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Paratypothorax (Aetosauria) cranial anatomy (Paratypothorax = adult Aetosaurus?)

Ben Creisler

A new paper:

Rainer R. Schoch & Julia B. Desojo (2016)
Cranial anatomy of the aetosaur Paratypothorax andressorum Long &
Ballew, 1985, from the Upper Triassic of Germany and its bearing on
aetosaur phylogeny.
Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen 279(1):  73-95
DOI: http://dx.doi.org/10.1127/njgpa/2016/0542

The large aetosaur Paratypothorax andressorum has so far been known
only by its osteoderms. Here we describe for the first time the skull
of a complete, articulated specimen of this taxon that was found in
the type horizon at Murrhardt, southwestern Germany. Paratypothorax
andressorum has the following cranial autapomorphies: (1) upper jaw
margin with deep notch between premaxilla and maxilla, (2)
maxilla-lacrimal suture with finger-like projection, (3) upper
temporal fenestra triangular, and (4) first paramedian cervical
osteoderms narrow and oval, much smaller than second row. Apart from
these features, the skull of P. andressorum closely resembles that of
the small aetosaur Aetosaurus ferratus known from the same horizons,
despite major differences in the morphology of osteoderms. Both taxa
share (1) the pointed, beak-shaped premaxilla which expands only
gently anterior to the nasal, (2) maxilla and lacrimal excluding jugal
from margin of antorbital fenestra, (3) exclusion of squamosal from
margin of infratemporal fenestra, and (4) posterior part of jugal not
downturned. Phylogenetic analysis reveals poorly resolved
relationships within Aetosauria, but exclusion of a problematic taxon
Coahomasuchus results in a much better resolution, with Paratypothorax
to nest with Rioarribasuchus, Tecovasuchus, Typothorax, and
Redondasuchus within a monophyletic Typothoracinae. Interestingly,
Aetosaurus and Stenomyti form successive sister taxa of this clade
rather than fall within an aetosaurine grade of basal aetosaurs, as
suggested by previous authors. The resemblance of Paratypothorax and
Aetosaurus in many cranial features, their close relationship as
suggested by the present analysis, and the immature state of all
available Aetosaurus specimens suggest two new alternative hypotheses:
(1) Aetosaurus is the juvenile of a close relative of Paratypothorax
or (2) it is itself the juvenile of Paratypothorax.