New dicynodonts (Therapsida, Anomodontia) from near the Permo-Triassic boundary of Laos: implications for dicynodont survivorship across the Permo-Triassic mass extinction and the paleobiogeography of Southeast Asian blocks

The dicynodonts are an emblematic group of herbivorous therapsids that survived the Permo-Triassic (P-Tr) crisis. Laotian dicynodonts from stratigraphically constrained beds, recently dated using the U-Pb zircon method, yield new insights into terrestrial faunas of Southeast Asia during the latest Permian and earliest Triassic. Summarily described, they were originally attributed to the genus <i>Dicynodon</i>. We provide a new phylogenetic analysis for Laotian dicynodonts, based on three well-preserved skulls, indicating that they belong to two new taxa: <i>Counillonia superoculis</i>, gen. et sp. nov., and <i>Repelinosaurus robustus</i>, gen. et sp. nov. Our phylogenetic analysis of Dicynodontia indicates that (1) <i>Counillonia</i> is closely related to some ‘<i>Dicynodon</i>’-grade taxa and (2) <i>Repelinosaurus</i> is a kannemeyeriiform. The phylogenetic affinities of these new Laotian dicynodonts allow discussion of the survivorship of multiple lineages (Kannemeyeriiformes and ‘<i>Dicynodon</i>’-grade dicynodontoids) across the P-Tr crisis. The Laotian dicynodonts also shed new light on the paleobiogeography of Southeast Asia from the late Paleozoic to the early Mesozoic, particularly the timing of collisions between the Indochina, the South China, and the North China blocks. The presence of dicynodonts in Laos, most likely in the Early Triassic, thus implies that the connection between the Indochina Block and the South China Block occurred no later than the latest Permian or earliest Triassic (i.e., when the dicynodonts provide direct evidence for a connection).

二齿兽类（dicynodonts）是一类极具标志性的植食性兽孔目（therapsids）类群，曾幸存于二叠纪-三叠纪（Permo-Triassic, P-Tr）灭绝事件。产自具明确地层层位岩层的老挝二齿兽类，近期通过U-Pb锆石定年法（U-Pb zircon method）测定了年代，为研究晚二叠世至早三叠世东南亚陆生动物群提供了全新见解。此前该类群仅被简要描述，最初被归属于二齿兽属（Dicynodon）。我们基于三件保存完好的头骨标本，对老挝二齿兽类开展了全新的系统发育分析，结果显示它们隶属于两个新分类单元：新属新种大眼库尼兽（*Counillonia superoculis*, gen. et sp. nov.）以及新属新种粗壮雷佩林兽（*Repelinosaurus robustus*, gen. et sp. nov.）。我们针对二齿兽亚目（Dicynodontia）的系统发育分析结果显示：其一，库尼兽属与部分“二齿兽型”类群亲缘关系密切；其二，雷佩林兽属属于肯氏兽形类（kannemeyeriiform）。基于这两个老挝新二齿兽类的系统发育亲缘关系，我们得以探讨多个类群（肯氏兽形类与“二齿兽型”二齿兽类）在二叠纪-三叠纪灭绝事件中的残存情况。老挝二齿兽类还为晚古生代至早中生代东南亚的古生物地理学研究提供了新视角，尤其针对印支地块（Indochina Block）、华南地块（South China Block）与华北地块（North China Block）的碰撞时间这一核心议题。老挝境内（其产出层位最可能属于早三叠世）存在二齿兽类这一事实，意味着印支地块与华南地块的陆块联结不晚于晚二叠世至早三叠世（即该二齿兽类类群提供了陆块联结直接化石证据的时期）。