Mestriner Gabriel, Funston Gregory F, Nesbitt Sterling J, Marsola Júlio C A, Evans David C, Sidor Christian A, Langer Max C, LeBlanc Aaron R H
Departamento de Biologia, Universidade de São Paulo, Ribeirão Preto, Brazil.
Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Ontario, Canada.
J Anat. 2025 Aug 27. doi: 10.1111/joa.70037.
The study of the connection between the teeth and the jaw is important for understanding the palaeobiology of vertebrates, but inconsistent terminology and incomplete sampling have made it difficult to assess the evolutionary significance of some of the related characters. Among archosauromorphs, tooth attachment in dinosaurs and crocodylians is nearly identical to that of mammals in featuring a ligamentous connection (gomphosis), whereas closely related forms appear to have teeth fused to the jaws (ankylosis), as in most other amniotes. Hence, studying tooth attachment of stem-archosaurs is pivotal to characterize the main shifts in tooth attachment seen in the lineage. Here, we analyze the tooth attachment of rhynchosaurs - a group of quadrupedal herbivorous archosauromorphs that played a key role as primary consumers in many Triassic communities. Their dentition consists of multiple rows of marginal teeth with posterolingual addition of teeth during growth, but their tooth attachment has not been documented in a modern context. Histological data from three rhynchosaur specimens from the Middle Triassic Manda Beds of Tanzania show that, although ankylosed, rhynchosaur teeth are surrounded by an extensive network of Sharpey's fibers, layers of cementum, and well-defined zones of alveolar bone. What has been previously described as "spongy bone of attachment" in fact encompasses the same attachment tissues present in mammals, dinosaurs, and crocodylians, albeit completely mineralized in mature teeth. Analysis of different stages of tooth development shows that ankylosis occurs by the growth of alveolar bone towards the cellular cementum, which eventually mineralizes the soft ligament. This suggests that the tissues conflated as "bone of attachment"-alveolar bone, periodontal ligament, and cellular cementum-are homologous across Archosauromorpha. Our data add to a growing body of evidence that heterochronic changes to the timing and extents of mineralization, not convergent evolution to mammal-like attachment tissues, led to the independent evolution of gomphosis across many amniote lineages, including archosauromorphs.
对牙齿与颌骨之间联系的研究对于理解脊椎动物古生物学十分重要，但术语不一致和采样不完整使得评估某些相关特征的进化意义变得困难。在主龙形类中，恐龙和鳄鱼的牙齿附着与哺乳动物几乎相同，其特点是有韧带连接（齿槽连接），而与之亲缘关系较近的类群似乎牙齿与颌骨融合（骨性结合），就像大多数其他羊膜动物那样。因此，研究基干主龙类的牙齿附着对于描述该谱系中牙齿附着的主要转变至关重要。在此，我们分析了喙头龙类的牙齿附着情况——喙头龙类是一群四足草食性主龙形类，在许多三叠纪群落中作为主要消费者发挥了关键作用。它们的齿列由多排边缘齿组成，在生长过程中牙齿向舌后侧添加，但它们的牙齿附着情况在现代背景下尚未有记录。来自坦桑尼亚中三叠世曼达组的三个喙头龙标本的组织学数据表明，尽管喙头龙的牙齿是骨性结合的，但它们被大量的沙比纤维、牙骨质层和明确的牙槽骨区域所包围。之前被描述为“附着性海绵骨”的结构实际上包含了与哺乳动物、恐龙和鳄鱼中相同的附着组织，尽管在成熟牙齿中这些组织完全矿化了。对牙齿发育不同阶段的分析表明，骨性结合是通过牙槽骨向细胞性牙骨质生长而发生的，最终使软韧带矿化。这表明被合并为“附着骨”的组织——牙槽骨、牙周韧带和细胞性牙骨质——在主龙形类中是同源的。我们的数据进一步补充了越来越多的证据，即矿化时间和程度的异时性变化，而非向类似哺乳动物附着组织的趋同进化，导致了包括主龙形类在内的许多羊膜动物谱系中齿槽连接的独立进化。
