The biogeographic history of the allokotosaurian archosauromorphs in the Triassic of Pangaea

Allokotosauria (Trilophosauridae + Azendohsauridae) is a clade of Early – Late Triassic quadruped, sprawling archosauromorphs with diverse dietary habits, known from Madagascar, India, Morocco, North America, and Europe. We conducted the first quantitative biogeographic analysis of the clade to reconstruct its ancestral areas and dispersal events. The results estimated the India + Tanzania area as ancestral for Allokotosauria. Eastern North America was reconstructed as the ancestral area of Trilophosauridae, implying an Early Triassic Southern to Northern Hemisphere dispersal. Subsequently, trilophosaurids were restricted to the Northern Hemisphere, and at least two dispersal events from Europe to western North America are estimated during the early Anisian and probably during the early Carnian, respectively. Azendohsauridae retained the ancestral area of India + Tanzania during much of its evolutionary history. Two dispersal events to western and eastern North America, respectively, are reconstructed during the Carnian – early Norian, likely during or in the aftermath of the Carnian Pluvial Episode (CPE). This pattern mirrors that of early dinosaurs, which were restricted to the Southern Hemisphere with dispersals to the Northern Hemisphere not recorded until the end of the CPE. This study informs about biogeographic patterns involved in the biotic recovery after the end-Permian mass extinction and the early evolutionary radiation of Archosauromorpha.

异索龙类（Allokotosauria，包含三脊龙科（Trilophosauridae）与阿真多龙科（Azendohsauridae））是一支生存于早三叠世至晚三叠世的四足匍匐型主龙形类（Archosauromorpha）动物，食性多样，化石发现于马达加斯加、印度、摩洛哥、北美及欧洲。本研究首次对该演化支开展定量生物地理学分析，以重建其祖先分布区与扩散事件。分析结果显示，异索龙类的祖先分布区为印度+坦桑尼亚区域。三脊龙科的祖先分布区被重建为北美东部，这暗示其在早三叠世曾发生从南半球向北半球的扩散事件。此后三脊龙类的分布被限制在北半球，研究估算至少存在两次从欧洲向北美西部扩散的事件，分别发生于安尼期早期与卡尼期早期。阿真多龙科在其演化的绝大多数时期都保留了印度+坦桑尼亚的祖先分布区。研究重建出两次分别向北美西部与北美东部扩散的事件，发生于卡尼期至诺利早期，大概率对应卡尼雨期（Carnian Pluvial Episode, CPE）发生期间或其之后。该分布模式与早期恐龙的情况高度相似：早期恐龙的分布同样被限制在南半球，且直到卡尼雨期结束前都未有向北半球扩散的记录。本研究为二叠纪末大灭绝后的生物复苏，以及主龙形类的早期演化辐射相关的生物地理学模式提供了新的认知。