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Aquatic adaptations in the four limbs of the snake-like reptile Tetrapodophis from the Lower Cretaceous of Brazil Lee et al., 2016

*This image is copyright of its original author

Credits: Michael Caldwell and Alessandro Palci (University of Alberta/Flinders University & South Australian Museum)

*This image is copyright of its original author

Reconstruction credits: Alessandro Palci and Michael Lee (Flinders University & South Australian Museum)

*This image is copyright of its original author

Fig. 3. (A) Forelimbs and (B) Hindlimbs of Tetrapodophis; a terrestrial squamate (Varanus); a marine dolichosaur (Pontosaurus), a marine mosasauroid (Tylosaurus); and a marine rhynchocephalian (Pleurosaurus). Numbers 1–5 identify highly distinctive limb similarities between Tetrapodophis and marine forms, discussed in the main text. Sources: Tetrapodophis after Martill et al. (2015) and Museum Solnhofen BMMS BK 2-2, Varanus albigularis after University of Alberta Museum of Zoology 947, Pontosaurus kornhuberi after Museo di Storia Naturale di Milano V3662, Tylosaurus proriger after Russell (1967), Pleurosaurus goldfussi after Cocude-Michel (1963), and Chalcides ocellatus after Young et al. (2009) and Field Museum of Natural History 164674. 

Abstract:
"The exquisite transitional fossil Tetrapodophis – interpreted as a stem-snake with four small legs from the Lower Cretaceous of Brazil – has been widely considered a burrowing animal, consistent with recent studies arguing that snakes had fossorial ancestors. We reevaluate the ecomorphology of this important taxon using a multivariate morphometric analysis and a reexamination of the limb anatomy. Our analysis shows that the body proportions are unusual and similar to both burrowing and surface-active squamates. We also show that it exhibits striking and compelling features of limb anatomy, including enlarged first metapodials and reduced tarsal/carpal ossification – that conversely are highly suggestive of aquatic habits, and are found in marine squamates. The morphology and inferred ecology of Tetrapodophis therefore does not clearly favour fossorial over aquatic origins of snakes."

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