con-
strained search used the monophyly con-
straint option to unite Proganochelys with
the archosauriform clade, including Eupar-
keria and Proterosuchus.

DESCRIPTION OF THE LATEROSPHE-
NOID IN PROGANOCHELYS

Following is a more complete description
of the left laterosphenoid in Proganochelys

2009 ARCHOSAUR-LIKE LATEROSPHENOID IN EARLY TURTLES 3

fenestra epiotica

pila antotica

fenestra
prootica (V)

metoptica

fenestra
metoptica (Ill)

B _ taenia medialis pila antotica

pila metoptica

fenestra metoptica (Ill) ¥.

fenestra prootica (V)
fenestra epiotica
planum supraseptale taenia marginalis
taenia medialis

pila metoptica

Figure 1. (A) Left laterosphenoid of Proganochelys quenstedti SMNS 15759 in lateral view, after Gaffney (1990).
(B) Right laterosphenoid of Proterosuchus fergusi NMQR 1484 in lateral view, reflected, after Clark et al. (1993). (C)
Chondrocranium of Crocodylus porosus after ref 1 with region ossified as laterosphenoid filled in. BS, basisphenoid;
FR, frontal; EP, epipterygoid; LS, laterosphenoid; OP, opisthotic; PA, parietal; PF, postfrontal; PO, postorbital;
PR, prootic; Q, quadrate.

4 BREVIORA

than was offered in the original monograph.
Our goal is to elucidate the developmental
origins of the turtle laterosphenoid and thus
demonstrate its exact correspondence to the
archosauriform morphology.

Mediolaterally, the laterosphenoid is thin,
especially near its periphery, and it is inclined
ventromedially, reflecting the angulation of
the wall of the membranous braincase within
which it ossified (Fig. 1A). It has three major
components. The first is a strut that extends
anterodorsally from the clinoid process of
the basisphenoid, but whose posterodorsal
portion forms a small contact with the
anterodorsal portion of the prootic. The
posterior margin of the strut forms the
anterior half of the border of the trigeminal
(prootic) foramen transmitting cranial nerve
V, which is fully encircled by virtue of its
dual contacts—the ventral, broad contact
with the clinoid process of the basisphenoid
and the dorsal, attenuate contact with the
prootic. The anterior margin of the strut
forms the posterior border of a ventrally
incomplete aperture that in life would have
been formed around cranial nerves III and
IV. Topologically and morphologically, this
strut corresponds exactly to the pila antotica
of the embryonic amniote chondrocranium
(Fig. 1C; Bellairs and Kamal, 1981), as
suggested but not fully explicated in the
description by Gaffney (1990). The meeting
with the prootic and thus closure of the
trigeminal foramen, however, is a unique
feature of laterosphenoids.

The second major component of the
laterosphenoid is a broad, dorsoventrally
oriented strut whose posterior margin arches
over to form the anterior half of the aperture
for cranial nerves III and IV (Fig. 1A). The
strut becomes anteroposteriorly wider at its
base and then ends, presumably where it
would have sat upon the unossified fused
trabeculae cranii. Its anterior margin forms
the lower portion of an emargination that

No. 518

would have formed around cranial nerve II
(optic nerve) and its associated neurovascu-
lar structures. Thus, topologically and mor-
phologically, this strut corresponds to the
pila metoptica of the embryonic amniote
chondrocranium (Fig. 1C), an observation
not made in the description by Gaffney
(1990).

The third major component of the latero-
sphenoid is an anterodorsally directed, ter-
minally expanded lobe (Fig. 1A) connected
basally to both of the other two components
whose broadly curved anteroventral margin
forms the majority of the emargination for
cranial nerve II and whose posterodorsal
margin borders an aperture that might
represent the fenestra epioptica of the
diapsid embryo (Bellairs and Kamal, 1981).
As noted in the description by Gaffney
(1990), the dorsal and anterior margins of
the lobe appear unfinished. This morphology
could represent breakage, but considering
the general completeness of the surrounding
elements, we think it more plausible that it is
instead the border between the ossified and
cartilaginous portions of the structure. The
rough but not jagged texture of the surfaces
supports this interpretation. The original
description emphasizes that there are no
signs on the parietal of a bony suture with
the laterosphenoid. Topologically and mor-
phologically, the lobe corresponds to the
taenia medialis and perhaps a portion of the
planum supraseptale of the chondrocranium
(Fig. 1C)—not the planum supraseptale ex-
clusively as suggested by Gaffney (1990).

COMPARATIVE NOTES

The laterosphenoid of Proganochelys is
identical to the laterosphenoid present in the
clade Archosauriformes (Clark et al., 1993)
with the sole exception that it retains an open
suture with the skull roof. The stem turtle
Kayentachelys aprix, closer to the crown than

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Proganochelys, also possesses laterosphe-
noids (again described as “pleurosphe-
noids’’), though in existing specimens their
detailed morphology is not discernable
(Sterli and Joyce, 2007). This distribution
suggests their ancestral presence in the turtle
lineage. Unfortunately, specimens of the
oldest known stem turtle, Odontochelys
semitestacea, are dorsoventrally crushed,
obscuring the relevant region (Li et al.,
2008). In modern turtles, ventral down-
growths of the parietal articulate directly
with the prootic and have thus obliterated
any remnant of the laterosphenoids.

The archosauriform laterosphenoid shows
the three components listed above in every
case where it is known, though there is some
variation in their relative prominence
(Fig. 1C). In crocodylians, for instance, and
particularly Alligator, the pila metoptica
component is reduced but present. The
morphology and topology of the latero-
sphenoid in turtles and Archosauriformes
are unique among all vertebrates. It appears
that the identity of the turtle laterosphenoid
has simply been overlooked. The element is
not mentioned in comprehensive reviews of
the archosaur condition (Clark et al., 1993).

As noted in the original description of
Proganochelys and in subsequent works,
stem reptiles had a more anterior spheneth-
moid ossification, Y-shaped or V-shaped in
transverse section. This ossification is lost in
diapsids (de Braga and Rieppel, 1997). There
is limited overlap between the region of
ossification of the stem reptile sphenethmoid
and the turtle/archosauriform laterosphe-
noid. The posteriormost interorbital region,
notably the base of the pila antotica, is only
ossified in Archosauriformes and turtles (de
Braga and Rieppel, 1997; Gaffney, 1990).

The stem reptile sphenethmoid and turtle
laterosphenoid were confounded and
claimed to be homologous in some recent
work, suggesting pareiasaur affinities for

ARCHOSAUR-LIKE LATEROSPHENOID IN EARLY TURTLES 3)

turtles (Lee, 1993, 1995, 1997). Interestingly,
a rebuttal of many of the conclusions of that
work (de Braga and Rieppel, 1997) asserted
that both stem turtles and pareiasaurs have
sphenethmoids. However, the anatomical
criteria they set out for a sphenethmoid
(e.g., complete enclosure of the optic nerve
foramen) do not describe the structure in
Proganochelys, although it does fit the
structure in pareiasaurs. Simultaneously,
their criteria for a true laterosphenoid
(“pleurosphenoid’’) precisely describe the
structure in Proganochelys. The only plausi-
ble explanation for this oversight is that the
authors of that paper accepted the homology
assessments of the study they were rebutting
(Lee, 1995) without referring to the descrip-
tion of Proganochelys by Gaffney (1990).

CHARACTER DISTRIBUTION ON THE
ARCHOSAUR STEM WITH
TURTLES INCLUDED

Not all stem archosaurs have a latero-
sphenoid—as described earlier, the bone is a
synapomorphy of Archosauriformes (Clarke
et al., 1993), which excludes protorosaurs,
rhynchosaurs, and Trilophosaurus (Dilkes,
1998; Modesto and Sues, 2004; Sues, 2003).
The presence of a laterosphenoid in turtles
suggests a close relationship to Archosaur-
iformes to the exclusion of non-archosauri-
form archosauromorphs. Additionally, the
presence of a tight suture of the latero-
sphenoid to the parietal might unite Arch-
osauriformes to the exclusion of turtles,
suggesting, on the basis of this character, a
sister-group relationship between the two.
Unfortunately, the highly derived nature of
the remainder of the turtle skull and post-
cranium results in widely inconsistent results
when turtles are included in morphological
character matrices taken from other studies
of reptilian relationships that did not initially
include turtles. Typically, these analyses have

6 BREVIORA

not included a large number of characters
within Archosauromorpha that would allow
the precise placement of turtles within that
clade (e.g., Muller and Reisz, 2006, and
references therein). A full analysis of rela-
tionships will require considerable additional
work.

As a preliminary exercise, we scored P.
quenstedti using the 144-character matrix by
Dilkes (1998), the most comprehensive arch-
osauromorph matrix in the literature. The
characters listed by Dilkes (1998) as candi-
dates for ordering were ordered. To his
matrix, we added three characters:

145. Laterosphenoid (0) not sutured to parietal or (1)
sutured to parietal.

146. Skull (0) broadly wedge-shaped or (1) tall and
mediolaterally narrow.

147. Mid-dorsal region dermal ossifications (0) absent or
(1) present.

We briefly discuss each of these in turn.
See the Appendix for individual character
scores.

The skull of Euparkeria and archosaurs is
tall and mediolaterally compressed com-
pared with that of non-Archosauriformes
and to an extent Proterosuchus. Progano-
chelys shows what appears to be the primi-
tive condition. Scoring of this character does
not affect the current analysis, but it is a
codification of this basic observation on
skull proportions and will be useful as more
taxa within Archosauria are added to the
analyses. A row of ossifications close to the
midline of the back is another overlooked
potential synapomorphy of turtles and Arch-
osauriformes. It is especially interesting
because Odontochelys has only the mid-
dorsal ossifications, the rest of the carapace
remaining unossified (Li et al, 2008). If this is
the primitive condition in the turtle lineage,
it would be even more similar to the state in
Archosauriformes, which have a pair of rows
of osteoderms running down the center of
the back (Gauthier et al., 1988). It is true that

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turtles appear to have a single row of discrete
ossifications, whereas Archosauriformes
have two, but despite this difference, they
share the presence of a longitudinal series of
dermal bone elements in the mid-dorsal
region.

In addition to the synapomorphies includ-
ed in the matrix, P. quenstedti has what
appears to be a typical diapsid infraorbital
foramen, despite the lack of a separate
ectopterygoid. This infraorbital foramen
becomes progressively smaller along the
lineage to extant turtles and is given the
name “foramen palatinum posterius”’ (Joyce,
2007). This terminology implies homology to
a very small vascular foramen present in
stem reptiles (Gaffney, 1990), despite the
greater resemblance of the large foramen of
plesiomorphic stem turtles to the diapsid
infraorbital foramen. Only more crown-ward
turtles have a very small foramen.

The first, unconstrained parsimony analy-
sis yielded a single most parsimonious tree of
397 steps and recovered P. quenstedti as
sister to Archosauromorpha (Fig. 2), sug-
gesting archosaurian affinities for turtles, but
a dual origin of the laterosphenoid. Synapo-
morphies supporting this placement are:
36(1), quadrate exposed laterally; 47(1),
crista prootica present; 107(1), entepicondy-
lar foramen absent; 122(1), fifth metatarsal
hooked without deflection. Unambiguous
synapomorphies along the lineage leading
to Archosauriformes, but lacking in Proga-
nochelys (requiring reversal if Proganochelys
is allied to Archosauriformes), are: 2(1),
snout greater than or equal to 50% of skull
length; 5(1), antorbital fenestra present; 8(1),
maxillary ramus of premaxilla extends as
posterodorsal process to form caudal border
of naris; 18(1), ratio of lengths of nasal and
frontal greater than 1.0; 29(0), postparietal
present; 37(1), quadrate emargination pre-
sent with conch; 43(1), orientation of basip-
terygoid processes lateral; 45(1), internal

2009

(NACE TNCT ST SIE

ARCHOSAUR-LIKE LATEROSPHENOID IN EARLY TURTLES 7

Prolacerta
Euparkeria
Archosauriformes % Proterosuchus
Mesosuchus
Howesia
Rhynchosaurus
Scaphonyx
Hyperodapedon
Stenaulorhynchus
Trilophosaurus
Macrocnemus
Langobardisaurus
Tanystropheus
Megalancosaurus
Drepanosaurus
Protorosaurus
Proganochelys
Squamata
Gephyrosaurus
Cteniogenys
Champsosaurus
Lazurussuchus
Youngina

Petrolacosaurus

Figure 2. Single most parsimonious tree resulting from unconstrained phylogenetic analysis with the use of

modified matrix from Dilkes (1998).

carotid foramina on ventral surface of
parasphenoid; 53(1), post-temporal fenestra
small; 75(1), upturned retroarticular process;
76(1), lateral mandibular fenestra; 79(0),
postaxial cervical intercentra present; 87(2),
second sacral rib bifurcate with caudal
process truncated sharply; 88(2), proximal
caudal neural spies very tall; 96(0), inter-
clavicle broad diamond; 97(1), notch in
interclavicle between clavicles; 104(1), ante-
rior apron of pubis present; 109(1), medial
centrale of carpus absent; 116(1), lateral
tuber of calcaneum; 126(1), pterygoids re-
main separate cranially.

For the second analysis, we constrained P.
quenstedti to be sister to Archosauriformes to

determine potential synapomorphies in the
case of a single origin of the laterosphenoid.
A single most parsimonious tree of 413 steps
was recovered (Fig. 3). In this tree, the
Proganochelys/Archosauriformes clade was
sister to the remaining archosauromorphs.
Synapomorphies supporting a sister-group
relationship between P. quenstedti and Arch-
osauriformes are: 14(1), septomaxilla absent;
50(1), laterosphenoid present; 74(2), retro-
articular process present, large, and formed
by articular; 77(1), slender and tapering
cervical ribs at low angle to vetebrae present;
83(1), notochordal canal absent in adult;
89(1), ratio of lengths of caudal transverse
processes and centra greater than 1.0; 102(1),

8 BREVIORA

Archosauromorpha

Figure 3.

No. 518

Proganochelys

Euparkeria
Archosauriformes” Proterosuchus
Macrocnemus
Langobardisaurus
Tanystropheus
Megalancosaurus
Drepanosaurus
Protorosaurus
Prolacerta
Trilophosaurus
Mesosuchus
Howesia
Rhynchosaurus
Scaphonyx
Hyperodapedon
Stenaulorhynchus
Cteniogenys
Champsosaurus
Lazurussuchus
Squamata
Gephyrosaurus
Youngina

Petrolacosaurus

Single most parsimonious tree resulting from phylogenetic analysis with Proganochelys quenstedti

constrained as sister to Archosauriformes with the use of modified matrix from Dilkes (1998).

dorsal margin of ilium with large posterior
process and smaller anterior process; 143(1),
distal ends of cervical neural spines expanded
in form of flat table; 147(1), mid-dorsal
region dermal ossifications present.

DISCUSSION

The tree recovered by our first (uncon-
strained) analysis agrees in its general topol-
ogy with the preferred tree discussed by
Dilkes (1998). This topology suggests a dual
origin of the laterosphenoid; note, however,
the caveats below about the overall topology
of the tree. Nevertheless, Proganochelys does

emerge on the basis of this dataset both as a
diapsid and as part of the archosaur stem
lineage. Constraining Proganochelys as sister
to Archosauriformes (and therefore forcing a
single origin of the laterosphenoid) pulls that
clade into a sister-taxon relationship with the
remaining archosauromorphs. That Proga-
nochelys would exert a pull toward the
archosauromorph base is unsurprising given
that the apparently primitive reptilian char-
acters of turtles generally place them as the
sister taxon to the remaining reptiles in
morphological phylogenetic analyses (Gau-
thier et al., 1988). Additionally, the positions
of Trilophosaurus and Prolacerta are labile,

2009

with Prolacerta jumping from an_ affinity
with Archosauriformes in the unconstrained
tree to a more traditional position allied with
other “primitive” archosauromorphs in the
constrained tree. Trilophosaurus is highly
autapomorphic and jumps from a sister-
taxon relationship to a “higher” archosaur-
omorph clade, including Archosauriformes,
in the unconstrained analysis to a position
sister to the “primitive” archosauromorph
clade in the constrained analysis. Note that
the new characters we added did not affect
the broad-scale topology of the tree exclusive
of Proganochelys.

Because of the lability of the trees recov-
ered using the matrix from Dilkes (1998) and
the incongruence among various hypotheses
of diapsid relationships, we consider that a
good deal of additional work is required to
create a truly comprehensive character list
allowing a robust placement of turtles among
fossil and extant taxa. The exercise described
above is directed only at examining, in a
preliminary way, the distribution of poten-
tially interesting characters within Archo-
sauromorpha if turtles have archosaur affin-
ities. The continued lack of consensus about
relationships within archosauromorphs is
why we are careful to distinguish between
physical identity between the laterosphenoids
of turtles and archosauriforms, which we
have shown, and homology between the
structures. We subscribe to the “historical”
homology concept, elegantly stated by Van
Valen (1982) as “‘continuity of information”
from ancestor to descendant. Thus, a con-
clusive homology statement depends on a
robust phylogenetic tree.

The laterosphenoids in turtles and arch-
osauriforms fulfill the requirements for a
hypothesis of homology as set forth by
Patterson (1982), including topology and
ontogeny. Ontogeny, however, has since
been discredited as a separate, special crite-
rion for homology or character polarity

ARCHOSAUR-LIKE LATEROSPHENOID IN EARLY TURTLES 9

determination (de Queiroz, 1985). Rather,
characters from different times in an organ-
ism’s existence simply represent additional
points of identity between putatively homol-
ogous structures. The total existence of every
organism in time consists of a series of
“frames” or semaphoronts (sensu Hennig,
1966), and points of identity that might be
homology relations can be sought between
any semaphoronts, no matter their relative
sequence. Interestingly, Owen (1848) already
understood, as stated explicitly in the intro-
duction to the cited work, that different
modes of development (early semaphoronts)
do not preclude homology of later struc-
tures.

Although the debate on turtle origins and
the evolution of their unique anatomy
remains unresolved, molecular studies over-
whelmingly indicate archosaurian affinities
for turtles. The preliminary analyses we ran
identified a number of interesting characters
that might represent synapomorphies of
turtles and various archosauromorph clades.
Yet, the laterosphenoid alone is a character
shared between turtles and a monophyletic
group within archosauromorphs that does
not appear elsewhere among vertebrates. It
represents potential morphological support
for the hypothesis that turtles are part of a
major stem archosaur radiation and another
example of the immense variety of the
archosaur lineage. —

ACKNOWLEDGMENTS

We thank Jacques Gauthier, Tyler Lyson,
and Farish Jenkins for discussion of the
structure in question. Jonathan Losos, José
Rosado, Farish Jenkins, and Bill Amaral
permitted access to comparative specimens.
Reviews by Chris Brochu, Michael Lee, and
Randy Irmis were uniformly insightful and
constructive.

10 BREVIORA

APPENDIX 1:
ADDITIONS TO DILKES (1998) CHAR-
ACTER MATRIX

For new characters, order is: Pe, Y, G, Sq,
Pr, Ma, Ta, Tr, Ho, Me, R, Sc, St, Hy, Ph, E,
Ch, Ct, L, Po, Mg, Ln, D. See Dilkes (1998)
for key to abbreviations.

Character 146: 0000000000000001000000?
Character 147: 00000000000000?10000000
Proganochelys quenstedti: 1022000010
1111200000 0007071011 000071212?

0012001011 0117720110 1272010021
0100111101 002010???? 1112000000
000007000? 0?10001

LITERATURE CITED

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Brocuu, C. A. 2001. Progress and future directions in
archosaur phylogenetics. Journal of Paleontology,
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Ciark, J. M., J. A. WeELMAN, J. GAUTHIER, AND M.
ParrRisH. 1993. The laterosphenoid bone of early
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Cao, Y., M. D. SORENSEN, Y. KUMAZAWA, D. P. MINDELL,
AND M. Hasecawa. 2000. Phylogenetic position of
turtles among amniotes: evidence from mitochondrial
and nuclear genes. Gene, 259: 139-148.

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