Goldsmith Erika R, Barta Daniel E, Kligman Ben T, Nesbitt Sterling J, Marsh Adam D, Parker William G, Stocker Michelle R
Department of Geosciences, Virginia Tech, Blacksburg, Virginia, USA.
Department of Anatomy and Cell Biology, Oklahoma State University College of Osteopathic Medicine at the Cherokee Nation, Tahlequah, Oklahoma, USA.
J Anat. 2025 Sep-Oct;247(3-4):556-575. doi: 10.1111/joa.14185. Epub 2024 Dec 3.
Fossils of embryonic and hatchling individuals can provide invaluable insight into the evolution of prenatal morphologies, heterochronies, and allometric trajectories within Archosauria but are exceptionally rare in the Triassic fossil record, obscuring a critical aspect of archosaurian biology during their evolutionary origins. Microvertebrate sampling at a single bonebed in the Upper Triassic Chinle Formation within Petrified Forest National Park has yielded diminutive archosauriform femora (PEFO 45274, PEFO 45199) with estimated and measured femoral lengths of ~31 mm and ~ 37 mm, respectively. These new specimens provide the unique opportunity to assess the preservation, body size, and growth dynamics of skeletally immature archosauriforms in North America and compare the growth dynamics of archosauromorphs within an evolutionary and ontogenetic context. We assign PEFO 45199 and PEFO 45274 to Phytosauria (Archosauriformes) based on their strongly sigmoidal shape in lateral view, the presence of proximal anterolateral and posteromedial tubera, the absence of an anteromedial tuber of the proximal end, a teardrop-shaped proximal outline, and a fourth trochanter that is not confluent with the proximal head. Osteohistological analyses of PEFO 45274 reveal a cortex comprising low vascularity, parallel-fibered bone composed of primary osteons that lacks a hatching line and any lines of arrested growth. We interpret PEFO 45274 as a slow-growing, post-hatching individual of less than 1 year of age. Surprisingly, osteohistology of some larger phytosaur femora implies faster growth rates in comparison to PEFO 45274 based on the occasional presence of woven bone and overall higher degrees of vascular density, suggesting the ontogenetic shift from rapid-to-slow growth rates might not occur simply or uniformly as expected in Phytosauria and that non-archosaurian archosauriforms may exhibit size-dependent histological characteristics. This study highlights the importance of including osteohistology from multiple body sizes to investigate non-archosaurian archosauriform ancestral growth rates given the phylogenetic position of phytosaurs near the divergence of Archosauria.
胚胎和幼体化石能够为深入了解主龙类产前形态、发育时间异速生长以及异速生长轨迹的演化提供极有价值的见解，但在三叠纪化石记录中极为罕见，这使得主龙类生物学在其演化起源阶段的一个关键方面变得模糊不清。在石化森林国家公园内上三叠统钦尔组的一个单一骨床进行的微体脊椎动物采样，获得了小型的主龙形股骨（PEFO 45274、PEFO 45199），其估计和测量的股骨长度分别约为31毫米和37毫米。这些新标本提供了一个独特的机会，来评估北美骨骼未成熟主龙形动物的保存情况、体型大小和生长动态，并在演化和个体发育背景下比较主龙形动物的生长动态。基于其在侧视图中强烈的S形、近端前外侧和后内侧结节的存在、近端无前内侧结节、泪滴形近端轮廓以及第四转子不与近端头部融合，我们将PEFO 45199和PEFO 45274归入植龙目（主龙形类）。对PEFO 45274的骨组织学分析显示，其皮质由低血管化的平行纤维骨组成，由初级骨单位构成，缺乏孵化线和任何生长停滞线。我们将PEFO 45274解释为一只孵化后生长缓慢、年龄小于1岁的个体。令人惊讶的是，一些较大的植龙股骨的骨组织学显示，与PEFO 45274相比，基于偶尔出现的编织骨和总体较高的血管密度，其生长速度更快，这表明在植龙目中，从快速生长到缓慢生长的个体发育转变可能不像预期的那样简单或一致发生，并且非主龙类主龙形动物可能表现出与体型大小相关的组织学特征。鉴于植龙在主龙类分歧附近的系统发育位置，本研究强调了纳入多种体型的骨组织学来研究非主龙类主龙形动物祖先生长速度的重要性。
