title: The Slothful Claw: Osteology and Taphonomy of Nothronychus mckinleyi and N. graffami (Dinosauria: Theropoda) and Anatomical Considerations for Derived Therizinosaurids.
abstract: 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.
authors.affiliation: Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA, United States of America.
authors.forename: Brandon P
authors.initials: BP
authors.lastname: Hedrick
authors.affiliation: North Carolina Museum of Natural Sciences, Raleigh, NC, United States of America; Department of Biological Sciences, North Carolina State University, Raleigh, NC, United States of America.
authors.forename: Lindsay E
authors.initials: LE
authors.lastname: Zanno
authors.affiliation: White Mountain Dinosaur Exploration Center, Springerville, AZ, United States of America.
authors.forename: Douglas G
authors.initials: DG
authors.lastname: Wolfe
authors.affiliation: Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA, United States of America; Department of Animal Biology, University of Pennsylvania, Philadelphia, PA, United States of America.
authors.forename: Peter
authors.initials: P
authors.lastname: Dodson
mesh_terms: D000818:Animals; D005075:Biological Evolution; D001842:Bone and Bones / Q000033:anatomy & histology*; D025061:Dinosaurs / Q000033:anatomy & histology* / Q000145:classification*; D010802:Phylogeny; D014070:Tooth / Q000033:anatomy & histology