Didactyl therizinosaur with a preserved keratinous claw from the Late Cretaceous of Mongolia

Therizinosaurs were a group of herbivorous/omnivorous theropod dinosaurs that lived in Asia and North America during the Cretaceous Period. These theropods are notable for their three-fingered hands sporting large claw-like unguals. Here, we describe a new and unusual species of the therizinosaurid <i>Duonychus tsogtbaatari</i>, recovered from the lower Upper Cretaceous Bayanshiree Formation of the Gobi Desert of Mongolia. This species is different from other therizinosaurs in that the hand possesses only two fingers, rather than three fingers, the typical condition for Avetheropoda, providing further insight into forelimb/hand reduction among theropods. Phylogenetically, <i>Duonychus</i> reveals a broader but still limited appearance of didactyly among avetheropodans. The manual ungual of <i>Duonychus tsogtbaatari</i> also preserves a nearly complete three-dimensional structure covering the ungual of the left manual digit I, which represents a keratinous manual claw. The strong curvature of this large claw and high ungual flexion indicate that <i>Duonychus</i> was likely amplectorial (grasping) and an effective grasper of vegetation, despite having only two functional fingers.

镰刀龙类（Therizinosaurs）是一类植食性/杂食性兽脚类恐龙，生存于白垩纪时期的亚洲与北美洲。这类兽脚类恐龙以其生有大型爪状指骨的三指手部而闻名。本文记述了镰刀龙科（Therizinosauridae）下一种新颖奇特的物种——<i>Duonychus tsogtbaatari</i>，其化石发现于蒙古国戈壁沙漠的上白垩统下部巴彦希勒组（Bayanshiree Formation）。该物种与其他镰刀龙类的区别在于其手部仅具两指，而非鸟兽脚类（Avetheropoda）典型的三指结构，这为探究兽脚类恐龙前肢/手部的退化演化提供了新的视角。系统发育分析显示，<i>Duonychus</i>表明两指型结构在鸟兽脚类类群中的分布范围更广，但整体仍较为局限。<i>Duonychus tsogtbaatari</i>的手部指骨还保存了近乎完整的三维结构，覆盖于左手第一指的指骨之上，该结构即为角质爪鞘。该大型爪鞘强烈的弯曲形态与较高的指骨弯曲度表明，尽管仅具两枚功能性手指，<i>Duonychus</i>仍大概率具备握持能力，可高效抓取植物。