

Tyrannoneustes lythrodectikos (metriorhynchid crocodylomorph) cranial osteology (free pdf)

Ben Creisler

A new paper:

Davide Foff & Mark T. Young (2014)
The cranial osteology of Tyrannoneustes lythrodectikos
(Crocodylomorpha: Metriorhynchidae) from the Middle Jurassic of
PeerJ 2:e608
doi: http://dx.doi.org/10.7717/peerj.608

Tyrannoneustes lythrodectikos is one of numerous metriorhynchid
crocodylomorph species known from the Oxford Clay Formation of England
(Callovian-Oxfordian; Middle-Late Jurassic). This taxon is of
evolutionary importance, as it is the oldest and most basal known
macrophagous metriorhynchid. It has a mosaic of plesiomorphic and
derived feeding related characteristics, including: teeth with
microscopic, poorly formed and non-contiguous denticles; increased
tooth apicobasal length; ventrally displaced dentary tooth row
(increased gape); reduced dentary tooth count; and a proportionally
long mandibular symphysis. However the type specimen, and current
referred specimens, all lack a preserved cranium. As such, the
craniofacial morphology of this taxon, and its potential feeding
ecology, remains poorly understood. Here we describe two skulls and
two lower jaws which we refer to T. lythrodectikos. Previously these
specimens were referred to 'Metriorhynchus' brachyrhynchus. They share
with the T. lythrodectikos holotype: the in-line reception pits on the
dentary, dorsal margin of the surangular is strongly concave in
lateral view, and the most of the angular ventral margin is strongly
convex. Based on our description of these specimens, the skull of T.
lythrodectikos has three autapomorphies: very long posterior processes
of the premaxilla terminating in line with the 4th or 5th maxillary
alveoli, deep lateral notches on the lateral surface of the maxillary
with reception pits for dentary teeth, and the premaxilla forms the
anterior margin of the first maxillary alveoli. Our description of the
cranial anatomy of Tyrannoneustes lythrodectikos confirms that some
macrophagous characteristics evolved during the Middle Jurassic, and
were not exclusive to the clade Geosaurini. Moreover, the skulls
further highlight the mosaic nature of Tyrannoneustes lythrodectikos
and wide-gape macrophagous evolution in Geosaurinae.