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New Triassic archosaur papers

From: Ben Creisler

New papers from the Geological Society Special Publications 379:
Anatomy, Phylogeny and Palaeobiology of Early Archosaurs and their

Sterling J. Nesbitt, Julia B. Desojo, and Randall B. Irmis (2013)
Anatomy, phylogeny and palaeobiology of early archosaurs and their kin.
Geological Society Special Publications 379:  Anatomy, Phylogeny and
Palaeobiology of Early Archosaurs and their Kin (advance online

Archosauria include two extant clades, crocodylians and avians.
However, the diversification of the avian and crocodylian crown groups
are relatively recent events that began during the Late Cretaceous
(Brochu 2001, 2003; Clarke et al. 2005), less than 100 Ma ago. These
crown diversifications are shallow in time compared to the much deeper
divergence of the crocodylian and avian stem lineages in the Triassic,
approximately 250 Ma ago (Butler et al. 2011; Nesbitt 2011; Nesbitt et
al. 2011). The early archosaurian diversification began in the wake of
the devastating end-Permian mass extinction, and the divergence of
both lineages and disparate body forms was rapid, occurring less than
10 Ma after the first archosaur (Nesbitt et al. 2010). This initial
diversification was severely affected by the end-Triassic mass
extinction approximately 50 Ma later, and only three major archosaur
clades – pterosaurs, dinosaurs and crocodylomorphs – survived to
diversify during the rest of the Mesozoic.
Since the cladistic revolution first affected studies of archosaur
phylogeny nearly 30 years ago (e.g. Benton 1985; Clark 1986; Gauthier
1986; Sereno 1986; Benton & Clark 1988), many workers attempted to
elucidate the early evolutionary history of archosaurs during the late
1980s and early 1990s (e.g. Benton & Clark 1988; Sereno 1991; Parrish
1993; Juul 1994). Whereas the pace of phylogenetic study for
pterosaurs, dinosaurs and crocodylomorphs continued unabated through
to the present day, early archosaur work reached little consensus
(Gower & Wilkinson 1996), and novel phylogenetic datasets attempting
to resolve the interrelationships of the major early clades were
comparatively rare after the mid-1990s. Nonetheless, discoveries of
new taxa and specimens continued throughout the 1990s and 2000s.


Jeremías R. A. Taborda, Ignacio A. Cerda, and Julia B. Desojo (2013)
Growth curve of Aetosauroides scagliai Casamiquela 1960 (Pseudosuchia:
Aetosauria) inferred from osteoderm histology.
Geological Society Special Publications 379:  Anatomy, Phylogeny and
Palaeobiology of Early Archosaurs and their Kin (advance online

Recent palaeohistological studies on paramedian osteoderms of
aetosaurs revealed the presence of growth lines (lines of arrested
growth or LAGs) and a minimal or nonexistent secondary remodelling in
the bone matrix of these elements. This feature allows the age of
individuals to be estimated through growth line count. In the present
contribution we study the growth curve of the South American aetosaur
Aetosauroides scagliai. We estimated the age (obtained from LAG
counting) and body size (body length and body mass were used as
proxies) of different aetosaur specimens in order to reconstruct the
growth curve of the South American species. The data obtained for
Aetosauroides scagliai were compared with that of other aetosaurs,
such as Neoaetosauroides engaeus, Aetosaurus ferratus,
Aetobarbakinoides brasiliensis, Typothorax coccinarum and
Paratypothorax sp. Our results indicate that, if body length is
considered as proxy, all studied aetosaur specimens have a similar or
almost identical growth rate. However, important variations arose
among aetosaur taxa if body mass is considered as proxy, which would
be related to a body morphology ranging from slender (e.g.
Aetobarbakinoides brasiliensis) to very wide (Typothorax coccinarum)
morphotypes. In comparison with extant pseudosuchians (i.e.
crocodylians), Aetosauroides scagliai possesses a relatively lower
growth rate.


María Jimena Trotteyn, Andrea B. Arcucci, and Tiago Raugust (2013)
Proterochampsia: an endemic archosauriform clade from South America.
Geological Society Special Publications 379:  Anatomy, Phylogeny and
Palaeobiology of Early Archosaurs and their Kin (advance online

Proterochampsia is a monophyletic group of crocodile-like
archosauriforms currently endemic to the late Middle and early Late
Triassic of South America considered as one of the potential
successive sister-taxa of the crown group Archosauria. The
proterochampsians come from the Ischigualasto-Villa Unión Basin in the
west of Argentina and the Parana Basin in the south of Brazil. The
traditional composition of the group includes the genera Cerritosaurus
Price 1946, Proterochampsa Reig 1959 (with two species: P.
barrionuevoi in Argentina and P. nodosa Barberena 1982 in Brazil),
Chanaresuchus Romer (with two species from Argentina: C. bonapartei
Romer and C. ischigualastensis Trotteyn et al. 2012), Gualosuchus
reigi Romer 1971 and Tropidosuchus romeri Arcucci 1990. After a
precladistic history of confusion about their relationships with
crocodilians, in the last 20 years new discoveries of taxa, and more
systematic and phylogenetic studies, have clarified their position as
non-archosaurian archosauriforms and their relationships with other
Triassic archosaurs.


Randall B. Irmis, Sterling J. Nesbitt, and Hans-Dieter Sues (2013)
Early Crocodylomorpha.
Geological Society Special Publications 379:  Anatomy, Phylogeny and
Palaeobiology of Early Archosaurs and their Kin (advance online

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 approximately 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 Mesozoic. 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
crocodylomorph phylogeny will provide a solid framework for
understanding their change in diversity and disparity through time,
potential biogeographic patterns, and the morphological transformation
leading to Crocodyliformes.