Extreme convergence in the body plans of an early suchian (Archosauria) and ornithomimid dinosaurs (Theropoda).
ABSTRACT Living archosaurs comprise birds (dinosaurs) and crocodylians (suchians). The morphological diversity of birds and stem group dinosaurs is tremendous and well-documented. Suchia, the archosaurian group including crocodylians, is generally considered more conservative. Here, we report a new Late Triassic suchian archosaur with unusual, highly specialized features that are convergent with ornithomimid dinosaurs. Several derived features of the skull and postcranial skeleton are identical to conditions in ornithomimids. Such cases of extreme convergence in multiple regions of the skeleton in two distantly related vertebrate taxa are rare. This suggests that these archosaurs show iterative patterns of morphological evolution. It also suggests that this group of suchians occupied the adaptive zone that was occupied by ornithomimosaurs later in the Mesozoic.
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Extreme convergence in the body plans of an
early suchian (Archosauria) and ornithomimid
Sterling J. Nesbitt1,2,*and Mark A. Norell1
1Division of Paleontology, American Museum of Natural History, Central Park West at 79th Street,
New York City, NY 10024, USA
2Lamont-Doherty Earth Observatory, Columbia University, 61 Rt. 9W Palisades, NY 10964, USA
Living archosaurs comprise birds (dinosaurs) and crocodylians (suchians). The morphological diversity of
birds and stem group dinosaurs is tremendous and well-documented. Suchia, the archosaurian group
including crocodylians, is generally considered more conservative. Here, we report a new Late Triassic
suchian archosaur with unusual, highly specialized features that are convergent with ornithomimid
dinosaurs. Several derived features of the skull and postcranial skeleton are identical to conditions in
ornithomimids. Such cases of extreme convergence in multiple regions of the skeleton in two distantly
related vertebrate taxa are rare. This suggests that these archosaurs show iterative patterns of
morphological evolution. It also suggests that this group of suchians occupied the adaptive zone that
was occupied by ornithomimosaurs later in the Mesozoic.
Keywords: convergent evolution; Effigia; Shuvosaurus; suchian; Triassic; archosaur
The specimens were discovered in unprepared blocks
originally excavated in the 1940s by American Museum of
Natural History field parties from the famous Ghost
Ranch Coelophysis Quarry (also known as Whitaker
Quarry). Until recently, this quarry was thought to
represent a nearly monospecific death assemblage of the
early theropod Coelophysis (Colbert 1989). However,
many different kinds of reptiles have recently been
reported (Long & Murry 1995; Clark et al. 2000; Harris
& Downs 2002). Several specimens are referred to the new
taxon. The skull has edentulous jaws, an enlarged orbit, a
posteroventrally rotated squamosal and an anteroventrally
angled quadrate. All of these features are identical to those
of derived ornithomimid dinosaurs. Additional derived
features of the pelvis, tail and vertebrae are convergent
features that diagnose nested clades within Theropoda.
2. SYSTEMATIC PALEONTOLOGY
Archosauria (Cope 1869)
Suchia (Krebs 1974 sensu Benton & Clark 1988) Effigia
okeeffeae gen. et sp. nov.
The generic name is derived from the Latin word effigia
(ghost). The specific name is in honour of Georgia
O’Keeffe for her numerous paintings of the badlands at
Ghost Ranch and her interest in the Coelophysis Quarry
when it was discovered.
An articulated skeleton including a nearly complete skull,
American Museum of Natural History Fossil Reptiles
(AMNH FR) 30587.
(c) Referred specimens
AMNH FR 30588, articulated pelvis, femur and caudal
vertebrae; AMNH FR 30589, a partial skull with
associated cervical vertebrae and partial hindlimb;
AMNH FR 30590, femur.
(d) Horizon and locality
Ghost Ranch, central New Mexico. From the Coelophysis
Quarry in the ‘siltstone member’ (Norian–?Rhaetian) of
the Chinle Formation (Stewart et al. 1972).
A suchian archosaur distinguishable from all other
suchians except Shuvosaurus by the presence of an
edentulous premaxilla, maxilla and dentary, a posteriorly
long anterodorsal process of the premaxilla, a long
preacetabular process of the ilium that connects to the
posterior process by a large thin flange and a pubic boot
that is 33% as long as the pubic shaft. Distinguished from
Shuvosaurus by the presence of a dorsal and posterior
process of the maxilla, small posterior process of the
premaxilla, fossa on the posterior side of the lacrimal,
absence of posterior process of the squamosal and a fossa
on the posterior side of the squamosal.
AMNH FR 30587 (figure 1) includes a nearly complete
skull missing only the quadratojugal and parts of the
braincase. The premaxilla, maxilla and dentary are
unusual in being edentulous, and are covered by an
Proc. R. Soc. B (2006) 273, 1045–1048
Published online 25 January 2006
The electronic supplementary material is available at http://dx.doi.
org/10.1098/rsbl.2005.3426 or via http://www.journals.royalsoc.ac.
*Author for correspondence (firstname.lastname@example.org).
Received 12 October 2005
Accepted 24 November 2005
q 2006 The Royal Society
extensive system of foramina that suggests the presence of
a horny beak or rhamphotheca. The premaxilla bears a
long posterior nasal process that meets the nasal dorsally
and forms the internarial bar. The premaxillae are
separated from the nares dorsally by anterior narial
processes of the nasals. Ventrally, the premaxillae meet
to form a small secondary palate that contacts the anterior
palatal shelves of the maxillae. The reduced maxilla has
(c)( f )
Figure 1. Skull reconstruction of Effigia okeeffeae (AMNH FR 30587). (a, b) Lateral view, (c) reconstruction, (d, e) dorsal view
and ( f ) reconstruction. Abbreviations: an, angular; d, dentary; f, frontal; j, jugal; l, lacrimal; m, maxilla; mf, mandibular
fenestra; n, nasal; p, parietal; pal; palatine; pf, prefrontal; pm, premaxilla; po, postorbital; q, quadrate; qj, quadratojugal; sq,
squamosal; sr, surangular.
(g)( f )
Figure 2. AMNH FR 30587. (a) Skeletal reconstruction of Effigia okeeffeae. The bones in grey are unknown. (b) Posterior caudal
vertebrae (AMNH FR 30589) with overlapping prezygapophyses in lateral view. Arrow indicates anterior direction for the
vertebrae only. (c) Articulated pelvis in lateral view. (d) Right coracoid in lateral view. (e) Anterior cervical vertebra in lateral
view. ( f ) Incomplete right manus scaled to the right pes in dorsal view. (g) Right pes in dorsal view. (h) Right proximal part of
the femur in medial view. (i) Articulation of the right ankle in posterior view. Abbreviations: as, astragalus; c, carpal; ca,
calcaneum; cf, coracoid foramen; fi, fibula; gl, glenoid; il, ilium; ns, neural spine; prz, prezygapophysis; pu, pubis; sa, sacrum; ti,
tibia. ScalesZ1 cm.
1046S. J. Nesbitt & M. A. Norell
Extreme convergence in archosaurs
Proc. R. Soc. B (2006)
both a dorsal process that separates the external naris from
the antorbital fenestra and a tapered posterior process that
contacts the jugal anterior to the antorbital bar.
A pronounced, raised external rim forms the anterior
and ventral margin of the antorbital fossa on the lateral
surface of the maxilla. The ventral edge of the premaxilla
and only the anterior half of the maxilla occlude with the
dentary. The length of the posteriorly shortened and
attenuated dentary corresponds to the beak-like premax-
illa and anterior part of maxilla. The dentaries meet at the
midline to form a posteriorly expanded symphysis that
corresponds to the area created by the premaxillae. This
area of the dentary is also covered with nutrient foramina,
again suggesting a rhamphotheca. A huge mandibular
fenestra is bounded dorsally and posteriorly by the
surangular, ventrally by the angular and anteriorly (only
slightly) by the dentary. A large surangular foramen is
present on the posterolateral side just anterior to the
quadrate shaft. The distal articular end of the anteroven-
trally projected quadrate articulates with the largely
convex articular. A slightly dorsolaterally expanded area
of the surangular also articulates with the quadrate (also
an unambiguous synapomorphy of Ornithomimosauria;
Makovicky et al. 2004).
Other specializations of the skull include an enlarged
orbit and external naris. The crescentic postorbital forms
both the posterior and part of the ventral and dorsal
border of the orbit. A long ventral process of the
postorbital and a large orbit are also found in derived
ornithomimids (Makovicky et al. 2004). The elongate
jugal creates most of the ventral border of the upper skull
and forms part of the postorbital bar. It lacks an anterior
dorsal process, so the lacrimal forms all of the antorbital
bar. The dorsal surface of the skull is completely smooth
andincludes an enlarged frontal andprefrontal, butclearly
lacks a postfrontal. The parietals form a sagittal crest and
articulate with posteroventrally directed squamosals. As
with Shuvosaurus, the parabasisphenoid is anteriorly
elongated as in theropods (Rauhut 1997, 2003).
In the axial skeleton, the posterior cervical vertebrae
bear posteriorly directed processes on each diapophysis
Makovicky et al. 2004). Cervical and dorsal vertebrae
are anteroposteriorly elongated and have complicated
transverse processes. The anterior cervical vertebrae have
true pleurocoels on the posterior half of the centrum (Britt
1997). The posterior caudal vertebrae bear long pre-
zygapophyses that overlap the adjacent vertebra.
The large scapula (figure 2) is long and triangular with
a blunt posterior apex, and the coracoid is equally
anteroposteriorly wide. The humerus, radius and ulna
are about the same size and are unreduced; the distal ends
of both the radius and ulna taper. Although the manus is
incomplete, the preserved elements are extremely reduced
(2 cm metacarpals), yet retain the relative proportions of a
typical Crocodylus manus (Mook 1921).
The elongated, posteriorly concave pubis has a large
boot that projects posteriorly. The appressed left and right
ischium are laterally compressed. The ilium bears a thin
dorsal flange that connects an elongated preacetabular
process to a large postacetabular process. The ilium is
oriented vertically and the acetabulum is closed medially.
The supra-acetabular rim covers the head of the femur on
both the dorsal and part of the lateral sides. The sacrum
consists offour centra fused into a stiff rod. All of the sacral
neural arches are fused.
The proximal portion of the femur bears two medial
condyles; the anteromedial condyle is knob-like and hooks
posteriorly, a condition reminiscent of the offset proximal
head of a dinosaurian femur. However, the ‘head’ of the
femur articulates anteriorly as in crocodylians and not
medially as in dinosaurs. The shafts of the femur, tibia and
fibula are thin-walled and hollow. The astragalus and
calcaneum articulate in a crocodile-normal configuration
and their morphology is similar to Alligator. The pes
Figure 3. (a) Phylogenetic positions of Effigia and Shuvo-
saurus among basal archosaurs. Unique tree shown (81
characters, lengthZ156, RIZ0.8320, CIZ0.5897). The
placement of Effigia within Theropoda requires the addition
of 27 steps and the placement sister to ornithomimids
requires the addition of 30 steps. The first number is the
decay value and the second number is the bootstrap value.
(b) The convergences listed below are present in Effigia and
are characters that describe the dinosaurian clade and nested
theropod clade. (i) Dinosauria. Reduction of arm/leg ration,
postfrontal absent, ‘offset femoral head’, elongated parabasi-
sphenoid. (ii) Neotheropoda.Enlargement of thepreacetabular
process of the ilium, incorporation of additional sacral
vertebrae, hollow limbs, ‘antitrochanter’, present on the
ilium, fusion of sacral vertebrae into rigid rod, fusion of the
sacral neural arches, left and right oppression of the ischium,
cervical vertebra pleurocoels. (iii) Coelurosauria. Increased
pubic boot size, elongated prezygapophyses in distal caudal
vertebrae, preacetabular fossa. (iv) Ornithomimosauria. Enlar-
gement of the orbit, posteroventral rotation of the squamosal,
anteroventrally angled quadrate, lacrimal–frontal contact
absent. (v) Clade within Ornithomimosauria. Posterior directed
process on each diapophysis of the posterior cervical
vertebrae, edentulous jaws.
Extreme convergence in archosaurs
S. J. Nesbitt & M. A. Norell1047
Proc. R. Soc. B (2006)
consists of five metatarsals. The recurved ungual is
Chatterjee (1993) named Shuvosaurus on the basis of a
skull from the Miller (Post) Quarry; the specializations of
its skull led him to attribute it to an ornithomimid
dinosaur. Later, it was argued (Long & Murry 1995;
Hunt et al. 1998) that the skull of Shuvosaurus belongs
with the postcrania of Chatterjeea, a suchian archosaur
found in the same quarry, although no overlapping
elements had been found. The articulated cranial and
skeletal material of Effigia strongly suggests that the skull
of Shuvosaurus belongs to the postcrania of Chatterjeea.
Therefore, Chatterjeea is very probably a junior subjective
synonym of Shuvosaurus. The fully developed crocodile-
normal ankle, crocodylomorph-like pes and articulation
between the femur and ilium clearly place Effigia as more
closely related to crocodiles than to birds. Effigia and
Shuvosaurus were inserted into a modified version of
Benton’s (1999) basal archosaur phylogenetic analysis
(see electronic supplementary material). The resultant
analysis (figure 3a) produced a single tree that placed
Effigia and Shuvosaurus within derived suchians. The
characters that Chatterjee (1993) used to assign the skull
of Shuvosaurus to the Ornithomimidae are now under-
stood to be convergent, according to the phylogenetic
analysis presented here. The postcrania of Effigia also
contain characters that are convergent with those of
theropods at more general taxonomic levels (figure 3b).
This is particularly apparent in the pelvis, where Effigia
shares the following characters with theropods: at least
four sacral vertebrae are fused into a stiffened rod, the
preacetabular process of the ilium is enlarged, a thin flange
of bone connects the preacetabular process to the ischiadic
wing of the ilium and the pubic boot is large and
Characters shared between Effigia and Shuvosaurus are
also present in the fragmentary taxon Sillosuchus (Alcober
& Parrish 1997) including the presence of a thin supra-
acetabular crest, thin dorsal flange of the ilium and similar
ischial morphology. From the Late Carnian of Argentina,
Sillosuchus indicates that Effigia-like taxa had a larger
distribution than just North America and furthermore,
Effigia-like taxa were present during most of the Late
The Late Triassic is marked by a number of body plans
convergent with later dinosaurs. The simple unspecialized
bodies of Late Triassic theropods such as Coelophysis
contrast strongly with the diversity of coeval crocodile-line
archosaurs. An iterative pattern of morphological evol-
ution suggests that some of the Late Triassic suchians may
have occupied similar adaptive zones to subsequent clades
of dinosaurs. The presence of ornithomimid-like suchians
(as well as the carnosaur-like suchians such as Postosuchus
(Chatterjee 1985) and ankylosaur-like suchians such as
aetosaurs) suggest that only in the Jurassic after most
clades of crocodile-line archosaurs became extinct, did
dinosaurs begin to explore new adaptive opportunities.
We thank Paul Olsen, Peter Makovicky, Randall Irmis, Kevin
Padian and Alan Turner for invaluable help and guidance in
preparing this manuscript. We thank Carl Mehling (AMNH)
and Sankar Chatterjee (TTM) for access to collections.
Technical assistance with figures was provided by Mick
Ellison. Preparation of the skull was generously completed by
Amy Davidson. This work was supported under a National
Science Foundation Graduate Research Fellowship and
support from AMNH.
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