tableS1_femora list_20220610.xlsx from The dinosaurian femoral head experienced a morphogenetic shift from torsion to growth along the avian stem

Significant evolutionary shifts in locomotor behaviour often involve comparatively subtle anatomical transitions. For dinosaurian and avian evolution, medial overhang of the proximal femur has been central to discussions. However, there is an apparent conflict with regard to the evolutionary origin of the dinosaurian femoral head, with neontological and palaeontological data suggesting seemingly incongruent hypotheses. To reconcile this, we reconstructed the evolutionary history of morphogenesis of the proximal end of the femur from early archosaurs to crown birds. Embryological comparison of living archosaurs (crocodylians and birds) suggests the acquisition of the greater overhang of the femoral head in dinosaurs results from additional growth of the proximal end in the medial-ward direction. On the other hand, the fossil record suggests that this overhang was acquired by torsion of the proximal end, which projected in a more rostral direction ancestrally. We reconcile this apparent conflict by inferring that the medial overhang of the dinosaur femoral head was initially acquired by torsion, which was then superseded by mediad growth. Details of anatomical shifts in fossil forms support this hypothesis, and their biomechanical implications are congruent with the general consensus regarding broader morpho-functional evolution on the avian stem.

运动行为的重大演化转变往往伴随相对细微的解剖学过渡。在恐龙与鸟类的演化历程中，股骨近端内侧悬突（medial overhang of the proximal femur）一直是相关学术讨论的核心议题。然而，关于恐龙类股骨头部（femoral head）的演化起源，目前存在一处明显矛盾：现生生物学（neontological）与古生物学（palaeontological）数据所支持的假说看似相互冲突。为调和这一矛盾，我们重建了从早期主龙类（archosaurs）到冠群鸟类（crown birds）的股骨近端形态发生（morphogenesis）演化历史。对现生主龙类（鳄形类crocodylians与鸟类）的胚胎学（embryological）比较研究表明，恐龙类股骨头部更大悬突的形成，源于股骨近端向内侧方向的额外生长。而另一方面，化石记录则显示，该悬突是通过股骨近端的扭转（torsion）获得的——在祖先类群中，股骨近端曾朝向更靠前的头侧方向（rostral direction）突出。我们通过推断解决了这一表观矛盾：恐龙类股骨头部的内侧悬突最初通过扭转机制形成，随后该机制被向内侧生长（mediad growth）的模式所替代。化石物种的解剖学演化细节佐证了这一假说，其对应的生物力学（biomechanical）意义也与鸟类干群（avian stem）整体形态功能演化的主流学术共识相一致。