Data from: A lower jaw of Palaeoxonodon from the Middle Jurassic of the Isle of Skye, Scotland, sheds new light on the diversity of British stem therians

The Middle Jurassic was a key interval of mammalian evolutionary history that witnessed the diversification of the therian stem-group. Great Britain has yielded a significant record of mammalian fossils from this interval, represented by numerous isolated jaws and teeth from the Bathonian of Oxfordshire and the Isle of Skye. This record captures a key period in early cladotherian evolution, with amphitheriids, peramurans and “stem zatherians” displaying intermediate talonid morphologies that document the evolutionary assembly of tribosphenic molars. We present a mandible with near-complete dentition from the late Bathonian (ca. 167.4–166.5 Ma) Kilmaluag Formation, near Elgol, Skye, representing the amphitheriid Palaeoxonodon ooliticus, previously known only from isolated teeth. The specimen sheds new light on the taxonomic diversity of British Middle Jurassic stem therians, as the morphological variation within the preserved tooth row encompasses that previously ascribed to three distinct species within two genera: Palaeoxonodon ooliticus, P. freemani, and Kennetheridium leesi. Thus, both P. freemani and Kennetheridium leesi are subjective junior synonyms of P. ooliticus. The dental formula of P. ooliticus (i4:c1:p5:m5) is intermediate between the primitively larger postcanine count (p5:m6-7) of Amphitherium and the reduced number in peramurans and tribosphenidans (p5:m3). Phylogenetic analyses of P. ooliticus generally confirm a close affinity with Amphitherium, but highlight the lack of strong empirical support for hypothesized patterns of divergences among early cladotherians.中侏罗世是哺乳动物演化历史的关键阶段，见证了兽类干群（therian stem-group）的辐射分化。英国产出了该时期极具科学价值的哺乳动物化石记录，以产自牛津郡巴通阶（Bathonian）和斯凯岛（Isle of Skye）的大量孤立颌骨与牙齿为代表。该记录涵盖了早期支系兽类（cladotherian）演化的关键时期：amphitheriid类群、peramuran类群以及“干群扎氏兽类”（stem zatherians）展现出过渡性的下跟座（talonid）形态，完整记录了磨楔式臼齿（tribosphenic molars）的演化组装过程。本次研究报道了一件产自斯凯岛埃尔戈尔（Elgol）附近晚巴通阶（约1.674亿~1.665亿年前）基尔马鲁格组（Kilmaluag Formation）的带有近乎完整齿列的下颌骨，该标本代表amphitheriid类群的古棱齿兽（Palaeoxonodon ooliticus）——该物种此前仅通过孤立牙齿被认知。该标本为英国中侏罗世兽类干群的分类多样性提供了新视角：保存在齿列中的形态变异范围，覆盖了此前被归为两个属内三个不同物种的特征：古棱齿兽（Palaeoxonodon ooliticus）、弗里曼古棱齿兽（P. freemani）以及利氏肯尼兽（Kennetheridium leesi）。因此，弗里曼古棱齿兽与利氏肯尼兽均为古棱齿兽的主观次同物异名。古棱齿兽的齿式为i4:c1:p5:m5，其处于Amphitherium的原始更大后齿列数目（p5:m6-7）与peramuran类群和磨楔齿类（tribosphenidans）的缩减数目（p5:m3）之间。针对古棱齿兽的系统发育分析总体证实了其与Amphitherium的近缘关系，但同时凸显出目前缺乏强有力的实证支持，以验证早期支系兽类间被提出的演化分化模式。