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Geological Society, London, Special Publications

Early Crocodylomorpha
Randall B. Irmis, Sterling J. Nesbitt and Hans-Dieter Sues

Geological Society, London, Special Publications 2013, v.379;

p275-302.
doi: 10.1144/SP379.24

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Early Crocodylomorpha
RANDALL B. IRMIS1,2*, STERLING J. NESBITT3,4 & HANS-DIETER SUES5
1
Natural History Museum of Utah, 301 Wakara Way, Salt Lake City, UT 84108-1214, USA
2
Department of Geology & Geophysics, University of Utah, Salt Lake City,
UT 84112-0102, USA
3
Department of Biology, University of Washington, Seattle, WA 98195-1800, USA
4
Division of Paleontology, American Museum of Natural History, Central Park
West at 79th Street, New York, NY 10024, USA
5
Department of Paleobiology, National Museum of Natural History, Smithsonian Institution,
NHB MRC 121, P.O. Box 37012, Washington, DC 20013-7012, USA
*Corresponding author (e-mail: irmis@umnh.utah.edu)

Abstract: Non-crocodyliform crocodylomorphs, often called ‘sphenosuchians’, were the earliest-

diverging lineages of Crocodylomorpha, and document the stepwise acquisition of many
of the features that characterize extant crocodylians. The first crocodylomorph fossils are approxi-
mately 230 million years old (upper Carnian, Late Triassic), and at least one of these early lineages
persisted until at least 150 million years ago (Late Jurassic). These taxa occupied a wide variety
of terrestrial environments from equatorial regions to high-paleolatitudes during the early Meso-
zoic. Despite a quarter-century of quantitative phylogenetic work, the interrelationships of early
crocodylomorphs remain in a state of flux, though recent studies suggest that these lineages are
paraphyletic with respect to Crocodyliformes, rather than forming a monophyletic early offshoot
of Crocodylomorpha as some previously hypothesized. Nearly all early crocodylomorphs were
upright quadrupedal small-bodied taxa, but lumping them all together as small cursorial faunivores
masks ecological and morphological disparity in diet and limb functional morphology. With the
accelerated pace of recent discovery of new specimens and taxa, future consensus on early croco-
dylomorph phylogeny will provide a solid framework for understanding their change in diversity
and disparity through time, potential biogeographic patterns, and the morphological transforma-
tion leading to Crocodyliformes.

Crocodylians, the group that includes alligators and respective faunas, these early crocodylomorphs
crocodiles, have long been perceived as ‘living were the origin of an important and extremely suc-
fossils’, thought of as a clade that has changed lit- cessful radiation; more than that, they were suc-
tle for two-hundred million years. Beginning with cessful in their own right, and representatives of
the pioneering work of Walker (1970), and an these early evolutionary branches co-existed with
explosion of work over the last twenty-five years, crocodyliforms for at least sixty-five million years.
it has become clear that many relatives of croco- These lineages survived the end-Triassic mass
dylians, including the earliest crocodylomorphs, extinction, and transitioned successfully from eco-
looked nothing like their younger kin – thus shatter- systems dominated by large pseudosuchians to
ing the myth of the crocodylian ‘living fossil’ those dominated by large dinosaurs.
(Brochu 2001, 2003). Placing them in a phylogenetic context, early
The earliest known crocodylomorph precursors crocodylomorphs, often called ‘sphenosuchians’,
of extant crocodylians appear in the fossil record do not appear to form a clade at the base of Cro-
during the Carnian (early Late Triassic), approxi- codylomorpha as originally thought; rather, they
mately 230 million years ago (Fig. 1). These taxa represent a series of separate lineages that collecti-
were small cursorial faunivores with an erect vely document a gradual evolution of the character
stance, a stark contrast to their Mesozoic and Ceno- states that are characteristic of Crocodyliformes.
zoic crocodyliform descendants, which included At a more detailed level, the individual lineages
herbivorous forms (e.g. Wu & Sues 1996a; Krause often show their own specializations, including
& Key 2010), and many heavily armored, sprawl- unusual locomotory characteristics and sometimes
ing, semi-aquatic taxa, typified by extant forms. the complete loss of dermal osteoderms (e.g. Clark
Despite being generally rare components of their et al. 2004). Thus, early crocodylomorphs record

From: Nesbitt, S. J., Desojo, J. B. & Irmis, R. B. (eds) 2013. Anatomy, Phylogeny and Palaeobiology of Early
Archosaurs and their Kin. Geological Society, London, Special Publications, 379, 275–302.
First published online June 11, 2013, http://dx.doi.org/10.1144/SP379.24
# The Geological Society of London 2013. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics
 Downloaded from http://sp.lyellcollection.org/ at University of Michigan on November 1, 2014

276 R. B. IRMIS ET AL.

-145
Tithonian
-152
Late
Kimmeridgian
-157.3
Oxfordian
Callovian
-163.5
-166
Middle
JURASSIC

Bathonian
Bajocian
-168.3
-170.3
Aalenian
-174
Toarcian
-182.7
Early

Pliensbachian
-190.8
Sinemurian
Hettangian
-199.3
-201.3
Rhaetian
-208.5
TRIASSIC
Late

Norian

-227
Carnian
-235 Ma

CROCODYLIFORMES
Hesperosuchus agilis
Redondavenator quayensis

Macelognathus vagans

Terrestrisuchus gracilis

Saltoposuchus connectens

Sphenosuchus acutus
Litargosuchus leptorhynchus

Dibothrosuchus elaphros
Phyllodontosuchus lufengensis
Cañadón Calcáreo taxon

Hesperosuchus CM
Trialestes romeri
ARGENTINA: Pseudhesperosuchus jachaleri

Kayentasuchus walkeri

Pedeticosaurus leviseuri

Junggarsuchus sloani
Dromicosuchus grallator

Hallopus victor
CM 73372

UNITED KINGDOM:

GERMANY:
STATES:

AFRICA:
UNITED

SOUTH

CHINA:

Fig. 1. Temporal and geographic distribution of valid early crocodylomorph taxa. Geological timescale follows
Walker & Geissman (2009) with modifications from Mundil et al. (2010) and Muttoni et al. (2010).

an important record of early disparity and evolu- Phylogenetic definitions

tionary experimentation that shaped the evolution
of the stem lineage leading to Crocodylia. Fur- Although early crocodylomorph fossils had been
thermore, they document the origin and mosaic known since the late 1800s (e.g. Hallopus vic-
evolution of many morphological features that tor Marsh 1877), it was not until the early 1970s
characterize extant Crocodylia. that it became clear these small cursorial,
 Downloaded from http://sp.lyellcollection.org/ at University of Michigan on November 1, 2014

EARLY CROCODYLOMORPHA 277

morphologically divergent forms were closely to crocodylomorphs than to other ‘rauisuchians’

related to extant crocodylians and their relatives such as poposauroids.
(Crocodyliformes) (e.g. Walker 1970; Bonaparte
1972a). The systematic nomenclature for this group Sphenosuchia
was used in a phylogenetic context in the late
1980s (Benton & Clark 1988), but many higher Stem-based definition: The most inclusive clade
taxon names were only formally converted to containing Sphenosuchus acutus Haughton 1915,
clade names in the past 15 years. Few names exist Dibothrosuchus elaphros Simmons 1965, and Ter-
for clades within non-crocodyliform crocodylomor- restrisuchus gracilis Crush 1984, but not Crocody-
phs because of the lack of consensus on the inter- lus niloticus Laurenti 1768 (modified from Sereno
relationships of these taxa (e.g. Fig. 2). Here, we et al. 2005).
review three widely-used taxonomic names per- Bonaparte (1972a) named the group Spheno-
tinent to early crocodylomorphs: Crocodylomor- suchia to include Pseudhesperosuchus jachaleri,
pha; Sphenosuchia; and Crocodyliformes. Sphenosuchus acutus, and Hesperosuchus agilis,
in the spirit of Haughton’s (1924) original nam-
Crocodylomorpha ing of Sphenosuchidae. The widely used term sphe-
nosuchian is derived from Bonaparte’s (1972a)
Stem-based definition: The most inclusive clade concept and usually refers to a group of early
containing Crocodylus niloticus Laurenti 1768, quadrupedal crocodylomorphs with a gracile build,
but not Rauisuchus tiradentes von Huene 1942, long limbs, and dorsoventally deep skulls, and phy-
Poposaurus gracilis Mehl 1915, Gracilisuchus logenetically outside of Crocodyliformes. How-
stipanicicorum Romer 1972, Prestosuchus chini- ever, a number of recent phylogenetic analyses
quensis Huene 1942, or Aetosaurus ferratus Fraas have questioned the monophyly of this clade (see
1877 (modified from Sereno et al. 2005; Nesbitt below). Our phylogenetic definition of the clade is
2011). essentially the same provided by Sereno et al.
The name Crocodilomorphi was first coined by (2005), as it is centred on three classic ‘sphenosu-
Hay (1930) and later emended to Crocodylomor- chians’, Sphenosuchus acutus, Dibothrosuchus ela-
pha by Walker (1968). Walker (1968) used this phros, and Terrestrisuchus gracilis. This definition
name for a group that included members of Cro- of Sphenosuchia is designed specifically to only
codylia and their close relatives; he limited the be used if there is a clade that includes at least Sphe-
taxonomic scope based on the presence of certain nosuchus acutus, Dibothrosuchus elaphros, and
apomorphies among fossil relatives of crocodi- Terrestrisuchus gracilis but is exclusive of all cro-
lians. The apomorphies of the clade have changed codyliforms (e.g. Protosuchus and Crocodylus).
through time, but in a later work, Walker (1990) The most recent phylogenetic analyses (e.g.
explicitly wanted Crocodylomorpha to include Nesbitt 2011) do not recover such a topology.
taxa with a quadrate that touches the braincase.
Even though Crocodylomorpha has been utilized Crocodyliformes
in a phylogenetic context since the early days of
archosaur cladistic phylogenies (Benton & Clark Node-based definition: The least inclusive clade
1988; Parrish 1991, 1993; Juul 1994; Benton 1999, containing Protosuchus richardsoni (Brown 1933),
2004), it was not phylogentically defined until and Crocodylus niloticus Laurenti 1768 (Sereno
Benton & Walker (2002), who defined it as ‘all et al. 2001).
archosaurs closer to Eusuchia than to Erpetosuc- Crocodyliformes has a similar history to that
hus or Ornithosuchus’. However, this definition of Crocodylomorpha. Crocodyliformes was coined
depends on two external specifiers whose system- by Hay (1930) and then used by Benton & Clark
atic position has been especially labile. Thus, (1988) in a phylogenetic context, but was not for-
depending on their phylogenetic relationships, this mally phylogenetically defined until Sereno et al.
definition could conceivably result in a Croco- (2001). This definition was followed by Nesbitt
dylomorpha that includes most of Suchia. Sereno (2011) and numerous other authors (e.g. Clark
et al. (2005), therefore, provided a revised def- et al. 2004; Pol & Norell 2004a, b; Gasparini
inition that utilized more phylogenetically stable et al. 2006; Pol et al. 2009; Turner & Sertich 2010).
external specificiers. Nesbitt (2011) conserved
much of the definition of Sereno et al. (2005), but
added the external specifier Rauisuchus tiradentes Fossil record
Huene 1942 to maintain the traditional and com- Distribution and biogeography
mon concept of Crocodylomorpha, so that it did
not include traditional ‘rauisuchian’ taxa like Post- As a whole, early crocodylomorph remains are
osuchus and Rauisuchus tiradentes that are closer rare in vertebrate assemblages through their
 Downloaded from http://sp.lyellcollection.org/ at University of Michigan on November 1, 2014

278 R. B. IRMIS ET AL.

(a) Trialestes (b) Poposauria

Saltoposuchus Saltoposuchus
Pseudhesperosuchus Pseudhesperosuchus
Dibothrosuchus Sphenosuchus
Sphenosuchus “Hesperosuchus” UCMP 129470
Kayenta Form Dibothrosuchus
Crocodyliformes Crocodyliformes
Crocodyliformes Outgroup
(c) Saltoposuchus (d) Saltoposuchus
Pedeticosaurus Pseudhesperosuchus
Terrestrisuchus Hesperosuchus
Sphenosuchus Sphenosuchus
Dibothrosuchus Dibothrosuchus
Pseudhesperosuchus Crocodyliformes
Alligator
Protosuchus
(e)
(BP/1/5237) = Litargosuchus
Terrestrisuchus
Pseudhesperosuchus
Hesperosuchus holotype
CM 29894
UCMP 129470
Saltoposuchus
Sphenosuchus
Dibothrosuchus
Stagonolepis Stagonolepis
(f) Gracilisuchus (g) Gracilisuchus
Postosuchus Postosuchus
Dibothrosuchus Hesperosuchus
Sphenosuchus Dromicosuchus
Saltoposuchus Saltoposuchus
Hesperosuchus Dibothrosuchus
Pseudhesperosuchus Sphenosuchus
Litargosuchus Kayentasuchus
Kayentasuchus Pseudhesperosuchus
Terrestrisuchus Terrestrisuchus
Protosuchus Litargosuchus
Alligator Protosuchus
Alligator
Gracilisuchus Rauisuchidae
(h) (i)
Stagonolepis CM 73372
Postosuchus Hesperosuchus holotype
Erpetosuchus Hesperosuchus CM
Pseudhesperosuchus Dromicosuchus
Terrestrisuchus Sphenosuchus
Hesperosuchus Dibothrosuchus
Saltoposuchus Terrestrisuchus
Kayentasuchus Litargosuchus
Dromicosuchus Kayentasuchus
Litargosuchus Protosuchus
Sphenosuchus Orthosuchus
Dibothrosuchus Alligator
Junggarsuchus
Protosuchus
Gobisuchus
Alligator
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EARLY CROCODYLOMORPHA 279

stratigraphic range; most named taxa are rep- member of New Mexico (Clark et al. 2001).
resented by a single specimen. Yet, in terms of spec- Although rare, crocodylomorphs have also been
imen completeness and quality, nearly all of the found in the Dockum Group (Long & Murry 1995;
taxa listed in Table 1 have skull material and Martz et al. in press). In Jurassic strata, early croco-
more than half of the specimens consist of well- dylomorphs are present in the Kayenta Formation in
preserved cranial and postcranial material (e.g. Arizona (Clark & Sues 2002) and have been found
Hesperosuchus agilis, Dromicosuchus grallator, in Colorado and Wyoming localities in the Upper
and Sphenosuchus acutus). The relative abundance Jurassic Morrison Formation (e.g. Ague et al.
of early crocodylomorphs might be under-reported, 1995; Göhlich et al. 2005) of the western United
because other than named holotypes, few authors States.
have reported isolated remains. The lack of pub- Elsewhere in the northern half of Pangaea, cro-
lished fragmentary material is unlikely to repre- codylomorphs have been found in the Triassic of
sent true rarity, but is partly the result of the both the Germanic Basin and the United Kingdom
difficulty to recognize diagnostic character states (Huene 1921; Crush 1984). Abundant skeletal
in isolated elements. material of a single species of crocodylomorph, Ter-
Crocodylomorpha likely originated in the early restrisuchus gracilis, was found in least five differ-
portion of the Late Triassic (Fig. 1) given that ent latest Triassic fissure fills in Carboniferous
both the observed fossil record begins during the limestones from southwestern England and Wales
late Carnian (see below) and their closest relatives (Crush 1984; Whiteside & Marshall 2008). Two
in the most recent early archosaur phylogenies of species, Dibothrosuchus elaphros and Phyllodon-
(Erpetosuchus in Brusatte et al. 2010 and Rauisu- tosuchus lufengensis, co-occur in the Lower Juras-
chidae in Nesbitt 2011) also appeared early in the sic Lufeng Formation of China, but have not
Late Triassic. They appear to be one of only a few been found at the same locality (Wu & Chatterjee
speciose clades of Triassic archosaurs that did not 1993; Harris et al. 2000). Junggarsuchus sloani is
originate by the Middle Triassic (Nesbitt 2011). known from the Middle Jurassic part of the Shishu-
No comprehensive biogeographic hypotheses gou Formation (Clark et al. 2004) and co-occurs
have focused specifically on early crocodylo- with a diversity of crocodyliform taxa (Wings
morphs, but they have been incorporated into lar- et al. 2010).
ger analyses of Triassic tetrapod distribution (e.g. The diversity of Gondwanan crocodylomorphs
Ezcurra 2010). Early crocodylomorphs were rela- from both the Triassic and Jurassic is similar to
tively widespread throughout Pangaea during both that of the northern Pangaea crocodylomorphs, but
the Triassic and Jurassic periods (Table 1 and nearly all occurrences are restricted to fossilifer-
Fig. 1). North America possesses the most diverse ous well-sampled intervals (e.g. ‘Stormberg Group’
assemblage of early crocodylomorphs ranging and Ischigualasto-Villa Union Basin). The Upper
from the early part of the Norian to the end of the Triassic sequence in the Ischigualasto–Villa Union
known stratigraphic range in the Late Jurassic. In Basin of western Argentina preserves the oldest
the Triassic, crocodylomorph specimens are pre- confirmed occurrence of a crocodylomorph (Tria-
sent in the Chinle Formation and Dockum Group lestes romeri, see below for details) in the Ischi-
across the western United States (Long & Murry gualasto Formation, and the oldest association of
1995) and from the Deep River Basin in North a non-crocodyliform crocodylomorph (Pseudhes-
Carolina, USA (Sues et al. 2003; Schneider et al. perosuchus jachaleri) with a crocodyliform (Hemi-
2008). Within the Chinle Formation, crocodylo- protosuchus leali) in the overlying middle Norian
morphs have been recovered throughout much Los Colorados Formation (Bonaparte 1969, 1972a;
of the unit’s deposition. They have been found Santi Malnis et al. 2011). The ‘Stormberg Group’
both in lower Chinle Fm. units such as the Blue preserves the most diverse record of taxa; Spheno-
Mesa (Parker & Irmis 2005) and Cameron suchus acutus and Litargosuchus leptorhynchus
members (Colbert 1952), the middle Sonsela are from the upper Elliot Formation (Haughton
Member (Parker & Martz 2011), as well as upper 1915; Clark & Sues 2002), associated with the
Chinle Fm. units such as the Petrified Forest early crocodyliforms Protosuchus haughtoni (Gow
Member in Arizona (Parrish 1991) and New 2000) and Orthosuchus stormbergi (Nash 1968),
Mexico (Irmis et al. 2007a), and at the end of depo- whereas Pedeticosaurus leviseuri (which might
sition of the Chinle Formation in the ‘siltstone’ also be an early crocodyliform; see Bonaparte

Fig. 2. Phylogenetic hypotheses for the interrelationships of early crocodylomorphs. Boxes with solid lines surround
a monophyletic Sphenosuchia whereas dashed lines indicate a paraphyletic Sphenosuchia. (a) Benton & Clark
(1988); (b) Parrish (1991); (c) Sereno & Wild (1992); (d) Wu & Chatterjee (1993); (e): Clark et al. (2001); (f) Clark &
Sues (2002); (g) Sues et al. 2003; (h) Clark et al. (2004); (i) Nesbitt (2011).
 Downloaded from http://sp.lyellcollection.org/ at University of Michigan on November 1, 2014

280 R. B. IRMIS ET AL.

Table 1. List of currently recognized valid early crocodylomorph taxa, including their geologic and
geographic occurrence, age, and preserved skeletal material assigned to each taxon

Taxon Occurrence Age Material

CROCODYLOMORPHA Hay (1930)

CM 73372 ‘siltstone’ mbr., Chinle Late Triassic: Articulated skeleton
Fm./New Mexico, ?Rhaetian missing the skull
SW USA
Hesperosuchus agilis Cameron Mbr., Chinle Late Triassic: early Partial skull and postcranial
Colbert (1952) Fm./Arizona, SW Norian skeleton
USA
CM ‘Hesperosuchus’ ‘siltstone’ mbr., Chinle Late Triassic: Complete skull and anterior
Clark et al. (2000) Fm./New Mexico, ?Rhaetian half of an articulated
SW USA skeleton
Dromicosuchus grallator Lithofacies Association Late Triassic: early Complete skull and most
Sues et al. (2003) II, Durham Sub-Basin or middle Norian of an articulated skeleton
of Deep River Basin/
North Carolina,
eastern USA
Sphenosuchus acutus upper Elliot Fm./South Early Jurassic: Complete skull and
Haughton (1915) Africa ?Hettangian portions of an articulated
skeleton
Dibothrosuchus elaphros Zhangjiawa Mbr., Early Jurassic: Complete skull and partial
Simmons (1965) Lufeng Fm./Yunnan, ?Hettangian postcranium (from two
China specimens)
Terrestrisuchus gracilis fissure fills, Late Triassic: Nearly entire skeleton
Crush (1984) Pant-y-ffynon and ?Rhaetian known from many
Cromhall quarries, articulated and
Wales disarticulated specimens
Litargosuchus upper Elliot Fm./South Early Jurassic: Complete skull and much
leptorhynchus Clark & Africa ?Hettangian of an articulated skeleton
Sues (2002)
Kayentasuchus walkeri silty facies, Kayenta Early Jurassic: Partial skull and postcranial
Clark & Sues (2002) Fm./Arizona, ?Sinemurian skeleton
SW USA
Saltoposuchus connectens Löwenstein Fm./ Late Triassic: Four partial skulls and
Huene (1921) Baden-Württemberg, middle-late partially articulated
Germany Norian skeletons
Hallopus victor (Marsh ‘upper’ mbr, Morrison Late Jurassic: Partially articulated
1877) Fm./Colorado, Kimmeridgian postcranial skeleton
western USA
Junggarsuchus sloani Shishugou Fm./ Middle Jurassic: Complete skull and anterior
Clark et al. (2004) Xinjiang, western Callovian half of an articulated
China skeleton
Unnamed taxon Pol et al. Cañadón Calcáreo Fm./ Late Jurassic: late Posterior half of skull and
(2011, in press) Chubut, Patagonia, Oxfordian-earliest partial postcranium
Argentina Kimmeridgian
Macelognathus vagans Brushy Basin Mbr, Late Jurassic: Partial skull and skeleton of
Marsh (1884) Morrison Fm./ Kimmeridgian several individuals
Wyoming &
Colorado, western
USA
Pedeticosaurus leviseuri upper Elliot Fm./South Early Jurassic: Skull and articulated
Van Hoepen (1915) Africa ?Hettangian skeleton as natural
mould
Phyllodontosuchus Zhangjiawa Mbr., Early Jurassic: Nearly complete skull
lufengensis Lufeng Fm./Yunnan, ?Hettangian
Harris et al. (2000) China
Pseudhesperosuchus Los Colorados Fm./La Late Triassic: late Complete skull and partial
jachaleri Bonaparte (1969) Rioja, NW Argentina Norian postcranial skeleton
(Continued)
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EARLY CROCODYLOMORPHA 281

Table 1. Continued

Taxon Occurrence Age Material

Redondavenator Redonda Fm., Dockum Late Triassic: late Anterior portion of the
quayensis Nesbitt et al. Grp./New Mexico, Norian skull and
(2005) SW USA scapulacoracoid
Trialestes romeri (Reig Ischigualasto Fm./NW Late Triassic: late Partial skull elements,
1963) Argentina Carnian including partial
mandible, and
postcranial skeleton
(based on at least two
specimens)
CROCODYLIFORMES Late Triassic (late
Hay (1930) Norian) to Present

1972b) is from the overlying Clarens Formation portion of the formation (Rogers et al. 1993),
(Clark & Sues 2002). which places it near the Carnian-Norian bound-
ary at c. 227 Ma (Olsen et al. 2011). Thus, these
Oldest record data demonstrate the presence of crocodylomor-
phs by at least 228 Ma, during the latest Carnian
The oldest confirmed record of Crocodylomorpha (Fig. 1).
is material from the Ischigualasto Formation of Traditionally, the lower part of the Chinle
northwestern Argentina (Fig. 1). This includes the Formation has been assigned a late Carnian age
type and referred material of Trialestes romeri (e.g. Lucas 1993, 1998), which would place the
(Reig 1963; Bonaparte 1982a; Clark et al. 2001; type specimen of Hesperosuchus agilis (Colbert
Ezcurra et al. 2008), and newly discovered unde- 1952) in contention for the oldest crocodylo-
scribed material (Ezcurra et al. 2011). The sys- morph, because it is from the Cameron or Blue
tematic identity of Trialestes romeri has been Mesa Member (Nesbitt 2011), low in the Chinle
controversial, and a full review is outside of the Formation (Lucas 1993). These strata in the vicin-
scope of this paper. Nevertheless, although some ity of Cameron, Arizona, have never been radio-
referred material is dinosaurian, it is clear that isotopically dated. Tetrapod remains from the
the type specimen (PVL 2561) comprises a single Cameron Member (Heckert et al. 2002) are sugges-
individual that preserves unambiguous crocodylo- tive of, but not diagnostic for, an Adamanian bio-
morph synapomorphies such as an elongate radiale stratigraphic age, though vertebrate fossils from the
and ulnare, and that the referred specimen PVL overlying Blue Mesa Member in the area include
3889 shares one or more autapomorphies with the Adamanian index taxa (Long & Murry 1995).
holotype (Ezcurra et al. 2008). However, this is still not very helpful, because the
The Ischigualasto Formation is well-dated by age of the base of the Adamanian biozone is uncon-
40
Ar/39Ar radioisotopic ages to c. 231– 225 Ma strained by geochronologic data (Irmis et al. 2010,
(Rogers et al. 1993; Furin et al. 2006; Martinez 2011). Nonetheless, new radioisotopic ages from
et al. 2011), which indicates it ranges from late the Chinle Formation (Irmis et al. 2011; Ramezani
Carnian to earliest Norian (Walker & Geissman et al. 2011) indicate that the formation is wholly
2009; Olsen et al. 2011). The type specimen of Tria- Norian in age, with evidence that the lowest units
lestes romeri comes from near Aguada de la Peña (e.g. Shinarump Member) are probably no older
(Reig 1963), and the possible new taxon mentioned than 227 Ma (Dickinson & Gehrels 2009; Irmis
by Ezcurra et al. (2011) is from the Cancha de et al. 2011; Ramezani et al. 2011). This strongly
Bochas Member, placing both of these specimens suggests that the type specimen of Hesperosuchus
in the lower third of the formation, which has a agilis, which is from somewhat higher in the for-
late Carnian age between 231– 228 Ma (Fig. 1). In mation, is early Norian in age, and thus younger
contrast, the referred specimen of Trialestes rom- than the oldest crocodylomorph specimens from
eri mentioned by Bonaparte (1978, 1982b, 1997) the Ischigualasto Formation (Fig. 1).
was collected much further north from near Cerro
Las Lajas, in La Rioja Province, which is also the Youngest record
type locality for the earliest known ornithischian
dinosaur, Pisanosaurus mertii (Casamiquela 1967; The youngest well-dated records of non-
Bonaparte 1997). Though direct correlation is dif- crocodyliform crocodylomorphs are from the
ficult, this area is thought to be in the middle Upper Jurassic Morrison Formation of the western
 Downloaded from http://sp.lyellcollection.org/ at University of Michigan on November 1, 2014

282 R. B. IRMIS ET AL.

United States. These include the type and referred with high analytical uncertainty, and the method
material of Macelognathus vagans (Marsh 1884; does not use the Mattinson pre-treatment techni-
Ostrom 1971; Göhlich et al. 2005) and the type que of chemical abrasion and thermal annealing,
and only known specimen of Hallopus victor which is the only reliable method for ameliorat-
(Marsh 1877; Walker 1970; Ague et al. 1995) ing the effects of lead loss (Mundil et al. 2004;
(Fig. 1). Hallopus victor is from Morrison For- Mattinson 2005). The potential bias of lead loss
mation exposures near Garden Park, Colorado. introduces additional uncertainty (≫1%) not
Although there has been quite a bit of historical included in the +2 Ma analytical error reported
controversy over whether the type and only known by Trujillo et al. (2006). Thus, although this new
specimen was found in the Morrison Formation age is intriguing, it does not provide unambiguous
or older underlaying strata, stoked in part by evidence that Quarry Nine and the ‘upper mem-
Marsh’s own uncertainty (Marsh 1877, 1891; Will- ber’ of the Morrison Formation in Wyoming is
iston 1905; Schuchert 1939; Ague et al. 1995), older than the Brushy Basin Member dated by
recent geologic work indicates it was found in the Kowallis et al. (1998). Therefore, the type specimen
upper portion of the informal ‘upper member’ of of Macelognathus vagans can only be constrained
the Morrison Formation (Ague et al. 1995). An to a broad age of 156–150 Ma (Kimmeridgian to
40
Ar/39Ar radioisotopic age of 152.77 + 0.30 Ma earliest Tithonian) (Fig. 1).
(Kowallis et al. 1998; recalculated to account for Referred material of Macelognathus vagans
revised age of standard and K –Ar decay constant from the Fruita Paleontological Area of western-
bias following the method of Renne et al. 2010) most Colorado is from the base of the Brushy
from the base of the ‘upper member’ in Garden Basin Member (Göhlich et al. 2005; Kirkland
Park provides a maximum age constraint for 2006). Although this particular section has not
Hallopus victor; and there is no indication that the been radioisotopically dated (though Kowallis
top of the Morrison Formation is younger than et al. 1998 discusses an unsuccessful attempt), an
c. 150 Ma (based on recalculated 40Ar/39Ar ages of age of 152.60 + 0.30 Ma from a nearby section in
Kowallis et al. 1998). This indicates that Hallopus Utah (recalculated from Kowallis et al. 1998)
victor is latest Kimmeridgian to earliest Tithonian indicates that the base of the Brushy Basin Mem-
in age (Walker & Geissman 2009) (Fig. 1). ber is 152–153 Ma, and therefore late Kimmer-
The type specimen of Macelognathus vagans idgian in age (Walker & Geissman 2009). This
is from Quarry Nine of Como Bluff in southeast- demonstrates that the Fruita material of Macelog-
ern Wyoming (Marsh 1884; Ostrom 1971; Göhlich nathus vagans is older than Hallopus victor, and
et al. 2005). This locality is within the middle therefore Hallopus victor is the youngest well-dated
portion of the ‘upper member’ (different from the specimen of a non-crocodyliform crocodylomorph,
informal unit in Garden Park, CO with the same between 152–150 Ma in age (Fig. 1).
name) of the Morrison Formation (Turner & Peter- The new Junggarsuchus-like taxon from the
son 1999). Unfortunately, there are no published Cañadón Calcáreo Formation of Chubut, Argen-
radioisotopic ages from the Morrison Formation tina (Pol et al. 2011, in press) is also Late Jurassic
of Wyoming (cf. Kowallis et al. 1998). The ‘upper in age. Although the middle part of the formation
member’ of southeastern Wyoming has been contains a palynomorph assemblage previously
broadly correlated with the Brushy Basin Member assigned a Cretaceous age (Volkheimer et al.
in Utah and western Colorado using a regional 2008, 2009), new precise CA-TIMS U– Pb ages
change in clay mineralogy (Turner & Peterson from nearly the same level as the crocodylomorph
1999) and a regionally extensive paleosol horizon indicate an age of 157–158 Ma (Cúneo et al. in
(Demko et al. 2004), though recent work suggests press), placing it near the Oxfordian-Kimmeridgian
that the clay-based correlation is not straight- boundary. Thus, the new crocodylomorph is almost
forward because there are multiple clay changes certainly several million years older than the Mor-
throughout the Morrison Formation whose pres- rison Formation crocodylomorphs described above.
ence/absence vary regionally (Trujillo 2003, 2006).
Trujillo et al. (2006) recently reported a Palaeoenvironmental context
SHRIMP U –Pb weighted mean age of 156.3 + 2
Ma from the same stratigraphic horizon as Quarry Early crocodylomorphs (i.e. those taxa outside of
Nine in southeastern Wyoming. At face value, this Crocodyliformes), span a wide temporal and geo-
would suggest that this portion of the ‘upper mem- graphic range, from 230 Ma during the early
ber’ is older than classic Brushy Basin Member Late Triassic (late Carnian) to c. 150 Ma in the
sections in Utah and western Colorado, which are latest Jurassic (Kimmeridgian-Tithonian), and five
dated to between 153 –150 Ma (based on recal- continents (North America, South America, Eur-
culated 40Ar/39Ar ages of Kowallis et al. 1998). ope, Africa, and Asia) (Table 1; Fig. 1). Thus, they
However, SHRIMP ages are small spot analyses encompass a wide variety of environments both
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EARLY CROCODYLOMORPHA 283

because of their latitudinal distribution and the (Kent & Tauxe 2005; Sellwood & Valdes 2006;
major tectonic changes that occurred during the Whiteside et al. 2011). These data indicate that
first half of the Mesozoic as the supercontinent at least within an order of magnitude, early Meso-
Pangaea began to break apart. zoic crocodylomorphs tolerated a wide range of
By the end of the Late Triassic, crocodylo- climatic conditions.
morphs had already achieved a widespread latitu- The local paleoenvironmental context of early
dinal distribution (Fig. 1), despite being relatively crocodylomorphs is dominated by fluvial and
rare components of assemblages. The lowest pala- lacustrine deposition. Late Triassic taxa are found
eolatitude record is that of Dromicosuchus gral- largely in fluvial and floodplain facies, including
lator, from the Durham Sub-Basin of the Deep Hesperosuchus agilis from the Chinle Formation
River Basin in North Carolina, USA. During much of Arizona (Stewart et al. 1972; Heckert et al.
of the Late Triassic, this area was equatorial (Kent 2002), taxa from Ghost Ranch in northern New
& Tauxe 2005; Whiteside et al. 2011), though the Mexico (Stewart et al. 1972; Dubiel 1989; Sch-
exact paleolatitude is unknown. Traditionally the wartz & Gillette 1994), and crocodylomorphs from
unit containing Dromicosuchus grallator (Litho- the Ischigualasto and Los Colorados formations
facies Association II) was thought to be early of northwestern Argentina (Caselli et al. 2001;
Norian because it was correlated with the Sanford Arcucci et al. 2004; Currie et al. 2009). Other speci-
Formation of the adjacent Sanford Sub-Basin mens are found in lacustrine margin sediments or
and Passaic Formation of the Newark Basin interbedded fluvial/lacustrine depositional envi-
(Huber et al. 1993; Olsen & Huber 1997), but new ronments, including Redondavenator quayensis
calibration of the Newark Basin Astrochronology (Hester & Lucas 2001; Nesbitt et al. 2005), Dro-
and Geomagnetic Polarity Time Scale (Newark micosuchus grallator (Olsen et al. 1989; Olsen &
AGPTS) demonstrates that these correlative units Huber 1997; Sues et al. 2003), and Saltoposuchus
are instead middle Norian in age (Muttoni et al. connectens (Hungerbühler 1998). Among all
2004; Olsen et al. 2011). This would place Dromi- early crocodylomorphs, the one outlier in deposi-
cosuchus grallator at 2– 38N palaeolatitude (Kent tional environment is the latest Triassic taxon
& Tauxe 2005; Whiteside et al. 2011); however, Terrestrisuchus gracilis from SW Britain, which is
the underlying correlations between the Durham preserved in fissure fill sediments from a lowland
Sub-Basin and other Newark Supergroup units area near the coast (Marshall & Whiteside 1980;
are very tentative (Olsen 1977; Olsen et al. 1989; Whiteside & Robinson 1983; Crush 1984; Fraser
Huber et al. 1993; Olsen & Huber 1997). An early 1994; Whiteside & Marshall 2008). Not only
Norian age for Dromicosuchus grallator would does this represent the only record that is not
place it at 2–48S palaeolatitude. Nonetheless, even from fluvio-lucustrine environments, but it is the
with such temporal uncertainty, it is apparent that only coastal record of a non-crocodyliform
this early crocodylomorph lived within five crocodylomorph.
degrees of the Late Triassic equator. Late Triassic Early Jurassic crocodylomorphs are also found
Carnian-Norian records from Argentina (Trialestes predominantly in fluvial and floodplain environ-
romeri and Pseudhesperosuchus jachaleri), the ments, particularly those from the upper Elliot
Early Jurassic of South Africa (Sphenosuchus Formation of southern Africa (Bordy et al. 2004a,
acutus, Litargosuchus leptorhynchus, and Pedetico- b; Smith & Kitching 1997; Smith et al. 2009) and
saurus leviseuri), and the late Norian of Germany the Kayenta Formation of Arizona (Clark & Fas-
(Saltoposuchus connectens) (Fig. 1) represent the tovsky 1986; Sues et al. 1994; Behrensmeyer &
highest-latitude early Mesozoic records at 40 –508 Whatley 2008). The depositional environment of
palaeolatitude in the southern and northern hemi- crocodylomorphs from the Zhangjiawa Member
spheres, respectively. Middle and Late Jurassic (‘deep red beds’) of the Lufeng Formation in
records of non-crocodyliform crocodylomorphs Yunnan Province (China) is considered fluvial and
(Fig. 1) are much rarer, and therefore little can be lacustrine in a general sense (Bien 1940, 1941;
said about latitudinal distribution other than they Young 1951; Sun et al. 1985; Luo & Wu 1994),
are at least present in middle (Hallopus victor and but there are no detailed sedimentological studies
Macelognathus vagans) and high (Junggarsuchus available either generally for the formation or
sloani and Cañadón Calcáreo taxon) latitudes specifically for the main fossil localities. The sole
during this time. Middle Jurassic record, Junggarsuchus sloani from
The latitudinal separation of early crocodylo- Xinjiang Province (China), was found in fluvial
morphs reflects in part their general climate toler- sediments (Eberth et al. 2001), as were the Late
ances. During the Late Triassic and Early Jurassic, Jurassic taxa Hallopus victor (Ague et al. 1995)
equatorial regions were generally warm and and Macelognathus vagans (Kirkland 2006) from
humid, whereas the middle latitudes were warm the Morrison Formation of the western United
and arid, and the high latitudes cooler and humid States. Similarly, the new Junggarsuchus-like
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284 R. B. IRMIS ET AL.

form discovered in the Upper Jurassic Cañadón material of Saltoposuchus connectens is mini-
Calcáreo Formation of Patagonia, Argentina (Pol mally several million years older, from the middle
et al. 2011, in press) is preserved in fluviolacustrine to late Norian Löwenstein Formation (middle Stu-
strata (Volkheimer et al. 2009). bensandstein) of Germany (Schoch & Wild 1999;
The predominance of early crocodylomorphs Seegis 2005). Second, the available ontogenetic
in fluviolacustrine rocks almost certainly reflects indicators are not entirely consistent. Extant croco-
largely a preservational control rather than envi- dylians show a posterior to anterior progression
ronmental/ecological preference. These environ- in the closure of neurocentral sutures in the verte-
ments are ideal for preserving small terrestrial bral column during ontogeny, where distal caudal
tetrapods (e.g. Schwartz & Gillette 1994; Kirkland sutures close first, and cervical sutures close last
2006; Behrensmeyer & Whatley 2008), and a more (Brochu 1996); this pattern may also apply to
detailed examination shows that they vary widely other psuedosuchian archosaurs (Irmis 2007). In
in specific environmental conditions. For exam- the holotype of Terrestrisuchus gracilis (NHMUK
ple, Trialestes romeri from the Ischigualasto For- R7557), the articulated distal caudal vertebrae
mation of NW Argentina lived in relatively cool display closed neurocentral sutures, whereas dis-
humid conditions (Currie et al. 2009), whereas articulated mid- and anterior caudal, dorsal, and cer-
taxa from the uppermost Chinle Formation of New vical vertebrae all display open sutures. Allen’s
Mexico lived in a warm and arid environment (2003a) referred specimen of Terrestrisuchus graci-
(Schwartz & Gillette 1994), as did Early Jurassic lis (NHMUK R10002) is the largest-known spec-
taxa from South Africa and Lesotho (Bordy et al. imen of the taxon, and only preserves the caudal
2004a, b; Smith et al. 2009). Like the paleolatitudi- part of the vertebral column. Proximal caudal ver-
nal data, these occurrences suggest early croco- tebrae 1–5 display open neurocentral sutures,
dylomorphs had a broad environmental tolerance. whereas the rest of the tail (mid- and distal cau-
dals) display closed sutures. In contrast, the holo-
Synonymy of Saltoposuchus and type of Saltoposuchus connectens (SMNS 12597)
preserves open sutures in the proximal caudal ver-
Terrestrisuchus tebrae through at least caudal number 8. This sug-
Given the recent popularity of proposing that gests that NHMUK R10002 could be more mature
some Mesozoic archosaurs from similar geographic than SMNS 12597, even though the type of Salto-
areas and geologic ages may represent different posuchus connectens is significantly larger, though
ontogenetic morphs of the same taxon (e.g. Knoll we cannot rule out that this difference is from
et al. 2009; Horner & Goodwin 2009; Scannella individual variation in the timing of suture closure
& Horner 2010; Campione & Evans 2011; Woo- (see discussion in Brochu 1996 and Irmis 2007).
druff & Fowler 2012), it is worth briefly addressing Nonetheless, open neurocentral sutures in the prox-
a recent hypothesis regarding early crocodylo- imalmost caudal vertebrae of all three specimens
morphs; namely, that Terrestrisuchus gracilis suggest they are all juvenile individuals.
from the latest Triassic fissure fills of the United A final difficulty with this hypothesis relates
Kingdom are juveniles of Saltoposuchus connec- to hindlimb scaling. The femur of the type of Ter-
tens from the Löwenstein Formation (Late Trias- restrisuchus gracilis (NHMUK R7557) is 57.5 mm
sic: Norian) of Germany (Allen 2003a, b). This long, with a mid-diaphyseal diameter of 2.65 mm,
hypothesis was based on the observation that the and the femur of NHMUK R10002 is 64 mm long
holotype (NHMUK R7557) and a referred speci- and has a mid-diaphyseal diameter of 3.6 mm. In
men (NHMUK R10002) of Terrestrisuchus graci- contrast, the femur of the type of Saltoposuchus
lis appear to display some juvenile characteristics, connectens (SMNS 12597) is 78.4 mm long with a
and therefore Allen (2003a, b) interpreted the dif- mid-diaphyseal diameter of 9.0 mm. If Terrestri-
ferences between Terrestrisuchus gracilis and suchus gracilis grew into Saltoposuchus connec-
Saltoposuchus connectens discussed by Clark et al. tens, it would suggest a remarkable allometric
(2001) to be ontogenetic rather than phylogenetic scaling ratio (0.3–0.4) between mid-shaft diameter
characters. It also appeared consistent with the his- and femur length, double the same scaling ratio
tology of a single Terrestrisuchus gracilis speci- in modern crocodylians (Dodson 1975; Bonnan
men described by de Ricqlès et al. (2003, 2008), et al. 2008). However, extant crocodylians have a
which displayed the fast growth expected for a sprawling stance, whereas early crocodylomorphs
juvenile. are thought be cursorial with an upright stance,
We hypothesize this synonymy is unlikely. so might this be the reason for such a different
First, the two taxa are from different localities and allometric relationship? It seems unlikely, because
geologic ages; Terrestrisuchus gracilis is from the modern upright cursorial quadrupeds have a scal-
Rhaetian of the United Kingdom (Whiteside & ing relationship between mid-shaft diameter and
Marshall 2008), whereas the type and referred femur length that is also very different (McMahon
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EARLY CROCODYLOMORPHA 285

1975) from that of a hypothesized Terrestrisuchus- of northern New Mexico (Clark et al. 2001), as
Saltoposuchus ontogenetic trajectory. Thus, we well as previously described specimens initially
conclude that the available evidence is inconsis- assigned to other taxa (e.g. UCMP 129470 from Pet-
tent with the idea that Terrestrisuchus gracilis is rified Forest National Park in Arizona). Previous
a juvenile of Saltoposuchus connectens (contra cladistic analyses were consistent with the idea
Allen 2003a, b). that all of these specimens are the same species
One of the difficulties in evaluating Saltopo- (Fig. 2e), as hypothesized by Clark et al. (2001),
suchus connectens is that the exact content of the or closely related separate taxa (Nesbitt 2011)
hypodigm from the ‘Weisser Steinbruch’ south of (Fig. 2i). We argue, following Nesbitt (2011), that
Pfaffenhofen, Baden-Württemberg (Germany), is the holotype and CM 29894 should be analyzed
controversial (e.g. Sereno & Wild 1992; Chatter- separatedly, for several reasons. First, some of the
jee 1993; Rauhut & Hungerbühler 2000; Clark earliest diverging crocodylomorphs have conser-
et al. 2001; Knoll 2008; Knoll & Rohrberg 2012). vative body plans and can only be differentiated
This is in part because of variable quality of pre- when comparing nearly complete material. For
servation and confusion caused by superficial example, there are only a few small differences
similarities between early theropods and early cro- between CM 29894, Dromicosuchus grallator
codylomorphs, but it is also because the specimens (Sues et al. 2003), and the holotype of Hesperosu-
in question do not preserve many overlapping chus agilis, yet they have differing placements
cranial elements, and when they do, the elements within early crocodylomorph phylogeny (Nesbitt
are often not preserved in the same anatomical 2011). Second, if both the holotype of Hespero-
view (i.e. dorsal v. ventral view). In addition, differ- suchus agilis and CM 29894 are referable to the
ential crushing has affected the various sets of same species, the range of Hesperosuchus agilis
cranial remains. Much of this material has been would extend through the entire deposition of the
extensively reprepared using modern techniques, Chinle Formation (c. 20 myr) (see Irmis et al. 2011
including the type skull of cf. Halticosaurus orbi- and Ramezani et al. 2011), which seems unlik-
toangulatus, which was recently reclassified as a ely. Because Triassic crocodylomorphs of North
crocodylomorph (Rauhut & Hungerbühler 2000); America were conservative in morphology for
new research indicates this taxon is a basal lori- much of their history, but do not form a monophy-
catan pseudosuchian rather than a crocodylo- letic clade (e.g. Nesbitt 2011), morphologically
morph (Sues & Schoch in press). These new data similar but taxonomically distinct taxa can easily
should help resolve the alpha taxonomy of the be mistaken for a single species-level taxon. There-
Pfaffenhofen crocodylomorph material and clarify fore, it is likely that the type of Hesperosuchus
the composition of the hypodigm of Saltoposuchus agilis and CM 29894 represent different taxa.
connectens.
Taxa once assigned to Early
The content of Hesperosuchus Crocodylomorpha
Hesperosuchus agilis was initially described as The clade Crocodylomorpha was defined phylo-
an ornithosuchid pseudosuchian (Colbert 1952) genetically only recently and as a result, there
but was recognized as an early crocodylian rela- have been a few taxa assigned to the group that
tive by Walker (1970). This revised systematic clearly do not belong to the taxon. The following
placement has been supported by all subsequent taxa were originally hypothesized to be relatives
analyses. Hesperosuchus agilis is known from a based overall similar body form.
partial skeleton (AMNH FARB 6758) that includes
portions of the skull and many parts of the post- Erpetosuchus granti. Although long-recongized as
cranial skeleton, but much of the skull and tail a pseudosuchian archosaur, the specific hypoth-
is missing. Recently located additional screen- esized phylogenetic relationships of Erpetosuchus
washed material, which was found with the speci- granti Newton 1894, from the Elgin Sandstone of
men during the original excavation, includes more Scotland, have been unstable for over a century.
of the skull and skeleton that was not previously Although Newton (1894) noted similarities with
described (S. J. Nesbitt, pers. obs). crocodyliforms, he thought it was more closely
For most of the last six decades, Hesperosuc- related to phytosaurs, which are now thought to be
hus agilis was an excellent model for compari- either early pseudosuchians or stem archosaurs
son with all other early crocodylomorphs given its (Nesbitt 2011). Walker (1968) proposed that Erpe-
relative completeness and exquisite preservation. tosuchus granti was an early crocodylomorph, but
For that reason, newly discovered specimens from then changed his mind and suggested it was not
the Chinle Formation were assigned to this taxon, a particularly close relative of crocodylomorphs
such as CM 29894 from the Coelophysis Quarry (Walker 1970). A well-preserved partial skull of
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286 R. B. IRMIS ET AL.

Erpetosuchus granti from the Upper Triassic earlier-diverging lineage near or just outside of the
(Norian) New Haven Formation of Connecticut base of Archosauria (Walker 1968; Nash 1968).
has provided additional information on the cranial This position outside Archosauria has been con-
styructure of this taxon (Olsen et al. 2001). Early firmed by nearly all phylogenetic analyses (e.g.
cladistic phylogenetic analyses did not include Benton & Clark 1988; Sereno 1991; Parrish 1993;
Erpetosuchus granti (e.g. Benton & Clark 1988; Juul 1994; Benton 1999; Dilkes & Sues 2009;
Sereno 1991; Juul 1994), but two studies with Nesbitt et al. 2009; Brusatte et al. 2010; Nesbitt
limited taxon and character sampling did recover 2011).
it as the closest sister taxon of Crocodylomor-
pha (Olsen et al. 2001; Benton & Walker 2002). Platyognathus hsui. Discovered in the Lower Juras-
A more recent analysis of basal archosaurs also sic Lufeng Formation of Yunnan Province, China,
recovered this position (Brusatte et al. 2010). In Platyognathus hsui Young 1944, based on an iso-
contrast, a modified version of Nesbitt’s (2011) lated lower jaw of the now lost holotype, was
analysis, which has by far the most extensive taxon initially considered a pseudosuchian archosaur of
and character sampling of any phylogenetic data- indeterminate affinities. Simmons (1965) referred
set for basal archosaurs, recovers Erpetosuchus additional material to the taxon and proposed that
granti in a well-supported position as an early it was intermediate between classic psuedosuchians
suchian, only distantly related to Crocodylomor- and ‘protosuchian’ crocodyliforms, a lineage we
pha, and sister taxon to Parringtonia gracilis from might today consider an early crocodylomorph or
the Middle Triassic of Tanzania (Nesbitt & Butler ‘sphenosuchian.’ Though many authors recognized
2013). the crocodyliform nature of Platyognathus hsui
(see review in Wu & Sues 1996b), Walker (1968,
Gracilisuchus stipanicicorum. The Middle Trias- 1970) also proposed that it was a ‘sphenosuchian’,
sic Argentine taxon Gracilisuchus stipanicicorum though he later agreed with a ‘protosuchian’ inter-
Romer 1972 has suffered a similar phylogenetic pretation (Walker 1990). Clark (1986) and Wu &
fate as Erpetosuchus granti; it has variously been Sues (1996b) reviewed all available material, and
regarded as an early-diverging pseudosuchian or though they concluded it represented at least two
a taxon very closely related to Crocodylomorpha. taxa (with Simmons’ material not referable to Pla-
Initially described as an ornithosuchid (Romer tyognathus hsui), both sets of authors did confirm
1972), Brinkman (1981) suggested a possible rela- that all specimens preserve a number of unambi-
tionship with crocodylomorphs. Most phylogenetic guous crocodyliform synapomorphies.
analyses have recovered Gracilisuchus stipanici-
corum in an intermediate position, as sister group Problematic taxa
to some combination of ‘rauisuchians’ (e.g. Posto-
suchus) and crocodylomorphs (Benton & Clark Barberenasuchus brasiliensis. Mattar (1987) named
1988; Parrish 1993; Juul 1994; Olsen et al. 2001; Barberenasuchus brasiliensis from a poorly pre-
Benton & Walker 2002), yet Brusatte et al. (2010) served skull and axis vertebra and hypothesized
found Gracilisuchus stipanicicorum as the sister a relationship among ‘sphenosuchians’ (see also
taxon to the clade Erpetosuchus granti + Croco- Mattar 1989). The rational for the taxonomic assign-
dylomorpha, and it has been used as an outgroup ment is unclear. Recently, two of the authors of
for a number of crocodylomorph phylogenetic this paper (RBI and SJN) personally observed this
analyses (Fig. 2) (e.g. Clark et al. 2001, 2004; Pol specimen. We could not find any crocodylomorph
& Norell 2004a, b; Gasparini et al. 2006; Pol & character states preserved in the specimen and an
Powell 2011). In contrast, recent exhaustive early assignment to a more exclusive clade than Archo-
archosaur phylogenetic analyses recover Gracilisu- sauriformes (an antorbital fenestra is present) is
chus stipanicicorum as an early-diverging suchian, not possible without further preparation. Further-
well-outside the least inclusive clade containing more, the teeth identified as belonging to the ptery-
crocodylomorphs and ‘rauisuchians’ sensu lato (i.e. goid by Mattar (1987, 1989) are actually in the
including poposauroids) (Nesbitt 2011; Nesbitt & left maxilla that has been crushed into the skull.
Butler 2013).
Dyoplax arenaceus. Dyoplax arenaceus Fraas (1867)
Proterochampsa barrionuevoi. Because of the is known from a single specimen preserved as a
overall appearance of its cranium (i.e. elongate natural cast in sandstone from the Stuttgart Forma-
and platyrostral snout), Proterochampsa barrionue- tion (Schilfsandstein) of southern Germany. The
voi Reig 1958 from the Upper Triassic Ischigual- details of the anatomy are poorly preserved, which
asto Formation of Argentina was initially thought has led to a considerable debate about its relation-
to be ancestral to crocodylians (e.g. Reig 1958; ships; the taxon was referred to aetosaurs (Zittel
Sill 1967), but it was quickly recognized to be an 1890; Huene 1902; McGregor 1906), a protosuchid
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EARLY CROCODYLOMORPHA 287

crocodyliform (Benton 1994), an erpetosuchid included a few early crocodylomorphs (Juul 1994;
(Walker 1961, 1968; Romer 1966; Carroll 1988), Benton 1999; Brusatte et al. 2010), and studies of
and most recently, a ‘sphenosuchian’ crocodylom- crocodyliforms only included one or two early cro-
orph (Lucas et al. 1998; Benton & Walker 2002). codylomorphs or the group ‘Sphenosuchidae’ as a
The middle Carnian age of Dyoplax arenaceus composite outgroup (e.g. Benton & Clark 1988;
(Lucas et al. 1998; Kozur & Bachmann 2010) is Clark 1994; Pol et al. 2004a, b). The pioneering
particularly important if the taxon is a crocodylo- work of Clark (1986) and Benton & Clark (1988)
morph because this would be the oldest occurrence followed by subsequent studies (Parrish 1991;
for the clade. The assignment to a protosuchid Sereno & Wild 1992; Wu & Chatterjee 1993;
crocodyliform was never properly justified and the Clark et al. 2001, 2004; Clark & Sues 2002; Sues
evidence for assignment to ‘Sphenosuchia’ (Lucas et al. 2003; Nesbitt 2011) focused specifically
et al. 1998) was weak (Clark et al. 2001; Göhlich on the anatomy and interrelationships of early cro-
et al. 2005). Therefore, the relationships of Dyo- codylomorphs (see below). From these studies, it
plax arenaceus are not clear, though it can be became clear that Crocodylomorpha is diagnosed
clearly assigned to Archosauriformes based on the by a number of apomorphies throughout the skull
presence of an antorbital fenestra. and skeleton that unambiguously distinguish this
clade from all other groups of pseudosuchians. For
Parrishia mccreai. The holotype of Parrishia example, in the skull, the quadrate attaches to the
mccreai consists of a single cervical vertebra from prootic of the braincase (Clark 1986; Walker
a disarticulated topotypic series of vertebrae found 1990), the bones of the braincase are highly pneu-
in the Placerias Quarry of the lower Chinle For- matized as a whole (Walker 1990; Clark et al. 2004),
mation in Arizona, USA (Long & Murry 1995). Cur- and the squamosal laterally overhangs much of the
rently, most authors that have commented on the posterior portion of the skull (Clark 1986; Clark &
validity of this species have concluded that it is Sues 2002) (Figs 3 & 4); in the postcranial skele-
not diagnostic (Clark et al. 2001; Parker & Irmis ton, crocodylomorphs have elongated proximal
2005; Irmis 2005) and it is not clear if the cervi- carpals (radiale and ulnare) (Fig. 5) (Clark 1986),
cal vertebra character states point to a crocodylo- but few other unique character states that differen-
morph affinity or to a more inclusive group within tiate them from their suchian relatives. This suite
Archosauria. of unique and plesiomorphic morphologies has
hampered the identification of the closest sister
Alwalkeria maleriensis. The type and only known group or successive outgroups of Crocodylomor-
specimen of Alwalkeria maleriensis (Chatterjee pha. As a result, the relationships of early crocody-
1987) was originally described as an early theropod lomorphs and the optimizations of character states
dinosaur (‘podokesaurid’), and until recently was varied tremendously across each analysis (Fig. 2).
considered a valid saurischian dinosaur taxon (e.g. The identification of the closest relatives of
Langer 2004), but it has never been included in Crocodylomorpha has been a difficult challenge.
a quantitative phylogenetic analysis. Remes & Among large basal archosaur phylogenetic ana-
Rauhut (2005) recently reexamined the material lyses, taxa commonly proposed as the sister taxon
and suggested that the holotype is a chimaera, and of Crocodylomorpha include those classically
at least some of the materials are referable to a cro- referred to as ‘rauisuchians’ (likely a para- or poly-
codylomorph, but this study has not yet been phyletic group, see Nesbitt 2011 and Nesbitt et al.
published. 2013) such as Postosuchus (Benton & Clark 1988;
Parrish 1993; Benton 2004; Weinbaum & Hunger-
buhler 2007), the enigmatic taxon Erpetosuchus
Relationships and evolution granti (Olsen et al. 2001; Benton & Walker 2002;
Origin of Crocodylomorpha Brusatte et al. 2010), and other clades of pseudo-
suchians such as Ornithosuchidae + Rauisuchia
Walker (1990) ended his comprehensive contri- (Nesbitt & Norell 2006; Nesbitt 2007) or Aetosau-
bution on the anatomy of the early crocodylomorph ria (Gower & Walker 2002; Gower 2002). Most
Sphenosuchus acutus with a short section on the recently, Nesbitt (2011) found a sister taxon rela-
origin of Crocodylomorpha, and specifically stated tionship between Rauisuchidae (sensu Sereno
that ‘little can be said on this topic.’ At the time, et al. 2005), which includes Postosuchus, and cro-
this statement was forthright and honest, given codylomorphs, in an analysis that used only
that few fossils of early crocodylomorphs were species-level taxa. In the hypothesis proposed by
known, and even fewer were completely prepared Nesbitt (2011), crocodylomorphs and rauisuchids
and/or fully described. Early crocodylomorphs such as Postosuchus kirkpatricki share a number
occupy a phylogenetic ‘middle ground’ where large of features in the postcranial skeleton including
phylogenetic analyses of early archosaurs only (but not limited to) an expanded anterior portion
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288 R. B. IRMIS ET AL.

(a) (b)

1 cm

1 cm

(c) (d)

1 cm
(f)
1 cm

(e)

1 cm

5 cm

(g) (h)

5 cm
5 cm

(i)

1 cm
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EARLY CROCODYLOMORPHA 289

of the lower jaw, posteriorly hooked proximal clade to the exclusion of Crocodyliformes, or are
head of the humerus, and a deep groove on the a paraphyletic assemblage with some forms more
ventral side of the coracoid. Shared cranial syna- closely related to crocodyliforms than others (Fig.
pormophies include a foreshortened lower temporal 2). Despite the discovery of new specimens with
fenestra (or expanded quadratojugal), eustachian nearly complete remains (e.g. Clark et al. 2001,
tubes partially enclosed in bone, and other fea- 2004; Sues et al. 2003), there seems to be no tem-
tures of the braincase (Nesbitt 2011). Even though poral pattern in the evolution of these datasets
rauisuchids are recovered as the closest relatives as to whether they recover a monophyletic (Ser-
by Nesbitt (2011), early crocodylomorphs are sig- eno & Wild 1992; Wu & Chatterjee 1993; Sues
nificantly disparate from this clade and conse- et al. 2003) or paraphyletic (Benton & Clark 1988;
quently well supported as a monophyletic group. Parrish 1991; Clark et al. 2001, 2004; Clark &
Recent work on Erpetosuchus granti (Nesbitt Sues 2002; Nesbitt 2011) Sphenosuchia (Fig. 2).
& Butler 2013) demonstrates that most character Even iterations of the same dataset that only dif-
states cited as synapomorphies of Erpetosuchus fer in the addition of new taxa (but not characters)
granti + Crocodylomorpha (Benton & Walker are not consistent in recovering the same phyloge-
2002) have either a much wider distribution or netic signal, with very poorly-supported topolo-
were interpreted based on the only known skull gies (e.g. Clark et al. 2001, 2004 Clark & Sues
of Erpetosuchus granti, which shows few details 2002; Sues et al. 2003) (Fig. 2f–h). With the dis-
because it is a natural mold in a rather coarse- covery of new ‘sphenosuchian’ taxa that share a
grained sandstone. The proposal that aetosaurs number of character-states with crocodyliforms
are the sister group of Crocodylomorpha (Gower (e.g. Clark et al. 2004; Pol et al. 2011, in press), it
& Walker 2002; Gower 2002) is an important seems likely that ‘Sphenosuchia’ sensu lato is para-
hypothesis driven by similarities in the braincase phyletic (Nesbitt 2011) (Fig. 2i), though there is a
between the aetosaur Stagonolepis robertsoni and chance that some early crocodylomorph taxa form
the crocodylomorph Sphenosuchus actutus (Wal- smaller clades along the spine of the tree.
ker 1972; Gower & Walker 2002; Gower 2002) Some of this phylogenetic instability might rel-
and several other cranial features (Gower 2002). ate to outgroup selection. Early on, authors examin-
Gower (2002) used a phylogenetic dataset com- ing early crocodylomorph relationships adopted
posed solely of braincase characters to illustrate what are now unorthodox approaches to outgroup
this relationship; these were later integrated into selection. These included an ‘all-zero hypotheti-
the larger analyses of Brusatte et al. (2010) and cal ancestor’ (Sereno & Wild 1992) (Fig. 2c) and
Nesbitt (2011). Aetosaurs were found as the sister compositely-coded taxa (Poposauria, Parrish 1991;
group to the clade of Erpetosuchus granti + Cro- Crocodyliformes, Wu & Chatterjee 1993) (Fig. 2b,
codylomorpha in Brusatte et al. (2010) whereas d). More recent analyses used multiple basal pseu-
aetosaurs were found to be basal suchians and dosuchian taxa (Clark et al. 2001 and iterations
more distantly related to Crocodylomorpha by Nes- of that dataset) (Fig. 2e–h), but until very recently
bitt (2011). it was not clear which pseudosuchians were most
appropriate outgroups. This lack of clarity in
Monophyly or paraphyly of Sphenosuchia early archosaur relationships (discussed in the
previous section) led to unusual polarization of
The interrelationships and basic structure of early crocodylomorph characters, and thus uncertainty
crocodylomorph phylogeny has little consensus in early crocodylomorph phylogenetic relation-
despite a large number of studies with significant ships. With more exhaustive character and taxon
overlap in taxon and character sampling (Fig. 2). sampling throughout early archosaurs, proximate
Regardless of the relationships of individual taxa, sister groups to crocodylomorphs are better resolved
early crocodylomorph phylogenetic disagreements (Nesbitt 2011), which should help with improved
centre on whether non-crocodyliform crocodylo- outgroup selection and character polarization for
morphs, i.e. ‘sphenosuchians’, form a monophyletic crocodylomorph phylogenetic studies.

Fig. 3. Skeletal material of early crocodylomorphs. (a) skull of CM 29894, ‘Hesperosuchus’ from the Coelophysis
Quarry, Ghost Ranch, New Mexico, USA (Clark et al. 2001); (b) PVL 3830, skull of the holotype of
Pseudhesperosuchus jachaleri; (c) SMNS 12352, referred partial skull of Saltoposuchus connectens; (d) IVPP V14010,
skull of the holotype of Junggarsuchus sloani; (e) NHMUK R7557, holotype partial skull and skeleton of
Terrestrisuchus gracilis; (f ) BP/1/5237, holotype skull and skeleton of Litargosuchus leptorhynchus; (g) UNC 15574
(now housed at North Carolina State Museum), holotype skull and skeleton of Dromicosuchus grallator; (h) NMMNH
P-25615, premaxillae, nasals, and anterior portion of the maxillae of the holotype of Redondavenator quayensis; and
(i) MCZ 6727, referred skull of the early crocodyliform Protosuchus richardsoni. All photos by the authors.
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290 R. B. IRMIS ET AL.

(a) ‘sphenosuchians’ such as Junggarsuchus sloani

(Clark et al. 2004; Nesbitt 2011; Pol et al. 2011,
in press) (Fig. 3d), fossil evidence indicates that
the crocodyliform lineage diverged much earlier
(see below, Fig. 1).
The earliest unambiguous crocodyliform fossil
is the only known skull and skeleton of Hemipro-
tosuchus leali from the upper Los Colorados For-
1 cm mation of Argentina (Bonaparte 1972a; Arcucci
et al. 2004). These strata have recently been dated
to 217–215 Ma (Santi Malnis et al. 2011), indicat-
(b) ing that Crocodyliformes originated by the middle
Norian, less than 15 million years after the earliest
crocodylomorphs. Yet, there is a significant gap in
the early crocodyliform fossil record, because no
other specimens have been described from Trias-
sic strata; the next record is Protosuchus richard-
soni from the earliest Jurassic upper part of the
Moenave Formation of the southwestern USA
(Colbert & Mook 1951; Sues et al. 1994; Lucas &
Tanner 2007; Donohoo-Hurley et al. 2010; Lucas
1 cm et al. 2011) (Figs 3i, 4c & 5b).
Hettangian –Pliensbachian fossil assemblages
(c)
throughout Pangaea are dominated by Protosuchus
richardsoni and related forms, with occurrences
in the southwestern USA (Colbert & Mook 1951;
Sues et al. 1994; Tykoski 2005; Irmis 2005), east-
ern North America (Shubin et al. 1994; Sues et al.
1996), southern Africa (Nash 1968, 1975; Whet-
stone & Whybrow 1983; Busbey & Gow 1984;
Gow 2000), and China (Simmons 1965; Wu &
Sues 1996b). Despite this preponderance of early-
diverging forms, there is excellent evidence that
mesoeucrocodylians (including Notosuchia and
1 cm
Neosuchia) had diversified by the end of the Sine-
murian, because Calsoyasuchus valliceps from
Fig. 4. Skull reconstructions of representative early the Kayenta Formation of Arizona, USA (Tykoski
crocodylomorphs. (a) Dromicosuchus grallator, et al. 2002) is well-supported as the earliest mem-
(b) Sphenosuchus acutus, and (c) the early ber of Goniopholididae, a clade well-nested within
crocodyliform Protosuchus richardsoni. Modified Neosuchia (e.g. Tykoski et al. 2002; Turner &
from Nesbitt (2011). Sertich 2010).
Calsoyasuchus valliceps is one of several croco-
dyliform lineages that record a significant increase
Origin of Crocodyliformes in crocodylomorph disparity and ecological spec-
ialization during the Early Jurassic. It is the ear-
The vast majority of Mesozoic crocodylomorph liest longirostral semi-aquatic crocodylomorph,
diversity and disparity, and all Cenozoic croco- and by the end of the Early Jurassic, a separate
dylomorphs, belong to the clade Crocodylifor- longirostral lineage, Thalattosuchia, diversified as
mes. The initial diversification of Crocodyliformes marine predators (e.g. Pol et al. 2009). On land, non-
includes the first clearly semi-aquatic taxa (e.g. mesoeucrocodylian crocodyliforms also became
Tykoski et al. 2002) that resemble the modern specialized; in the Kayenta Formation, the unna-
crocodylian bauplan, and multiple lineages of unu- med sister taxon of Edentosuchus (Sues et al.
sually disparate omnivorous and/or herbivor- 1994) had a blunt snout and unusual bulbous teeth
ous terrestrial taxa (e.g. Sues et al. 1994; Wu & that suggest an omnivorous or herbivorous diet.
Sues 1996a; Krause & Key 2010). Though most All of these taxa lived alongside small cursorial
recent phylogenetic work indicates that the clos- ‘sphenosuchians’ (e.g. Kayentasuchus walkeri
est sister taxa to Crocodyliformes are a variety from the Kayenta Formation – see Clark & Sues
of small-bodied cursorial Early-Late Jurassic 2002) that persisted until the Late Jurassic.
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EARLY CROCODYLOMORPHA 291

(a)

10 cm

(b)

10 cm

Fig. 5. Skeletal reconstructions of representative early crocodylomorphs. (a) Terrestrisuchus gracilis (modified from
Sereno & Wild 1992), (b) Protosuchus richardsoni.

Recently, Wilberg (2010, 2012) suggested 2004; Erickson et al. 2001; de Ricqlès et al.
that thalattosuchians diverged prior to the split 2003). In general, the closest relatives of croco-
between Protosuchus and other crocodyliforms, dylomorphs, such as Rauisuchidae and Aetosauria,
thus placing this clade outside of Crocodyliformes. have not been extensively sampled across their
This result was partly because in contrast to pre- available taxonomic diversity. The few sampled
vious analyses (e.g. Pol & Norell 2004a, b; Pol outgroups of crocodylomorphs (e.g. Postosuchus
et al. 2009; Turner & Sertich 2010), he used a and material referred to the aetosaur Desmatosu-
more proximate outgroup (Postosuchus v. the previ- chus) display predominantly longitudinal vascular-
ously used early suchian Gracilisuchus) to polarize ization with paralleled-fiber bone (de Ricqlès et al.
characters, and increased taxon sampling within 2003), characteristic of slower growing reptiles,
Thalattosuchia (Wilberg 2010, 2012). This hypoth- but not as slow as extant crocodylians (Enlow &
esis is intriguing in part because it reduces the Brown 1957; Enlow 1969; Padian et al. 2001, 2004).
inferred ghost lineage for some mesoeucrocodylian Although sampling among early crocodylo-
lineages, but increases the ghost lineage for the stem morphs is poor, a few studies have hinted at intri-
of Thalattosuchia. It nonetheless requires further guing patterns. The first early crocodylomorph to
testing; even though Wilberg (2010, 2012) included be studied histologically was a humerus from the
two early crocodylomorph taxa in his analysis Rhaetian fissure-fills of Wales (Whiteside & Mar-
(Sphenosuchus acutus and Dibothrosuchus elaph- shall 2008) tentatively identified as Terrestrisu-
ros), he did not include any of the taxa recovered chus gracilis (de Ricqlès et al. 2003); its isolated
as most closely related to Crocodyliformes (e.g. nature makes it difficult to assign to Terrestrisuchus
Litargosuchus leptorhynchus, Kayentasuchus wal- or Crocodylomorpha using autapomorphies or a
keri, or Junggarsuchus sloani) in recent early cro- unique combination of character states. The ele-
codylomorph phylogenetic analyses (e.g. Clark ment is highly vascularized with woven-fibered
et al. 2004; Nesbitt 2011; Pol et al. in press). bone not typical of other sampled pseudosuchians,
and de Ricqlès et al. (2003) used these charac-
teristics to hypothesize that bone growth rate of
Terrestrisuchus was significantly higher than that
Palaeobiology of other early pseudosuchians and extant crocody-
Growth and body size lians. Soon after, Nesbitt et al. (2006) used the
histological characteristics of Terrestrisuchus gra-
Histological sampling of long bones from extinct cilis in combination with apomorphies to identify
vertebrate animals (e.g. Enlow & Brown 1957), the gut contents of the allegedly cannibalistic Coel-
particularly archosaurs, has revolutionized our ophysis bauri (AMNH FARB 7224) as an early
understanding of the growth dynamics and palaeo- crocodylomorph. They argued that the neotype of
biology of these clades (e.g. Padian et al. 2001, Coelophysis bauri had ingested a crocodylomorph
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292 R. B. IRMIS ET AL.

like Hesperosuchus rather than a member of its closely related to Crocodyliformes, such as Terres-
own species. trisuchus gracilis (Fig. 3e), Litargosuchus leptor-
In attempts to section another early crocodylo- hynchus (Fig. 3f ), and Kayentasuchus walkeri are
morph, de Ricqlès et al. (2008) reported on a his- small-bodied (,1 m in length) taxa with greatly
tological section of a long bone from the holotype elongated, gracile limbs (e.g. Crush 1984; Clark
of Hesperosuchus agilis (AMNH FARB 6758). & Sues 2002), whereas others are slightly larger
The section of Hesperosuchus agilis drastically dif- (1–2 m; e.g. Junggarsuchus). The earliest crocody-
fers in fundamental growth characters from that of liforms such as Protosuchus richardsoni (Figs 3i
Terrestrisuchus gracilis (Ricqlès et al. 2003) and & 4c) and Hemiprotosuchus leali were also small-
cf. Hesperosuchus from the Coelophysis Quarry bodied (c. 1 m in length or less), but Early Juras-
of Ghost Ranch, New Mexico (Nesbitt et al. sic mesoeucrocodylians such as the goniopholidid
2006). Specifically, this section has little vasculari- Calsoyasuchus (Tykoski et al. 2002) and thalat-
zation, a large number of lines of arrested growth tosuchians (e.g. Steneosaurus) easily reached 3 or
(LAGs), and only parallel-fibered bone, all char- more metres in length (Westphal 1961; Farlow
acteristics of slow growth. However, the identifi- et al. 2005).
cation of the element sectioned for Hesperosuchus
agilis is ambiguous. Most of the limb elements Feeding
of the holotype Hesperosuchus agilis are thin-
walled like those of early dinosaurs, and unlike the All extant crocodylians are semi-aquatic carni-
sectioned element. Furthermore, the holotype of vores. They have dorsoventrally flattened skulls,
Hesperosuchus agilis was mostly weathered at dis- whereas the skulls of all known non-crocodyliform
covery and was collected through screen-washing crocodylomorphs lack dorsoventral flattening and
(Colbert 1952). During the recovery process, at pronounced dermal sculpturing, and the antorbi-
least five other taxa were collected (S. J. Nesbitt tal fossa and fenestra are still well developed, a
pers. obs.), so it is very possible that the sectioned part of the skull thought to be pneumatic (Witmer
element actually belongs to another Triassic tetra- 1997). Sphenosuchus acutus (Fig. 4b), Dibothrosu-
pod. Only further sectioning of identifiable ele- chus elaphros, and Junggarsuchus sloani are suc-
ments that clearly belong to the type specimen of cessively more similar to Crocodyliformes in their
Hesperosuchus agilis will resolve this issue. In cranial structure although they all still lack the
sum, the few histologically-assessed specimens of principal features of the skull in the latter clade
early crocodylomorphs display a growth rate higher (Clark et al. 2004). Junggarsuchus sloani (Fig. 3d)
than that observed in extant crocodylians. already has various crocodyliform features such as
The body size of early crocodylomorphs var- the consolidation of potential zones of intracranial
ies considerably, but none approach the gigantic mobility, a decrease in the size of the supratem-
sizes of contemporary rauisuchids or dinosaurs. poral fenestrae, and a large surface for the insertion
Some of the earliest diverging crocodylomorphs of the posterior pterygoideus muscle on the antero-
(see Nesbitt 2011) such as the type of Hesperosu- lateral aspect of the retroarticular process (Clark
chus agilis, a specimen referred to Hesperosuchus et al. 2004). These features suggest more rapid
agilis (CM 29894) (Fig. 3a), and Trialestes romeri and powerful biting as in crocodylians (Schumacher
measure between 2–3 m in length (depending on 1973).
the estimated length of the tail), and are signifi- The tooth crowns of most non-crocodyliform
cantly larger than the closest outgroups of Croco- crocodylomorphs are labiolingually flattened and
dyliformes. Yet, there is no detectable trend of have finely serrated mesial and distal carinae, indi-
decreasing body size at the origin of Crocodylomor- cating a carnivorous diet (Abler 1992). Most early
pha, during the evolution of early crocodylomor- crocodylomorphs were cursorial predators that
phs, or at the origin of Crocodyliformes (Turner & likely fed on other small to medium-sized animals,
Nesbitt 2013) The poorly known Redondavenator and might best be considered generalist ‘fauni-
quayensis (Fig. 3h) hint at the presence of large- vores.’ This interpretation is not directly testable,
bodied crocodylomorphs prior to the end of the but is a reasonable hypothesis given the diets of
Triassic; the skull length of Redondavenator quay- extant juvenile crocodylians (e.g. Cott 1961). One
ensis was estimated to be over 60 cm (Nesbitt exception is Redondavenator quayensis from the
et al. 2005), a size more similar to that of large latest Triassic of New Mexico (Fig. 3h), which
rauisuchids from the Late Triassic (e.g. Postosu- had an estimated skull length of at least 60 cm
chus kirkpatricki). Unfortunately, the systematic (Nesbitt et al. 2005) and may have been a terrestrial
position of Redondavenator quayensis is unclear, apex predator, possibly during the replacement of
and the only preserved cranial remains in the ‘rauisuchian’ pseudosuchians by theropod dino-
type and only known specimen is the anterior end saurs in this ecological niche near or at the end of
of the snout. Many early crocodylomorphs most the Triassic.
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EARLY CROCODYLOMORPHA 293

Some basal crocodylomorphs apparently devi- vagans and Terrestrisuchus gracilis, the tibia is
ated from strict carnivory. For example, Phyllo- longer than the femur, which indicates cursoriality
dontosuchus lufengensis from the Early Jurassic (Walker 1970). However, the tibia is shorter than
of China has a distinctive, heterodont dentition the femur in larger forms such as Dromicosuchus
comprising anterior teeth with recurved, pointed grallator and Hesperosuchus agilis. Further
crowns and more posterior teeth with small, leaf- material is needed to test whether the femur/tibia
shaped crowns that have fine crenulations along length ratio is size-related and/or subject to onto-
the distal carinae (Harris et al. 2000). Similar teeth genetic change, as in extant crocodylians (Dodson
in other archosaurs imply an omnivorous or herbi- 1975). The geologically youngest known non-
vorous diet (Barrett 2000; Irmis et al. 2007b; crocodyliform crocodylomorphs, Hallopus victor,
Nesbitt et al. 2010; Barrett et al. 2011). Mace- Macelognathus and Junggarsuchus were highly
lognathus vagans has a dorsoventrally flattened, adapted cursors. The femora of Hallopus victor and
spatulate and anteriorly edentulous mandibular Macelognathus vagans have a ball-shaped round
symphysis as well as tooth crowns without serrated head that is clearly separated from the shaft by a
carinae (Göhlich et al. 2005). dorsomedially projecting neck and, in Macelog-
There exists intriguing evidence that the hun- nathus vagans, is directed medially (Walker 1970;
ters occasionally became the hunted. Based on Göhlich et al. 2005); nothing is known about the
gross morphology and osteohistology, Nesbitt et al. hind limb of Junggarsuchus sloani. The humerus
(2006) demonstrated that bones preserved in the of Junggarsuchus sloani has a hemispherical head
abdominal region of the neotype of the latest Trias- that projects perpendicular to the shaft (Clark
sic theropod Coelophysis bauri are not those of et al. 2004). The convex articular surface of the
a juvenile conspecific (as argued by Colbert 1989) head contacted the posteroventrally concave glen-
but actually belong to an early crocodylomorph. oid facet of the scapula. Movement of the carpus
The holotype of Dromicosuchus grallator (Figs 3a and functionally tridactyl manus was in line with
& 4a) has two areas of conspicuous damage on its other arm bones and ruled out splaying of the
left mandibular ramus and on the cervical armour, manus as in extant crocodylians. Clark et al.
which match the size and shape of teeth from an (2004) cited the presence of procoelous vertebral
associated skeleton of the rauisuchid Postosuchus centra, dorsal vertebrae with short transverse pro-
alisonae (Sues et al. 2003). Similarly, Walker (1990) cesses and steeply inclined zygapophyses, and the
cited possible tooth marks and other defects on likely absence of dorsal osteoderms in Junggarsu-
bones of the holotype of Sphenosuchus acutus as chus as evidence for greater vertical axial mobil-
evidence of either predation or scavenging. ity compared to extant crocodylians, which have
greater lateral axial mobility for rotary gait and
Locomotion and functional morphology swimming (Frey 1988).

Modern crocodylians have a long trunk region Skeletal pneumaticity

with a medio-laterally broad ribcage, extensive dor-
sal and ventral dermal armour, rather short but Cranial and postcranial pneumatic features are
muscular limbs, a sprawling stance, and a long, widespread in the skeletons of many avian-line
mediolaterally flattened tail. By contrast, early cro- archosaurs (e.g. Witmer 1997; O’Connor & Claes-
codylomorphs have dorsal armor limited to two sens 2005; O’Connor 2006; Sampson & Witmer
parasagittal longitudinal rows of osteoderms and 2007; Claessens et al. 2009; Wedel 2009). Crocody-
elongate, very slender limbs that were held directly lomorphs are often thought as of more ‘reptilian’
beneath the body in an erect stance and moved in physiology than their avian-line sister clade,
with a parasagittal gait. The carpus and metapodials but the recent discovery that an ‘avian-style’ flow-
are compact in taxa closer to Crocodyliformes through lung is present in extant Alligator and is
(Nesbitt 2011), and both the manus and pes appear likely plesiomorphic for Archosauria (Farmer &
to be digitigrade. Based on skeletal structure and Sanders 2010) has reignited interest in potential
associated biotas, most authors agree that early pseudosuchian skeletal pneumaticity, because orni-
crocodylomorphs had a terrestrial mode of life thodiran postcrania are often pneumatized by diver-
(Huene 1921; Walker 1970, 1990; Crush 1984; Par- ticula from this type of lung (e.g. O’Connor &
rish 1986; Clark et al. 2004). The sprawling stance Claessens 2005; Claessens et al. 2009; Wedel
and rotary gait of present-day crocodylians are 2009). Yet, no postcranial skeletal pneumaticity has
now interpreted as secondary adaptations to a semi- been reported for any crocodylomorphs, despite
aquatic mode of life (Parrish 1986; Padian et al. recent detailed surveys of pseudosuchians (e.g.
2010). Butler et al. 2012).
In the small-bodied crocodylomorphs Hallopus Early crocodylomorphs do display extensive
victor,Litargosuchusleptorhynchus,Macelognathus pnuematicization of the braincase elements,
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294 R. B. IRMIS ET AL.

particularly within the parabasisphenoid (e.g. revolutionized our understanding of this group over the
Walker 1972, 1990; Wu & Chatterjee 1993), and past 35 years. Special thanks to J. Desojo for the invitation
crocodyliforms and their closest outgroups share to contribute to this volume, and to participate in the ‘Early
apomorphic pneumatic features of the quadrate Archosaurs’ symposium at the IV Congreso Latinoameri-
cano de Paleontologı́a de Vertebrados. Funding was pro-
(e.g. Walker 1990; Nesbitt 2011) (Fig. 4c). These vided by the University of Utah (R. B. I.) and NSF
pneumatic openings are paralleled in many birds, award EAR 1024036 (to C. A. Sidor and S. J. N.). We
leading to the proposal that crocodylomorphs and thank J. Clark and D. Pol for helpful reviews. Discussions
birds shared a close common ancestor (Walker with A. Turner, J. M. Leardi, and J. Clark were extremely
1972), but careful comparative anatomical study helpful.
(Walker 1985) and more advanced phylogenetic
sampling and analysis (e.g. Nesbitt 2011) demon-
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