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

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.懒爪龙属（Nothronychus）是首个在北美被发现的确定的镰刀龙类（Therizinosauria），目前也是北美特化程度最高的镰刀龙属类群。该属目前已知两个物种：产自新墨西哥州中西部莫雷诺希尔组（Moreno Hill Formation，土仑阶中期）的N. mckinleyi，以及犹他州中南部热带页岩组（Tropic Shale，土仑阶早期）的N. graffami。两个物种均有部分乃至近乎完整的骨架化石，为阐明镰刀龙类的演化趋势提供了关键依据。尽管这两个分类单元（taxa）兼具生物地理与演化研究价值，但二者均未得到详细的形态学描述。本文针对N. mckinleyi与N. graffami开展全面的形态描述，其中N. graffami是目前已知保存最完整的镰刀龙科（Therizinosauridae）骨架标本。我们基于本次获取的新数据，修正了此前对两个物种的初步描述，并通过深入的形态学分析修订了既往的特征状态。此外，我们梳理了两件懒爪龙属标本的沉积历史，并对比了二者的埋藏学（taphonomic）模式。研究证实，这两个物种不仅存在地理隔离，时间分布上也存在显著差异：基于菊石生物带（ammonoid biozones）推算，二者的生存间隔至少为150万年，最长可达300万年。随后我们探讨了成岩（diagenetic）变形对骨骼形态的影响，并结合新数据重新评估了潜在的分类鉴别特征。例如，N. mckinleyi的尺骨（ulna）呈弯曲形态，而此前学界认为N. graffami的尺骨为直形，这一特征最初被用于区分两个物种。但本文研究表明，该差异更可能源于N. graffami标本所经历的成岩挤压作用，而非真实的生物学种间差异。最后，本文附有多张两个类群所有主要骨骼的三维表面扫描件以供参考。