A new diapsid with a unique tooth structure from the Middle Triassic (Ladinian) of Germany

The Middle Triassic was an important time in the evolutionary history of reptiles because it was during this time that many modern groups originated and initially diversified. In this context, the rich fossiliferous deposits of the Ladinian-age Erfurt Formation in southern Germany play an important role in elucidating reptilian diversity during this interval. Here we present an interesting record of a reptile with an unusual dentition. The material comprises a fragment of a left dentary. The teeth are tightly spaced and have chisel-shaped crowns, with the posterolingually extending apices abutting those of the successive teeth and forming a continuous cutting edge. Micro-computed tomography scanning of the jaw fragment revealed the teeth have expanded roots and sit on an open, shallow groove. The Meckelian canal is partially open lingually and a second canal for the inferior alveolar nerve and artery extends parallel to it. Although the preserved anatomical features of the material are insufficient for precise phylogenetic placement, the structure of the robust dentary and its teeth resembles those of some lepidosaurs and certain early Mesozoic marine diapsids. The distinctive morphology of the teeth, combined with the apparent lack of tooth wear, makes it difficult to assess their functional significance. It is interesting that extinct tetrapods with similar tooth morphologies come from strata representing freshwater and brackish-water paleoenvironments, which may serve as an additional line of evidence for future Morpho-functional assessments.

中三叠世是爬行动物演化历史上的关键时期，诸多现代爬行动物类群在此期间起源并初步实现辐射演化。在此研究背景下，德国南部拉丁期（Ladinian）埃尔福特组（Erfurt Formation）产出的富含化石的沉积序列，对于阐明该时期的爬行动物多样性具有重要意义。本研究报道了一件具有特殊齿系的爬行动物化石记录，该标本仅保存有左侧齿骨碎片。该标本的牙齿排列紧密，齿冠呈凿状，舌后向延伸的齿尖与后续牙齿的齿尖相互贴合，形成了连续的切割刃。通过对该齿骨碎片开展显微计算机断层扫描（Micro-computed tomography）分析，发现其牙齿具有膨大的齿根，且附着于一条开放的浅凹槽之上。梅克尔管（Meckelian canal）在舌侧部分开放，另有一条用于通行下牙槽神经与动脉的第二管道与其平行延伸。尽管该标本保存的解剖学特征不足以实现精准的系统发育定位，但其粗壮的齿骨及其牙齿的结构与部分鳞龙类（lepidosaurs）以及部分早中生代海洋双弓类动物具有相似性。值得关注的是，具有相似牙齿形态的已灭绝四足类动物均产自代表淡水与微咸水古环境的地层中，这或许可为未来的形态功能学评估提供额外的证据线索。