Osteohistology and paleobiological inferences of proterochampsids (Eucrocopoda: Proterochampsia) from the Chañares Formation (late Ladinian–early Carnian), La Rioja, Argentina

Proterochampsids were a group of stem archosaurs from the Middle–Late Triassic of South America. Using for the first time in proterochampsids quantitative microanatomical and morphological data and an inference model, we analyze the microstructure of postcranial bones of proterochampsids to infer life-history traits dealing with growth dynamics, ontogenetic changes, dermal armor histogenesis and lifestyle. We studied proterochampsid individuals from the Middle–Late Triassic Chañares Formation: three specimens of <i>Chanaresuchus bonapartei</i>: PVL 4575 (osteoderms and femur), CRILAR-Pv 80 (osteoderm) and CRILAR-Pv 81 (humerus), and a femur of a non-identified Rhadinosuchinae specimen (CRILAR-Pv 488). The osteoderms show a compact-uniform composition of parallel-fibered bone and lines of arrested growth (LAGs). The long bones are composed mainly of highly vascularized, parallel-fibered bone and isolated areas of woven-fibered bone in the innermost portion of cortex (only in <i>C. bonapartei</i> PVL 4575). The absence of an external fundamental system (EFS) indicates that all of the sampled individuals were somatically immature at time of death. The absence of mineralized bundles of collagenous fibers supports intramembranous ossification as the developmental origin for the osteoderms. With an inference model used for the first time in proterochampsids, the lifestyle inferred for <i>C. bonapartei</i> is terrestrial. Histological evidence indicates that sexual maturity is reached before both somatic and skeletal maturity. The growth patterns among proterochampsids exhibit some degree of variation even within a single species. Such variation can be due to different causes, including possibly phenotypic plasticity.

前弓龙类（Proterochampsids）是一类产自南美洲中晚三叠世的基干主龙类（stem archosaurs）类群。本研究首次将定量显微解剖学与形态学数据及推断模型应用于前弓龙类研究，通过分析其躯干骨骼的微观结构，以推断其生长动态、个体发育变化、真皮骨板组织发生以及生活方式等生活史特征。本次研究的标本采自中晚三叠世查尼亚雷斯组（Chañares Formation），包括3件博氏查纳雷龙（Chanaresuchus bonapartei）标本：PVL 4575（真皮骨板与股骨）、CRILAR-Pv 80（真皮骨板）以及CRILAR-Pv 81（肱骨），另有1件未鉴定的拉迪诺龙亚科（Rhadinosuchinae）标本的股骨（CRILAR-Pv 488）。该类群的真皮骨板呈现出由平行纤维骨构成的致密均一结构，并保留有生长停滞线（lines of arrested growth, LAGs）。长骨主要由高血管化平行纤维骨构成，仅在博氏查纳雷龙PVL 4575的骨皮质最内层区域可见孤立的编织纤维骨分布。未发现外源性基本生长系统（external fundamental system, EFS），表明所有采样个体在死亡时仍处于躯体发育未成熟阶段。未发现矿化胶原纤维束，这支持真皮骨板的发育起源为膜内成骨（intramembranous ossification）。本研究首次在前弓龙类研究中应用推断模型，推断得出博氏查纳雷龙的生活方式为陆生。组织学证据显示，其性成熟早于躯体成熟与骨骼成熟。即使在同一物种内，前弓龙类的生长模式也存在一定程度的变异，这类变异可由多种因素导致，其中可能包括表型可塑性（phenotypic plasticity）。