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Thescelosaurus skull snatomy (free pdf)

Ben Creisler

A new paper in open-access PeerJ:

Clint A. Boyd (2014)
The cranial anatomy of the neornithischian dinosaur Thescelosaurus neglectus.
PeerJ 2:e669
doi: http://dx.doi.org/10.7717/peerj.669

Though the dinosaur Thescelosaurus neglectus was first described in
1913 and is known from the relatively fossiliferous Lance and Hell
Creek formations in the Western Interior Basin of North America, the
cranial anatomy of this species remains poorly understood. The only
cranial material confidently referred to this species are three
fragmentary bones preserved with the paratype, hindering attempts to
understand the systematic relationships of this taxon within
Neornithischia. Here the cranial anatomy of T. neglectus is fully
described for the first time based on two specimens that include
well-preserved cranial material (NCSM 15728 and
TLAM.BA.2014.027.0001). Visual inspection of exposed cranial elements
of these specimens is supplemented by detailed CT data from NCSM 15728
that enabled the examination of otherwise unexposed surfaces,
facilitating a complete description of the cranial anatomy of this
species. The skull of T. neglectus displays a unique combination of
plesiomorphic and apomorphic traits. The premaxillary and ‘cheek’
tooth morphologies are relatively derived, though less so than the
condition seen in basal iguanodontians, suggesting that the high tooth
count present in the premaxillae, maxillae, and dentaries may be
related to the extreme elongation of the skull of this species rather
than a retention of the plesiomorphic condition. The morphology of the
braincase most closely resembles the iguanodontians Dryosaurus and
Dysalotosaurus, especially with regard to the morphology of the
prootic. One autapomorphic feature is recognized for the first time,
along with several additional cranial features that differentiate this
species from the closely related and contemporaneous Thescelosaurus
assiniboiensis. Published phylogenetic hypotheses of neornithischian
dinosaur relationships often differ in the placement of the North
American taxon Parksosaurus, with some recovering a close relationship
with Thescelosaurus and others with the South American taxon
Gasparinisaura, but never both at the same time. The new morphological
observations presented herein, combined with re-examination of the
holotype of Parksosaurus, suggest that Parksosaurus shares a closer
relationship with Thescelosaurus than with Gasparinisaura, and that
many of the features previously cited to support a relationship with
the latter taxon are either also present in Thescelosaurus, are
artifacts of preservation, or are the result of incomplete preparation
and inaccurate interpretation of specimens. Additionally, the overall
morphology of the skull and lower jaws of both Thescelosaurus and
Parksosaurus also closely resemble the Asian taxa Changchunsaurus and
Haya, though the interrelationships of these taxa have yet to be
tested in a phylogenetic analysis that includes these new
morphological data for T. neglectus.