A new Late Cretaceous iguanomorph from North America and the origin of New World Pleurodonta (Squamata, Iguania)

Iguanomorpha (stem + crown Iguania) is a diverse squamate clade with members that predominate many modern American lizard ecosystems. However, the temporal and palaeobiogeographic origins of its constituent crown clades (e.g. Pleurodonta (basilisks, iguanas, and their relatives)) are poorly constrained, mainly due to a meagre Mesozoic-age fossil record. Here, we report on two nearly complete skeletons from the Late Cretaceous (Campanian) of North America that represent a new and relatively large-bodied and possibly herbivorous iguanomorph that inhabited a semi-arid environment. The new taxon exhibits a mosaic of anatomical features traditionally used in diagnosing Iguania and non-iguanian squamates (i.e. Scleroglossa; e.g. parietal foramen at the frontoparietal suture, astragalocalcaneal notch in the tibia, respectively). Our cladistic analysis of Squamata revealed a phylogenetic link between Campanian-age North American and East Asian stem iguanomorphs (i.e. the new taxon + Temujiniidae). These results and our evaluation of the squamate fossil record suggest that crown pleurodontans were restricted to the low-latitude Neotropics prior to their early Palaeogene first appearances in the mid-latitudes of North America.

鬣蜥形类（Iguanomorpha，涵盖干群鬣蜥亚目与冠群鬣蜥亚目）是一个物种多样的有鳞类支系，其成员在诸多现代美洲蜥蜴生态系统中占据主导地位。然而，其所含冠群支系（例如侧齿蜥类（Pleurodonta，包括背鳞蜥、鬣蜥及其近缘类群））的时间与古生物地理起源仍缺乏精确限定，这一问题主要源于中生代化石记录极为匮乏。本文报道了两件采自北美洲晚白垩世坎潘期（Campanian）的近乎完整的骨架标本，它们代表一种全新的、体型相对较大且可能为植食性的鬣蜥形类，该类群栖息于半干旱环境之中。该新分类单元展现出一套镶嵌分布的解剖特征，这些特征传统上分别用于诊断鬣蜥亚目与非鬣蜥类有鳞类（即硬舌亚目（Scleroglossa）；例如分别位于额顶骨缝的顶骨孔、胫骨的距跟骨切迹）。我们针对有鳞类开展的分支系统学分析显示，坎潘期北美洲与东亚的干群鬣蜥形类（即本新分类单元与特蒙古蜥科（Temujiniidae））之间存在系统发育关联。结合本次研究结果与对有鳞类化石记录的综合评估，我们认为冠群侧齿蜥类在古近纪早期首次现身北美洲中纬度地区之前，仅局限于低纬度的新热带区。