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Abstract 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 seamore »snakes. Our findings suggest that heterochronic changes (resulting in pedomorphosis) have driven cranial shape convergence in response to dietary specializations in sea snakes.« less

Comparative osteological analyses of extant organisms provide key insight into major evolutionary transitions and phylogenetic hypotheses. This is especially true for snakes, given their unique morphology relative to other squamates and the persistent controversy regarding their evolutionary origins. However, the osteology of several major snake groups remains undescribed, thus hindering efforts to accurately reconstruct the phylogeny of snakes. One such group is the Atractaspididae, a family of fossorial colubroids. We herein present the first detailed description of the atractaspidid skull, based on fully segmented micro‐computed tomography (micro‐CT) scans ofAtractaspis irregularis. The skull ofAtractaspispresents a highly unique morphology influenced by both fossoriality and paedomorphosis. This paedomorphosis is especially evident in the jaws, palate, and suspensorium, the major elements associated with macrostomy (large‐gaped feeding in snakes). Comparison to scolecophidians—a group of blind, fossorial, miniaturized snakes—in turn sheds light on current hypotheses of snake phylogeny. Features of both the naso‐frontal joint and the morphofunctional system related to macrostomy refute the traditional notion that scolecophidians are fundamentally different from alethinophidians (all other extant snakes). Instead, these features support the controversial hypothesis of scolecophidians as regressed alethinophidians, in contrast to their traditional placement as the earliest‐diverging snake lineage. We propose thatAtractaspisand scolecophidians fall alongmore »a morphological continuum, characterized by differing degrees of paedomorphosis. Altogether, a combination of heterochrony and miniaturization provides a mechanism for the derivation of the scolecophidian skull from an ancestral fossorial alethinophidian morphotype, exemplified by the nonminiaturized and less extreme paedomorphAtractaspis.

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We compared the head skeleton (skull and lower jaw) of juvenile and adult specimens of five snake species [Anilios(=Ramphotyphlops)bicolor,Cylindrophis ruffus,Aspidites melanocephalus,Acrochordus arafurae, andNotechis scutatus] and two lizard outgroups (Ctenophorus decresii,Varanus gilleni). All major ontogenetic changes observed were documented both qualitatively and quantitatively. Qualitative comparisons were based on high‐resolution micro‐CTscanning of the specimens, and detailed quantitative analyses were performed using three‐dimensional geometric morphometrics. Two sets of landmarks were used, one for accurate representation of the intraspecific transformations of each skull and jaw configuration, and the other for comparison between taxa. Our results document the ontogenetic elaboration of crests and processes for muscle attachment (especially for cervical and adductor muscles); negative allometry in the braincase of all taxa; approximately isometric growth of the snout of all taxa exceptVaranusandAnilios(positively allometric); and positive allometry in the quadrates of the macrostomatan snakesAspidites,AcrochordusandNotechis, but also, surprisingly, in the iguanian lizardCtenophorus. Ontogenetic trajectories from principal component analysis provide evidence for paedomorphosis inAniliosand peramorphosis inAcrochordus. Some primitive (lizard‐like) features are described for the first time in the juvenileCylindrophis. Two distinct developmental trajectories for the achievement of the macrostomatan (large‐gaped) condition in adult snakes are documented, driven either by positive allometry of supratemporal and quadrate (in pythons),more »or of quadrate alone (in sampled caenophidians); this is consistent with hypothesised homoplasy in this adaptive complex. Certain traits (e.g. shape of coronoid process, marginal tooth counts) are more stable throughout postnatal ontogeny than others (e.g. basisphenoid keel), with implications for their reliability as phylogenetic characters.

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