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Sigilmassasaurus (theropod) redescribed with new material (free pdf)

Ben Creisler

New in PeerJ:

Serjoscha W. Evers, Oliver W.M. Rauhut, Angela C. Milner, Bradley
McFeeters & Ronan Allain (2015)
A reappraisal of the morphology and systematic position of the
theropod dinosaur Sigilmassasaurus from the “middle” Cretaceous of
PeerJ 3:e1323
doi: https://dx.doi.org/10.7717/peerj.1323

Sigilmassasaurus brevicollis is an enigmatic theropod dinosaur from
the early Late Cretaceous (Cenomanian) of Morocco, originally based on
a few isolated cervical vertebrae. Ever since its original
description, both its taxonomic validity and systematic affinities
were contentious. Originally considered to represent its own family,
Sigilmassasauridae, the genus has variously been suggested to
represent a carcharodontosaurid, an ornithischian, and, more recently,
a spinosaurid. Here we describe new remains referrable to this taxon
and re-evaluate its taxonomic status and systematic affinities. Based
on the new remains, a re-evaluation of the original materials, and
comparisons with other spinosaurids, the holotype of Sigilmassasaurus
brevicollis is identified as an anterior dorsal, rather than a
cervical vertebra, and differences between elements referred to this
taxon can be explained by different positions of the elements in
question within the vertebral column. Many characters used previously
to diagnose the genus and species are found to be more widespread
among basal tetanurans, and specifically spinosaurids. However, the
taxon shows several autapomorphies that support its validity,
including the presence of a strongly rugose, ventrally offset
triangular platform that is confluent with a ventral keel anteriorly
in the mid-cervical vertebral centra and a strongly reduced lateral
neural arch lamination, with no or an incomplete distinction between
anterior and posterior centrodiapophyseal laminae in the posterior
cervical and anterior dorsal vertebrae. We argue furthermore that
Spinosaurus maroccanus, also described on the basis of isolated
cervical vertebrae from the same stratigraphic unit and in the same
paper as Sigilmassasaurus brevicollis, is a subjective synonym of the
latter. Both a detailed comparison of this taxon with other theropods
and a formal phylogenetic analysis support spinosaurid affintities for
Sigilmassasaurus. However, we reject the recently proposed synonymy of
both Spinosaurus maroccanus and Sigilmassasurus brevicollis with
Spinosaurus aegyptiacus from the Cenomanian of Egypt, as there are
clear differences between the vertebrae of these taxa, and they do not
share any derived character that is not found in other spinosaurids.
Together with a comparison with other spinosaurid vertebral material
from the Kem Kem, this suggests that more than one taxon of
spinosaurid was present in the Kem Kem assemblage of Morocco, so the
referral of non-overlapping material from this unit to a single taxon
should be regarded with caution.