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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 reference.

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The Slothful Claw: Osteology and Taphonomy of Nothronychus mckinleyi and N. graffami (Dinosauria: Theropoda) and Anatomical Considerations for Derived Therizinosaurids