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Nothronychus osteology and derived therizinosaurid anatomy

Ben Creisler

New in PLoS ONE:

Brandon P. Hedrick , Lindsay E. Zanno, Douglas G. Wolfe & Peter Dodson (2015)
The Slothful Claw: Osteology and Taphonomy of Nothronychus mckinleyi
and N. graffami (Dinosauria: Theropoda) and Anatomical Considerations
for Derived Therizinosaurids.
PLoS ONE 10(6): e0129449.

Nothronychus was the first definitive therizinosaurian discovered in
North America and currently represents the most specialized North
American therizinosaurian genus. It is known from two species, No.
mckinleyi from the Moreno Hill Formation (middle Turonian) in
west-central New Mexico, and No. graffami from the Tropic Shale (early
Turonian) in south-central Utah. Both species are represented by
partial to nearly complete skeletons that have helped elucidate
evolutionary trends in Therizinosauria. In spite of the
biogeographical and evolutionary importance of these two taxa, neither
has received a detailed description. Here, we present comprehensive
descriptions of No. mckinleyi and No. graffami, the latter of which
represents the most complete therizinosaurid skeleton known to date.
We amend previous preliminary descriptions of No. mckinleyi and No.
graffami based on these new data and modify previous character states
based on an in-depth morphological analysis. Additionally, we review
the depositional history of both specimens of Nothronychus and compare
their taphonomic modes. We demonstrate that the species were not only
separated geographically, but also temporally. Based on ammonoid
biozones, the species appear to have been separated by at least 1.5
million years and up to 3 million years. We then discuss the impacts
of diagenetic deformation on morphology and reevaluate potentially
diagnostic characters in light of these new data. For example, the
ulna of No. mckinleyi is curved whereas the ulna of No. graffami was
considered straight, a character originally separating the two
species. However, here we present the difference as much more likely
related to diagenetic compression in No. graffami rather than as a
true biologic difference. Finally, we include copies of
three-dimensional surface scans of all major bones for both taxa for