Data from: New insights from bone microanatomy of the Late Triassic Hyperodapedon (Archosauromorpha, Rhynchosauria): implications for archosauromorph growth strategy

Bone microanatomy of multiple postcranial skeletal elements of several individuals of Hyperodapedon collected from India is reported. This reveals that fibrolamellar bone tissue is predominant in the mid- and inner cortices, whereas the peripheral region of the cortex is composed of either parallel-fibred and/or lamellar bone. The pattern of primary osteons mostly ranges between laminar and subplexiform. Such predominance of fibrolamellar bone tissue in the cortex suggests an overall fast growth, which slowed down considerably later in ontogeny. Four distinct ontogenetic stages are identified based on the bone microstructure. During the juvenile stage, growth was fast and continuous, but it became punctuated during the early and late sub-adult stages. In adult individuals, growth was slow and showed periodic interruption but did not stop completely, suggesting that Hyperodapedon had an indeterminate growth strategy. Interelemental histovariations affecting cortical thickness, organization of the vascular network, incidence of growth rings and extent of secondary reconstruction are noted. Throughout ontogeny, the femora show higher cortical thickness than humeri and tibiae, suggesting differential appositional growth rate between the skeletal elements. Differences in cortical thickness are noted in the ribs, which suggest differential functional constraints based on anatomical site-specific occurrences. Although fibrolamellar bone tissue became progressively more dominant towards the archosaurs, there are considerable variations in the growth patterns of the archosauromorphs. This is exemplified by the bone microstructure of Hyperodapedon, which deviates from the generalized slow-growth pattern proposed for all basal archosauromorphs, suggesting that rapid growth was already present in the archosauromorphs. The cortical thickness of various long bones of Hyperodapedon bears similarity with that of several extant terrestrial quadrupeds, suggesting that Hyperodapedon was essentially a terrestrial quadruped.本文报道了采自印度的多件海德坡齿龙（Hyperodapedon）个体的多件颅后骨骼元素的骨显微解剖学特征。研究结果显示，纤层骨组织（fibrolamellar bone tissue）在皮质骨的中层和内层占主导地位，而皮质骨的外周区域则由平行纤维骨和/或板层骨构成。初级骨单位（primary osteons）的形态主要介于层状与亚丛状之间。皮质骨中纤层骨组织的这种优势占比，整体反映出该物种曾具有快速生长模式，且在个体发育后期生长速率显著放缓。基于骨显微结构，本研究识别出四个明确的个体发育阶段：幼体阶段生长快速且持续，但在早亚成体和晚亚成体阶段生长变为间断式；成体阶段生长缓慢且呈现周期性中断，但并未完全停止，这表明海德坡齿龙具有不定生长策略。本研究还记录了骨骼元素间的组织学变异，这些变异会影响皮质骨厚度、血管网络排布、生长环出现频率以及次生重建范围。在整个个体发育过程中，股骨的皮质骨厚度均高于肱骨和胫骨，这说明不同骨骼元素的外积生长速率存在差异。肋骨的皮质骨厚度也存在差异，这提示不同解剖部位的骨骼受到了差异化的功能约束。尽管纤层骨组织在主龙类（archosaurs）中逐渐占据主导地位，但主龙形类（archosauromorphs）的生长模式存在显著变异。海德坡齿龙的骨显微结构正是这一现象的例证：其生长模式偏离了针对所有基干主龙形类提出的通用慢速生长模式，这表明快速生长在主龙形类中就已存在。海德坡齿龙多种长骨的皮质骨厚度与若干现存陆生四足动物的皮质骨厚度具有相似性，这说明海德坡齿龙本质上为陆生四足动物。