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

Brandon P Hedrick,Douglas G Wolfe,Lindsay E Zanno,Peter Dodson,Matthew C Mihlbachler

doi:10.1371/journal.pone.0129449

Brandon P Hedrick, Douglas G Wolfe + Show 3 more

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https://doi.org/10.1371/journal.pone.0129449

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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.

Highlights

Therizinosaurians are enigmatic theropods known from a substantial number of poorly preserved or only preliminarily described taxa
Prior to 2005, there was little consensus regarding the evolutionary relationships of Therizinosauria, and the increasing number of new taxa discovered in Asia and North America in the past decade permitted the first data-rich phylogenetic analyses of the clade [1, 2], monographic descriptions are still lacking
[16] Etymology: Nothros (Greek): slothful; onyx (Greek): claw Diagnosis: Nothronychus can be differentiated from all other therizinosaurians by the following autapomorphies: 1) distinctly subcircular obturator process [2]; 2) ovoid, dorsoventrally elongate obturator foramen [2]; 3) contact between pubis and ischium restricted to proximal half of the obturator process [2]; and 4) presence of a deep notch between the ventral aspect of the obturator process and cranial ischial shaft [2]

Summary

Introduction

Therizinosaurians are enigmatic theropods known from a substantial number of poorly preserved or only preliminarily described taxa. Prior to 2005, there was little consensus regarding the evolutionary relationships of Therizinosauria, and the increasing number of new taxa discovered in Asia and North America in the past decade permitted the first data-rich phylogenetic analyses of the clade [1, 2], monographic descriptions are still lacking Such descriptions of therizinosaurian taxa are necessary in order to help resolve some of the remaining ambiguities within the clade. Barsbold [4] and Perle [5] corrected this misidentification on the basis of more complete materials including a referred hindlimb and forelimb They identified Therizinosaurus as a member of Theropoda, linking it with Segnosaurus, which was described in 1979 [6]. Early descriptions and phylogenetic analyses of these taxa and others from Asia improved our knowledge of Therizinosauria [13,14,15], they did not include robust phylogenetic analyses, largely because most species were still known from fragmentary remains

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