Morris Zachary S, Vliet Kent A, Abzhanov Arhat, Pierce Stephanie E
Museum of Comparative Zoology and Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts, USA.
Department of Biology, University of Florida, Gainesville, Florida, USA.
Anat Rec (Hoboken). 2022 Oct;305(10):2838-2853. doi: 10.1002/ar.24802. Epub 2021 Oct 25.
The dorsoventrally flattened skull typifies extant Crocodylia perhaps more than any other anatomical feature and is generally considered an adaptation for semi-aquatic feeding. Although the evolutionary origins of caniofacial flattening have been extensively studied, the developmental origins have yet to be explored. To understand how the skull table and platyrostral snout develop, we quantified embryonic development and post-hatching growth (ontogeny) of the crocodylian skull in lateral view using geometric morphometrics. Our dataset (n = 103) includes all but one extant genus and all of the major ecomorphs, including the extremely slender-snouted Gavialis and Tomistoma. Our analysis reveals that the embryonic development of the flattened skull is remarkably similar across ecomorphs, including the presence of a conserved initial embryonic skull shape, similar to prior analysis of dorsal snout shape. Although differences during posthatching ontogeny are recovered among ecomorphs, embryonic patterns are not distinct, revealing an important shift in developmental rate near hatching. In particular, the flattened skull table is achieved by the end of embryonic development with no changes after hatching. Further, the rotation of skull roof and facial bones during development is critical for the stereotypical flatness of the crocodylian skull. Our results suggest selection on hatchling performance and constraints on embryonic skull shape may have been important in this pattern of developmental conservation. The appearance of aspects of cranial flatness among Jurassic stem crocodylians suggests key aspects of these cranial developmental patterns may have been conserved for over 200 million years.
背腹扁平的头骨或许比其他任何解剖特征都更能代表现存的鳄目动物，通常被认为是对半水生觅食的一种适应。尽管对颅面扁平的进化起源已进行了广泛研究，但其发育起源尚未得到探索。为了解颅骨板和扁平吻部是如何发育的，我们使用几何形态测量学从侧面量化了鳄目动物颅骨的胚胎发育和孵化后生长（个体发育）。我们的数据集（n = 103）包括了除一个现存属之外的所有属以及所有主要的生态形态，包括吻部极其细长的恒河鳄和马来鳄。我们的分析表明，在各种生态形态中，扁平头骨的胚胎发育非常相似，包括存在一种保守的初始胚胎头骨形状，这与之前对吻部背面形状的分析结果类似。尽管在孵化后的个体发育过程中，不同生态形态之间存在差异，但胚胎模式并无明显区别，这揭示了孵化前后发育速率的重要转变。特别是，扁平的颅骨板在胚胎发育结束时就已形成，孵化后不再变化。此外，头骨顶部和面部骨骼在发育过程中的旋转对于鳄目动物头骨典型的扁平形状至关重要。我们的结果表明，对幼体性能的选择以及对胚胎头骨形状的限制可能在这种发育保守模式中起到了重要作用。侏罗纪鳄形类干群中颅骨扁平特征的出现表明，这些颅骨发育模式的关键方面可能已经保存了超过2亿年。
