Abstract
The inner ear morphology of 80 snake and lizard species, representative of a range of ecologies, is here analysed and compared to that of the fossil stem snake Dinilysia patagonica, using three-dimensional geometric morphometrics. Inner ear morphology is linked to phylogeny (we find here a strong phylogenetic signal in the data that can complicate ecological correlations), but also correlated with ecology, with Dinilysia resembling certain semi-fossorial forms (Xenopeltis and Cylindrophis), consistent with previous reports. We here also find striking resemblances between Dinilysia and some semi-aquatic snakes, such as Myron (Caenophidia, Homalopsidae). Therefore, the inner ear morphology of Dinilysia is consistent with semi-aquatic as well as semi-fossorial habits: the most similar forms are either semi-fossorial burrowers with a strong affinity to water (Xenopeltis and Cylindrophis) or amphibious, intertidal forms which shelter in burrows (Myron). Notably, Dinilysia does not cluster as closely with snakes with exclusively terrestrial or obligate burrowing habits (e.g. scolecophidians and uropeltids). Moreover, despite the above similarities, Dinilysia also occupies a totally unique morphospace, raising issues with linking it with any particular ecological category.
Highlights
The debate on the ecological driver(s) that led to the origin of snakes is one of the most long-lasting and controversial in biology and evolution
The principal components analysis (PCA) of the five randomly selected species, each landmarked five times in order to estimate the magnitude of the error introduced by inconsistencies in landmark placement, shows that the selected landmarking scheme is robust and produces only small stochastic variations in the placement of the taxa in shape space
The tests for phylogenetic signal rejected the null hypothesis regardless of the inclusion of fossils and the placement of Dinilysia (H0 = no phylogenetic signal present [34]) (K = 0.4008, p = 0.001 when fossils were excluded; K = 0.4047, p = 0.001 when fossils were included and Dinilysia is a stem alethinophidian; K = 0.4112, p = 0.001 when fossils were included and Dinilysia is a stem ophidian), which implies that the shape data are affected by evolutionary history
Summary
The debate on the ecological driver(s) that led to the origin of snakes is one of the most long-lasting and controversial in biology and evolution. To test the hypothesis and conclusions of [2], we generated an expanded dataset consisting of 81 digital endocasts of the inner ear of squamate reptiles (about twice as many species as those in the earlier study), and refined the landmarking scheme and ecological categories. The implications of our finding on the origin and early evolution of snakes, and in particular the possible ecologies of Dinilysia, are discussed at length. We examine the epistemic issues related to drawing general inferences on the evolution of this group of reptiles based on a single extinct species
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