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

Macrostomy is the anatomical feature present in macrostomatan snakes that permit the ingestion of entire preys with high cross-sectional area. It depends of several anatomical traits in skeleton and soft tissues, 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 preys with low cross-sectional area such as annelids, insects or elongated vertebrates and 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 have been shaped during multiple episodes of occupation of underground and surface macrohabitats throughout the evolution of snakes.巨口解剖特征（macrostomy）是巨口类蛇类（macrostomatan snakes）特有的解剖结构，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性状，其中颌骨复合体（gnathic complex）的延长与方骨（quadrate）的后向旋转是关键的骨骼学先决条件。本研究探讨了这些颅骨结构的出生后发育规律，及其与宏观生境和食性的关联。与栖息于地下宏观生境的蜥蜴及基干蛇类（basal snakes）的情况相反，多数占据地表宏观生境的巨口类蛇类，其颌骨复合体各组分在出生后生长过程中相较于颅骨其余部分呈现显著延长，同时方骨进一步发生后向旋转。值得注意的是，多个小型隐蔽栖息巨口类演化支（clades）会逆转上述出生后形态转变，转而以横截面积较小的猎物为食，例如环节动物（annelids）、昆虫或细长脊椎动物，其解剖特征因此趋近于地下栖息的基干蛇类。多数巨口类蛇类所表现出的食性个体发育转变（dietary ontogenetic shift），与该个体发育轨迹直接相关，表明这种转变是随着出生后个体发育过程中颌骨复合体延长、方骨后向旋转而逐步获得的。在地下栖息巨口类中观察到的颌骨复合体与猎物选择方向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先决条件，均支持基干蛇类是曾具备巨口适应性骨骼结构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，在蛇类的演化历程中，巨口解剖特征是通过多次占据地下与地表宏观生境的演化事件逐步塑造而成的。