Senckenberg Centre for Human Evolution and Palaeoenvironment (HEP) an der Eberhard-Karls-Universität Tübingen, Sigwartstraße 10, 72076, Tübingen, Germany.
Fachbereich Geowissenschaften, Eberhard-Karls-Universität Tübingen, Hölderlinstraße 12, 72074, Tübingen, Germany.
BMC Ecol Evol. 2021 Jul 5;21(1):136. doi: 10.1186/s12862-021-01835-4.
Placodontia is a Triassic sauropterygian reptile group characterized by flat and enlarged crushing teeth adapted to a durophagous diet. The enigmatic placodont Henodus chelyops has numerous autapomorphic character states, including extreme tooth count reduction to only a single pair of palatine and dentary crushing teeth. This renders the species unusual among placodonts and challenges identification of its phylogenetic position.
The skulls of two Henodus chelyops specimens were visualized with synchrotron tomography to investigate the complete anatomy of their functional and replacement crushing dentition in 3D. All teeth of both specimens were segmented, measured, and statistically compared to reveal that H. chelyops teeth are much smaller than the posterior palatine teeth of other cyamodontoid placodonts with the exception of Parahenodus atancensis from the Iberian Peninsula. The replacement teeth of this species are quite similar in size and morphology to the functional teeth.
As other placodonts, Henodus chelyops exhibits vertical tooth replacement. This suggests that vertical tooth replacement arose relatively early in placodont phylogeny. Analysis of dental morphology in H. chelyops revealed a concave shape of the occlusal surface and the notable absence of a central cusp. This dental morphology could have reduced dental wear and protected against failure. Hence, the concave teeth of H. chelyops appear to be adapted to process small invertebrate items, such as branchiopod crustaceans. Small gastropods were encountered in the matrix close to both studied skulls.
板龙形类是三叠纪的主龙类爬行动物群，其特征为扁平且增大的压碎牙齿，适应于吞食硬物体的饮食方式。神秘的板龙形类爬行动物亨氏喙头龙具有许多独特的特征状态，包括牙齿数量极端减少，仅有一对腭骨和齿骨压碎牙齿。这使得该物种在板龙形类中与众不同，并挑战了其系统发育位置的识别。
利用同步加速器断层扫描技术对两个亨氏喙头龙标本的头骨进行可视化，以研究其功能性和替换性压碎齿列的完整解剖结构。对两个标本的所有牙齿进行了分割、测量和统计学比较，结果表明亨氏喙头龙的牙齿比其他基亚门龙形类爬行动物的后腭牙齿小得多，只有伊比利亚半岛的副喙头龙属的 Parahenodus atancensis 除外。该物种的替换牙齿在大小和形态上与功能性牙齿非常相似。
与其他板龙形类爬行动物一样，亨氏喙头龙表现出垂直的牙齿替换方式。这表明垂直的牙齿替换在板龙形类的进化历史中出现得相对较早。对亨氏喙头龙的牙齿形态进行分析表明，其咬合面呈凹形，且明显没有中央尖。这种牙齿形态可能减少了牙齿磨损，并起到了防止牙齿失效的保护作用。因此，亨氏喙头龙的凹形牙齿似乎适应于处理小型无脊椎动物食物，如介形类甲壳动物。在靠近两个研究头骨的基质中发现了小型腹足动物。
