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畸头龙（Hyperodapedon）个体的多处躯干后骨骼元素的骨显微解剖结构。研究显示，纤层骨组织（fibrolamellar bone tissue）在皮质的中部和内层占主导地位，而皮质外周区域则由平行纤维骨和/或板层骨构成。初级骨单位的形态多介于层状与亚丛状之间。皮质中纤层骨组织的这种主导性，整体表明该类群曾经历快速生长，且在个体发育后期生长速率显著放缓。基于骨显微结构，本次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、晚亚成体阶段生长变为间断性；成体个体的生长较为缓慢且存在周期性中断，但并未完全停止，这表明畸头龙具有不定向生长策略。本次研究还记录了不同骨元素间的组织学变异，这些变异涉及皮质厚度、血管网络排布、生长纹出现频率以及次生重建程度。在整个个体发育过程中，股骨的皮质厚度均高于肱骨与胫骨，这表明不同骨骼元素的附加生长速率存在差异。肋骨的皮质厚度也存在差异，这表明不同解剖位置的肋骨受到的功能约束存在分化。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。畸头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所提出的普遍慢速生长模式，这表明快速生长在主龙形类中就已经存在。畸头龙各类长骨的皮质厚度与若干现存陆生四足动物的皮质厚度具有相似性，这表明畸头龙本质上为陆生四足动物。