Data from: New remains of the aquatic turtle Hydromedusa casamayorensis (Pleurodira, Chelidae) from the middle Eocene of Patagonia: taxonomic validation and phylogenetic relationships

Twenty- seven new specimens recovered from lower levels of the Sarmiento Formation (Casamayoran SALMA, middle Eocene) in the ¨Cañadón Hondo¨ area, Chubut (Argentina), bring new information on the Palaeogene long-necked chelid Hydromedusa casamayorensis.The discovery of cranial and postcranial remains allows a complete description of the only known extinct species of the genus Hydromedusa recognizing diagnostic characters that revalidate the taxonomic status of this species. Hydromedusa casamayorensis is incorporated into a cladistic framework of chelid turtles. The unconstrained and constrained phylogenetic analyses recovered H. casamayorensis as a sister taxon of Hydromedusa maximiliani plus H. tectifera. A Danian age is proposed at least for the origin of the Hydromedusa clade, while a Late Cretaceous is suggested for the age of the split from the Yaminuechelys clade. Great differences in size are documented for H. casamayorensis based on the various specimens recovered from this study. The osteohistological study in H. casamayorensis and H. tectifera shows similarities in the internal cortex and cancellous bone of both taxa, but some differences are observed in the external cortex of H. casamayorensis (e.g. a parallel fibered bone on the more external surface of this cortex).

从阿根廷丘布特省（Chubut）坎洪多谷（Cañadón Hondo）地区萨尔米恩托组（Sarmiento Formation）下伏地层（时代为始新世中期卡萨马约兰期南美陆生哺乳动物化石年代单元（SALMA））中发现的27件新标本，为古近纪长颈侧颈龟类（Chelidae）的卡氏溪龟（Hydromedusa casamayorensis）提供了全新研究资料。本次发现的颅骨及颅后骨骼遗骸，使得研究人员能够对溪龟属（Hydromedusa）目前仅知的灭绝物种展开完整形态描述，并识别出可重新确立该物种分类学地位的鉴定特征。研究将卡氏溪龟纳入侧颈龟科的支序分类学分析框架之中。无约束与约束条件下的系统发育分析均显示，卡氏溪龟为马克西米利安溪龟（Hydromedusa maximiliani）与泰氏溪龟（Hydromedusa tectifera）构成支系的姊妹群。研究提出溪龟支系的起源时间至少可达达宁期（Danian），而其与亚明努伊侧颈龟属（Yaminuechelys）支系的分化时间则被推定为晚白垩世。基于本次研究采集的多件标本，研究记录了卡氏溪龟存在显著的个体体型差异。对卡氏溪龟与泰氏溪龟的骨组织学研究显示，二者的内皮层骨组织与松质骨结构具有相似性，但卡氏溪龟的外皮层骨组织存在若干差异（例如该皮层更表层区域存在平行纤维骨结构）。