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Almadasuchus, new Jurassic crocodylomorph from Argentina



From: Ben Creisler
bcreisler@gmail.com


A new online paper:


Diego Pol, Oliver W. M. Rauhut, Agustina Lecuona, Juan M. Leardi, Xing
Xu & James M. Clark (2013)
A new fossil from the Jurassic of Patagonia reveals the early
basicranial evolution and the origins of Crocodyliformes.
Biological Reviews (advance online publication)
DOI: 10.1111/brv.12030
http://onlinelibrary.wiley.com/doi/10.1111/brv.12030/abstract

Extant crocodylians have a limited taxonomic and ecological diversity
but they belong to a lineage (Crocodylomorpha) that includes basal and
rather generalized species and a highly diverse clade,
Crocodyliformes. The latter was among the most successful groups of
Mesozoic tetrapods, both in terms of taxonomic and ecological
diversity. Crocodyliforms thrived in terrestrial, semiaquatic, and
marine environments, and their fossil diversity includes carnivorous,
piscivorous, insectivorous, and herbivorous species. This remarkable
ecological and trophic diversity is thought only to occur in forms
with a completely akinetic skull, characterized by a functionally
integrated and tightly sutured braincase-quadrate-palate complex.
However, the patterns of evolutionary change that led to the highly
modified skull of crocodyliforms and that likely enabled their
diversification remain poorly understood. Herein, a new basal
crocodylomorph from the Late Jurassic of Patagonia is described,
Almadasuchus figarii gen. et sp. nov. The new taxon is known from a
well-preserved posterior region of the skull as well as other
craniomandibular and postcranial remains. Almadasuchus figarii differs
from all other crocodylomorphs in the presence of six autapomorphic
features, including the presence of a large lateral notch on the upper
temporal bar, an otic shelf of the squamosal that is wider than long,
a deep subtriangular concavity on the posterolateral surface of the
squamosal, and an elongated pneumatopore on the ventral surface of the
quadrate. Phylogenetic analysis focused on the origin of
Crocodyliformes places Almadasuchus as the sister group of
Crocodyliformes, supported by synapomorphic features of the skull
(e.g. subtriangular basisphenoid, absence of basipterygoid process,
absence of a sagittal ridge on the frontal, and a flat anterior skull
roof with an ornamented dorsal surface). New braincase information
provided by Almadasuchus and other crocodylomorphs indicates that most
of the modifications on the posterior region of the skull of
crocodyliforms, including the strongly sutured braincase, quadrate,
and the extensive secondary palate appeared in a stepwise manner, and
pre-dated the evolutionary changes in the snout, jaws, and dentition.
This indicates that the progressively increased rigidity of the skull
provided the structural framework that allowed the great ecological
diversification of crocodyliforms during the course of the Mesozoic.
The phylogenetic pattern of character acquisition inferred for the
strongly sutured (akinetic) skull and the appearance of more diverse
feeding behaviours that create high mechanical loads on the skull
provides another interesting parallel between the evolution of
Mesozoic crocodyliforms and the evolutionary origins of mammals.