School of Biological Sciences, The University of Adelaide, Adelaide, South Australia 5005, Australia.
South Australian Museum, North Terrace, Adelaide, South Australia 5000, Australia.
Integr Comp Biol. 2019 Sep 1;59(3):616-624. doi: 10.1093/icb/icz033.
Morphological variation among the viviparous sea snakes (Hydrophiinae), a clade of fully aquatic elapid snakes, includes an extreme "microcephalic" ecomorph that has a very small head atop a narrow forebody, while the hind body is much thicker (up to three times the forebody girth). Previous research has demonstrated that this morphology has evolved at least nine times as a consequence of dietary specialization on burrowing eels, and has also examined morphological changes to the vertebral column underlying this body shape. The question addressed in this study is what happens to the skull during this extreme evolutionary change? Here we use X-ray micro-computed tomography and geometric morphometric methods to characterize cranial shape variation in 30 species of sea snakes. We investigate ontogenetic and evolutionary patterns of cranial shape diversity to understand whether cranial shape is predicted by dietary specialization, and examine whether cranial shape of microcephalic species may be a result of heterochronic processes. We show that the diminutive cranial size of microcephalic species has a convergent shape that is correlated with trophic specialization to burrowing prey. Furthermore, their cranial shape is predictable for their size and very similar to that of juvenile individuals of closely related but non-microcephalic sea snakes. Our findings suggest that heterochronic changes (resulting in pedomorphosis) have driven cranial shape convergence in response to dietary specializations in sea snakes.
胎生海蛇（Hydrophiinae）是完全水生的眼镜蛇科的一个分支，其成员形态多样，其中包括一种极端的“小头”生态型，其头部非常小，身体前段狭窄，而身体后段则非常粗壮（最粗处可达前段周长的三倍）。之前的研究已经证明，这种形态是由于对穴居鳗鱼的专门化饮食而至少进化了九次，同时也研究了支撑这种体型的脊柱的形态变化。本研究要解决的问题是，在这种极端的进化变化过程中，头骨会发生什么变化？在这里，我们使用 X 射线微计算机断层扫描和几何形态测量方法来描述 30 种海蛇的头骨形状变化。我们研究了颅形状多样性的个体发育和进化模式，以了解颅形状是否由饮食特化来预测，并检查小头物种的颅形状是否是由异时性过程造成的。我们表明，小头物种的微小颅尺寸具有与穴居猎物专门化相关的趋同形状。此外，它们的颅形状可以根据其大小进行预测，并且与亲缘关系密切但非小头的海蛇的幼年个体非常相似。我们的研究结果表明，异时性变化（导致幼态持续）是海蛇对饮食专门化的响应，导致了颅形状的趋同。
