[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Early Archosaur Anatomy, Phylogeny and Palaeobiology (Special Publication)

From: Ben Creisler

Various papers have appeared in advance of publication of the
Geological Society Special Publication 379:

Anatomy, Phylogeny and Palaeobiology of Early Archosaurs and their Kin
Edited by S.J. Nesbitt, J.B. Desojo and R.B. Irmis
Geological Society Special Publication 379


Here are the abstracts for the currently posted papers in the volume
(more to come I assume):

William G. Parker and Sterling J. Nesbitt (2013)
Cranial remains of Poposaurus gracilis (Pseudosuchia: Poposauroidea)
from the Upper Triassic, the distribution of the taxon, and its
implications for poposauroid evolution.
Anatomy, Phylogeny and Palaeobiology of Early Archosaurs and their Kin
Geological Society Special Publication 379 (advance online publication)
doi: 10.1144/SP379.3

The partial postcrania of Poposaurus gracilis, a bipedal poposauroid
convergent with theropod dinosaurs, has been known for nearly a
century, but the skull of P. gracilis has proven elusive. P. gracilis
is part of a clade of morphologically divergent pseudosuchians
(poposauroids) whose members are sometimes bipedal, lack dentition
(i.e. beaks) and some have elongated neural spines (i.e. sails).
However, the timing and acquisition of these character states is
unknown given the uncertainty of the skull morphology of the
‘mid-grade’ poposauroid P. gracilis. Here, we present the first
confirmed skull remains of P. gracilis directly associated with
diagnostic pelvic elements that overlap with the holotype. The
incomplete skeleton (PEFO 34865) from the Chinle Formation of
Petrified Forest National Park (Arizona, USA) includes a left maxilla
with a large, mediolaterally compressed tooth, left dentary, right
prearticular and a partial postcranium. The character states of P.
gracilis (bipedal, ‘sail-less’ and toothed) demonstrate that the
evolution of bipedalism, the origin/loss of a dorsal ‘sail’ and the
shift to an edentulous beak are complex in poposauroids. P. gracilis
is widespread in the Upper Triassic formations in the western USA and
is restricted temporally prior to the Adamanian–Revueltian faunal
turnover during the Norian.


M. Belén Von Baczko and Martín D. Ezcurra (2013)
Ornithosuchidae: a group of Triassic archosaurs with a unique ankle joint.
Geological Society Special Publication 379 (advance online publication)
doi: 10.1144/SP379.4

The ornithosuchids were a group of archosaurs with body lengths
ranging from 2 to 4 m recorded from Upper Triassic beds in Argentina
and Scotland. The group was defined as a node-based clade including
Ornithosuchus longidens, Riojasuchus tenuisceps, Venaticosuchus
rusconii and all descendants of their most recent common ancestor. The
ornithosuchids are diagnosed by the following apomorphies observed in
the three known species of the clade: downturned premaxilla;
premaxilla–maxilla contact with a diastema in the alveolar margin
equal in length to two teeth; palatine–pterygoid fenestra; and orbit
with a distinct ventral point surrounded by ‘V’-shaped dorsal
processes of the jugal. The most remarkable postcranial apomorphy of
the group is the presence of the so-called crocodile reversed ankle
joint, a condition that seems to be unique for the ornithosuchids
among amniotans. The systematic history of Ornithosuchidae is complex
and Ornithosuchus was allied with dinosaurs or phytosaurs prior to the
implementation of numerical phylogenetic analyses. Currently, there is
consensus that Ornithosuchidae is positioned within Pseudosuchia, but
their phylogenetic position within the group remains strongly debated.
Nevertheless, all hypotheses agree in inferring an extremely long
ghost lineage at the base of the clade. The presence of derived
pseudosuchians in the late Olenekian produces a ghost lineage of
c.16–18 millions of years for Ornithosuchidae, indicating that only
the late evolutionary history of the clade is currently sampled in the
fossil record.


Michelle R. Stocker and Richard J. Butler (2013)
Geological Society Special Publication 379 (advance online publication)
doi: 10.1144/SP379.5

Phytosauria is a nearly cosmopolitan clade of large, quadrupedal,
carnivorous archosauriforms. They are known unambiguously from Late
Triassic deposits, although the clade’s ghost lineage extends at least
to the late Early Triassic. Their nares are uniquely located close to
the orbits rather than anteriorly in the rostrum as in modern
crocodylians, and the rostrum is formed by elongated premaxillae
bearing many teeth. Phytosaurs have roughly triangular, ornamented
paramedian osteoderms, rounder appendicular osteoderms, and a unique
‘gular shield’ assembled from multiple, irregular osteoderms under the
throat. Phytosaurs are reconstructed as semi-aquatic because of their
general similarity to modern crocodylians and common preservation in
fluvial and shallow-marine deposits. Currently, over thirty species
are recognized. New specimens continue to be collected, some
representing new taxa. The taxonomic status of other named taxa is
uncertain and requires re-investigation. Since their discovery,
phytosaurs have been used as biostratigraphic and biochronological
index taxa for correlating Late Triassic sediments worldwide. Recent
systematic and taxonomic revisions cast doubt on some of those
correlations. Our understanding of the evolution of Phytosauria is far
from complete. With reevaluation of well-known specimens, rigorous and
comparative morphological descriptions, and robust phylogenetic
hypotheses of ingroup relationships, studies of phytosaurs can address
larger palaeobiological questions.


Sterling J. Nesbitt, Stephen L. Brusatte, Julia B. Desojo, Alexandre
Liparini, Marco A. G. De França, Jonathan C. Weinbaum and David J.
Gower (2013)
Geological Society Special Publication 379 (advance online publication)
doi: 10.1144/SP379.1

‘Rauisuchia’ comprises Triassic pseudosuchians that ranged greatly in
body size, locomotor styles and feeding ecologies. Our concept of what
constitutes a rauisuchian is changing as a result of discoveries over
the last 15 years. New evidence has shown that rauisuchians are
probably not a natural (monophyletic) group, but instead are a number
of smaller clades (e.g. Rauisuchidae, Ctenosauriscidae, Shuvosauridae)
that may not be each other's closest relatives within Pseudosuchia.
Here, we acknowledge that there are still large gaps in the basic
understanding in the alpha-level taxonomy and relationships of these
groups, but good progress is being made. As a result of renewed
interest in rauisuchians, an expanding number of recent studies have
focused on the growth, locomotor habits, and biomechanics of these
animals, and we review these studies here. We are clearly in the midst
of a renaissance in our understanding of rauisuchian evolution and the
continuation of detailed descriptions, the development of explicit
phylogenetic hypotheses, and explicit palaeobiological studies are
essential in advancing our knowledge of these extinct animals.


Roland B. Sookias and Richard J. Butler (2013)
Geological Society Special Publication 379 (advance online publication)
doi: 10.1144/SP379.6

Euparkeria capensis has long been considered an archetype for the
ancestral archosaur morphology, and has been placed just outside of
crown Archosauria by nearly all cladistic analyses. Six species are
currently considered to be putative members of a clade Euparkeriidae,
and have been collected from Olenekian- or Anisian-aged deposits in
South Africa (Euparkeria capensis – the only definitive member of the
group), China (Halazhaisuchus qiaoensis, Wangisuchus tzeyii,
‘Turfanosuchus’ shageduensis), Russia (Dorosuchus neoetus) and Poland
(Osmolskina czatkowicensis). Four other species (Turfanosuchus
dabanensis, Xilousuchus sapingensis, Platyognathus hsui, Dongusia
colorata) were historically assigned to Euparkeriidae, but have been
removed by recent work. Recent authors deemed Osmolskina
czatkowicensis and Dorosuchus neoetus to be the most likely taxa to
form a euparkeriid clade with Euparkeria capensis, but Osmolskina
czatkowicensis and Euparkeria capensis were not found as sister taxa
by the only cladistic analysis to have tested euparkeriid monophyly.
Euparkeria capensis was small (<1 m), insectivorous or carnivorous,
probably had vision adapted to low-light conditions and a semi-erect
crocodile-like stance, and may have been facultatively bipedal. Bone
histology demonstrates that Euparkeria capensis had a slow growth
rate, which has been suggested to have been an adaptation to
relatively stable environmental conditions.


Casey M. Holliday and Sterling J. Nesbitt (2013)
Morphology and diversity of the mandibular symphysis of archosauriforms.
Geological Society Special Publication 379 (advance online publication)
doi: 10.1144/SP379.2

Archosauromorphs radiated into numerous trophic niches during the
Mesozoic, many of which were accommodated by particular suites of
cranial adaptations and feeding behaviours. The mandibular symphysis,
the joint linking the mandibles, is a poorly understood
craniomandibular joint that may offer significant insight into skull
function and feeding ecology. Using comparative data from extant
amniotes, we investigated the skeletal anatomy and osteological
correlates of relevant soft tissues in a survey of archosauromorph
mandibular symphyses. Characters were identified and their evolution
mapped using a current phylogeny of archosauriforms with the addition
of non-archosauriform archosauromorphs. Extinct taxa with the simple
Class I condition (e.g. proterochampsids, ‘rauisuchians’), rugose
Class II (aetosaurs, protosuchians, silesaurids) and interdigitating
Class III symphyses (e.g. phytosaurs, crocodyliforms) and finally
fused Class IV (avians) build the joints in expected ways, although
they differ in the contributions of bony elements and Meckel’s
cartilage. Optimization of the different classes of symphyses across
archosauromorph clades indicates that major iterative transitions from
plesiomorphic Class I to derived, rigid Class II–IV symphyses occurred
along the lines to phytosaurs, aetosaurs, a subset of poposauroids,
crocodyliformes, pterosaurs and birds. These transitions in symphyseal
morphology also appear to track changes in dentition and potentially