Abstract
Madtsoiids are among the most basal snakes, with a fossil record dating back to the Upper Cretaceous (Cenomanian). Most representatives went extinct by the end of the Eocene, but some survived in Australia until the Late Cenozoic. Yurlunggur and Wonambi are two of these late forms, and also the best-known madtsoiids to date. A better understanding of the anatomy and palaeoecology of these taxa may shed light on the evolution and extinction of this poorly known group of snakes and on early snake evolution in general. A digital endocast of the inner ear of Yurlunggur was compared to those of 81 species of snakes and lizards with known ecological preferences using three-dimensional geometric morphometrics. The inner ear of Yurlunggur most closely resembles both that of certain semiaquatic snakes and that of some semifossorial snakes. Other cranial and postcranial features of this snake support the semifossorial interpretation. While the digital endocast of the inner ear of Wonambi is too incomplete to be included in a geometric morphometrics study, its preserved morphology is very different from that of Yurlunggur and suggests a more generalist ecology. Osteology, palaeoclimatic data and the palaeobiogeographic distribution of these two snakes are all consistent with these inferred ecological differences.
Highlights
Several recent studies have shown a close correlation between the shape of part or all of the inner ear apparatus and ecological preferences in modern squamate reptiles, i.e. lizards and snakes (e.g. [1,2,3])
Because the effect of ontogenetic variation on the shape of inner ear endocasts of squamate reptiles is currently unknown, we provide here, for the first time, a quantitative analysis of ontogenetic trajectories in a selection of eleven taxa, inclusive of both lizards and snakes
Landmark coordinates for the inner ears of 79 squamate reptiles were taken from the electronic supplementary material in [3,25]
Summary
Several recent studies have shown a close correlation between the shape of part or all of the inner ear apparatus (sacculus, lagena and semicircular canals) and ecological preferences in modern squamate reptiles, i.e. lizards and snakes (e.g. [1,2,3]). Several recent studies have shown a close correlation between the shape of part or all of the inner ear apparatus (sacculus, lagena and semicircular canals) and ecological preferences in modern squamate reptiles, i.e. lizards and snakes The Madtsoiidae is a totally extinct snake lineage, which lived between the Upper Cretaceous (Cenomanian) and the Late Pleistocene This family mostly had a Gondwanan distribution [4,5,6,7,8,9,10], and by the end of the Eocene disappeared everywhere except in Australia and Argentina [11,12]. These snakes were initially considered to be closely related to pythons and boas (e.g. [13,14]), but most recent phylogenetic analyses placed them in a more basal position, which makes them a potentially pivotal group for our understanding of snake origins and evolution [7,8,15,16,17,18] (but see [19,20,21], who place madtsoiids within Alethinophidia)
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