Data from: Postnatal ontogeny and the evolution of macrostomy in snakes

Macrostomy is the anatomical feature present in macrostomatan snakes that permits the ingestion of entire prey with high cross-sectional area. It depends on several anatomical traits in the skeleton and soft tissues, of which the elongation of gnathic complex and backward rotation of the quadrate represent crucial skeletal requirements. Here, the relevance of postnatal development of these skull structures and their relationship with macrohabitat and diet are explored. Contrary to the condition present in lizards and basal snakes that occupy underground macrohabitats, elements of the gnathic complex of most macrostomatan snakes that exploit surface macrohabitats display conspicuous elongation during postnatal growth, relative to the rest of the skull, as well as further backward rotation of the quadrate bone. Remarkably, several clades of small cryptozoic macrostomatans reverse these postnatal transformations and return to a diet based on prey with low cross-sectional area such as annelids, insects or elongated vertebrates, thus resembling the condition present in underground basal snakes. Dietary ontogenetic shift observed in most macrostomatan snakes is directly linked with this ontogenetic trajectory, indicating that this shift is acquired progressively as the gnathic complex elongates and the quadrate rotates backward during postnatal ontogeny. The numerous independent events of reversion in the gnathic complex and prey type choice observed in underground macrostomatans and the presence of skeletal requirements for macrostomy in extinct non-macrostomatan species reinforce the possibility that basal snakes represent underground survivors of clades that had the skeletal requirements for macrostomy. Taken together, the data presented here suggest that macrostomy has been shaped during multiple episodes of occupation of underground and surface macrohabitats throughout the evolution of snakes.

巨口适应性（Macrostomy）是巨口类蛇类所具备的解剖学特征，使其能够吞食横截面积较大的完整猎物。该特征依赖于骨骼与软组织中的多项解剖学性状，其中颌骨复合体（gnathic complex）的伸长与方骨（quadrate）的后向旋转是关键的骨骼学前提。本研究探讨了这些头骨结构在出生后发育中的相关性，及其与大生境和食性的关联。与栖息于地下大生境的蜥蜴及基干蛇类（basal snakes）的性状不同，多数利用地表大生境的巨口类蛇类，其颌骨复合体在出生后的生长过程中，相较于头骨其余部分呈现出显著的伸长，同时方骨进一步发生后向旋转。值得注意的是，多个小型隐蔽栖息（cryptozoic）的巨口类蛇类演化支（clades）逆转了这些出生后发育的形态转变，重新以横截面积较小的猎物为食，例如环节动物、昆虫或细长脊椎动物，其食性状况与地下基干蛇类一致。多数巨口类蛇类所观察到的食性个体发育转变（dietary ontogenetic shift），与这一个体发育轨迹直接相关，表明该食性转变是随着颌骨复合体在出生后个体发育过程中伸长、方骨后向旋转而逐步获得的。在地下生境的巨口类蛇类中观察到的颌骨复合体与猎物选择模式的多次独立逆转事件，以及已灭绝的非巨口类物种所具备的巨口适应性骨骼学前提，均支持基干蛇类是曾具备巨口适应性骨骼学特征的演化支的地下孑遗类群这一推论。综合来看，本研究提供的数据表明，在蛇类的整个演化历程中，巨口适应性曾多次在对地下与地表大生境的占据过程中被塑造演化。