A new specimen of <i>Platypterygius sachicarum</i> (Reptilia, Ichthyosauria) from the Early Cretaceous of Colombia and its phylogenetic implications

<i>Platypterygius sachicarum</i> is one of the few Lower Cretaceous ichthyosaurian species described from the Hauterivian–Aptian-aged Paja Formation, the most complete Lower Cretaceous sedimentary sequence in northern South America. To date, <i>P. sachicarum</i> has been described only from a single skull, limiting morphological, stratigraphic, and phylogenetic comparisons. Here, we describe a new skull and associated postcranium of upper Barremian age from Villa de Leyva, Colombia, which represent the first documented postcranial remains of this species and enable detailed comparison with other Early Cretaceous ophthalmosaurid ichthyosaurs. <i>Platypterygius sachicarum</i> shares many morphological similarities with contemporaneous taxa from Europe but differs in both skull and tooth morphology from coeval South American species from northern South America and from the eastern Pacific. The additional data provide new diagnostic characters for this species and resolve the position of <i>P. sachicarum</i> as part of a polytomy of other species historically referred to <i>Platypterygius</i>. However, as with previous analyses, all recovered clades are poorly supported. The rich vertebrate fossil record of the Paja Formation provides an unparalleled opportunity to explore paleobiogeographic and paleoecological questions pertaining to Cretaceous marine reptiles; however, in most cases, a robust stratigraphic and phylogenetic framework remains elusive.萨氏扁鳍鱼龙（Platypterygius sachicarum）是产自南美洲北部保存最为完整的早白垩世沉积序列——豪特里维阶至阿普第阶（Hauterivian–Aptian）帕哈组（Paja Formation）的少数早白垩世鱼龙类（ichthyosaurian）物种之一。迄今为止，萨氏扁鳍鱼龙仅根据单一头骨化石被正式报道，这极大限制了针对该物种的形态学、地层学与系统发育学对比研究的开展。本次研究描述了产自哥伦比亚莱瓦镇（Villa de Leyva）的一件巴列姆阶（Barremian）上部的头骨及关联颅后骨骼标本材料，该标本为该物种首个被正式记录的颅后遗骸，也为其与其他早白垩世大眼鱼龙科（Ophthalmosauridae）鱼龙类开展详细形态学对比提供了可能。萨氏扁鳍鱼龙与欧洲同期的鱼龙类类群具有诸多形态学相似性，但在头骨与牙齿形态上，均与南美洲北部及东太平洋地区同时期的南美本土鱼龙物种存在显著差异。本次新增的标本数据为该物种提供了新的鉴定特征，并明确了萨氏扁鳍鱼龙的系统发育位置：其隶属于历史上被归入扁鳍鱼龙属（Platypterygius）的多个物种所构成的多歧支（polytomy）类群。但与既往研究分析结果一致，本次研究所恢复的所有演化支（clade）均支持度较低。帕哈组产出的丰富脊椎动物化石记录，为探索与白垩纪海洋爬行动物相关的古生物地理学（paleobiogeography）与古生态学（paleoecology）问题提供了得天独厚的研究机遇；然而在多数情况下，构建可靠的地层学与系统发育学研究框架仍颇具挑战。