The turtles from ‘Gilmore’s slab’: evolution and diversity of the extinct turtle <i>Echmatemys</i> (Testudines: Testudinoidea: Geoemydidae) from the central United States

This contribution presents a complete revision of the extinct geoemydid <i>Echmatemys</i> from North America, based on new information and a re-evaluation of previously published specimens. <i>Echmatemys</i> is the most diverse and widely distributed geoemydid turtle in the fossil record of North America, known from numerous shells. The different morphotypes are assigned to the currently accepted valid species and their evolutionary history is evaluated in a cladistic context. Their function is further evaluated with an anatomical network analysis. This re-appraisal is based mainly on ‘Gilmore’s slab’, a slab containing several turtle shells collected from the Bridgerian of Levett Creek (Wyoming, USA). The fossils from this slab are described in detail for the first time and are completely re-identified. As this slab represents a snapshot of the turtle community in past Wyoming during the Bridgerian it offers a unique opportunity to discuss the evolution and diversity of <i>Echmatemys</i>. The morphological variability in the shells of <i>Echmatemys</i> challenges its recovery as a monophyletic group under parsimony analysis. Also, most <i>Echmatemys</i> spp. are recovered in a derived position within Geoemydidae, a result that poses new problems in the interpretation of the origin and the palaeobiogeographical evolution of this clade in North America.本研究基于最新发现的资料，并重新评估了此前已发表的标本，对北美已灭绝的地龟科（Geoemydidae）厄赫马龟属（*Echmatemys*）类群开展了全面修订。厄赫马龟属是北美化石记录中多样性最高、分布最广的地龟科龟类类群，目前已发现大量完整龟壳化石。本研究将不同的形态型归入当前公认的有效物种，并在分支系统学框架下探讨其演化历史；同时借助解剖网络分析进一步解析其功能特征。本次重新评估的核心材料为“吉尔莫石板（Gilmore’s slab）”，该石板出土于美国怀俄明州莱维特溪的布里杰阶地层，内含多块龟类甲壳化石。本研究首次对该石板中的化石进行了详细描述，并完成了全面的重新鉴定。由于该石板完整呈现了布里杰期怀俄明地区的龟类群落面貌，为探讨厄赫马龟属的演化与多样性提供了独一无二的研究契机。厄赫马龟属甲壳的形态多样性，使得简约法分析下难以将其恢复为单系群；此外，绝大多数厄赫马龟属物种在分支系统学分析中被置于地龟科的衍生位置，这一结果为解释该类群在北美的起源与古生物地理演化带来了新的科学问题。