Fangs are a putative key innovation that revolutionized prey capture and feeding in snakes, and – along with their associated venom phenotypes – have made snakes perhaps the most medically-significant vertebrate animals. Three snake clades are known for their forward-positioned fangs, and these clades (Elapidae, Viperidae, and Atractaspidinae) contain the majority of snakes that are traditionally considered venomous. However, many other snakes are “rear-fanged”: they possess potentially venom-delivering teeth situated at the rear end of the upper jaw. Quantification of fang phenotypes – and especially those of rear-fanged species – has proved challenging or impossible owing to the small size and relative rarity of many such snakes. Consequently, it has been difficult to understand the evolutionary history of both venom and prey-capture strategies across extant snakes. We quantified variation in the dentition of 145 colubriform (“advanced”) snake species using microCT scanning and compared dental characters with ecological data on species’ diet and prey capture method(s) to understand broader patterns in snake fang evolution. Dental traits such as maxilla length, tooth number, and fang size show strong phylogenetic signal across Colubriformes. We find extreme heterogeneity and evolutionary lability in the rear-fanged phenotype in colubrid (colubrine, dipsadine, and natricine lineages) and lamprophiid snakes, in contrast to relative uniformity in the front fanged phenotypes of other groups (vipers and, to a lesser extent, elapids). Fang size and position are correlated with venom-use in vipers, elapids, and colubrid snakes, with the latter group shifting fangs anteriorly by shortening the entire maxillary bone. We find that maxilla length and tooth number may also be correlated with the evolution of dietary specialization. Finally, an ancestral state reconstruction suggests that fang loss is a recurring phenomenon in colubrid snakes, likely accompanied by shifts in diet and prey capture mode. Our study provides a framework for quantifying the complex morphologies associated with venom use in snakes. Our results suggest that fang phenotypes, and particularly the rear-fanged phenotype, in snakes are both diverse and labile, facilitating a wide range of ecological strategies and contributing to spectacular radiations of these organisms in tropical and subtropical biomes worldwide.

中文翻译：

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獠牙是由什么组成的？后牙蛇牙齿进化的系统发育和生态控制。

毒牙被认为是一项关键创新，它彻底改变了蛇的猎物捕获和喂养方式，并且连同其相关的毒液表型，使蛇可能成为最具医学意义的脊椎动物。三个蛇类分支以其前向的毒牙而闻名，这些蛇类（蛇蛇科、蝰蛇科和白蛇蛇科）包含大多数传统上被认为有毒的蛇。然而，许多其他蛇都是“后牙”：它们的上颌后端有可能输送毒液的牙齿。由于许多此类蛇的体型较小且相对稀有，对毒牙表型（尤其是后毒牙物种的毒牙表型）进行量化已被证明具有挑战性或不可能。因此，很难理解现存蛇的毒液和捕获猎物策略的进化历史。我们使用 microCT 扫描量化了 145 种蛇形（“高级”）蛇种的齿列变化，并将牙齿特征与物种饮食和猎物捕获方法的生态数据进行比较，以了解蛇牙进化的更广泛模式。上颌骨长度、牙齿数量和尖牙大小等牙齿特征在蛇形目中显示出强烈的系统发育信号。我们发现游蛇（colubrine、dipsadine 和 natricine 谱系）和蜥蜴类蛇的后牙表型具有极端的异质性和进化不稳定性，而其他类群（毒蛇以及较小程度上的毒蛇）的前牙表型则相对一致。埃拉皮兹）。毒牙的大小和位置与毒蛇、蛇类和游蛇的毒液使用相关，后者通过缩短整个上颌骨将毒牙向前移动。我们发现上颌骨长度和牙齿数量也可能与饮食专业化的进化相关。最后，祖先状态重建表明，毒牙丧失是游蛇中反复出现的现象，可能伴随着饮食和猎物捕获模式的变化。我们的研究提供了一个量化与蛇毒液使用相关的复杂形态的框架。我们的研究结果表明，蛇的毒牙表型，特别是后毒牙表型，既多样化又不稳定，促进了广泛的生态策略，并促成了这些生物在全世界热带和亚热带生物群落中的壮观辐射。