[Paleontology • 2016] Aquatic Adaptations in the Four Limbs of the Snake-like Reptile Tetrapodophis from the Lower Cretaceous of Brazil

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.

Keywords: Squamata; Ophidia; Serpentes; Evolution; Cretaceous; Paleoecology

Conclusions

The results of our multivariate morphometric analyses (PCA, LDA) intriguingly suggest that Tetrapodophis shows body form metrics similar to those of modern burrowers as well as surface-active but cryptozoic lizards (e.g. the diploglossid Celestus). The burrowers most similar to Tetrapodophis are relatively generalised forms that move through loose soil or leaf litter, so if Tetrapodophis is accepted as a stem snake, it would suggest that snake ancestors were not heavily specialised fossors. An examination of the anatomical details of Tetrapodophis conversely revealed features that may be indicative of aquatic habits (expanded first metapodials, lack of hooked fifth metatarsal, and reduced tarsal/carpal ossification). The small size of these limbs is not consistent with a major role in propulsion; however, it is possible that they have been reduced in size in association with a shift to anguilliform locomotion, as extreme limb reduction is a common phenomenon within dolichosaurs (e.g. Dal Sasso and Pinna, 1997 and Palci and Caldwell, 2007). An occasional grasping function for the limbs is not inconsistent with an aquatic habit. The morphology and inferred ecology of this fossil therefore does not clearly favour fossorial over aquatic origins for snakes.

Tetrapodophis does not clearly fall into any of the known ecological categories, but the presence of the limb features discussed above is difficult to explain as anything but holdovers from a more aquatically-adapted ancestor. Tetrapodophis therefore represents a truly enigmatic animal, combining the body proportions of an elongate squamate with the limbs of a swimmer (or former swimmer). The precise ecology of this iconic fossil will thus continue to be debated. Perhaps the reality lies in between, and Tetrapodophis may have been both fossorial and aquatic, a lifestyle exhibited by some living snakes such as neotropical pipesnakes (Anilius) (e.g. Murphy, 2010).


Michael S.Y. Lee, Alessandro Palci, Marc E.H. Jones, Michael W. Caldwell, James D. Holmes and Robert R. Reisz. 2016. Aquatic Adaptations in the Four Limbs of the Snake-like Reptile Tetrapodophis from the Lower Cretaceous of Brazil. Cretaceous Research. 
DOI:   10.1016/j.cretres.2016.06.004

Did snakes evolve from ancient sea serpents? http://phy.so/385370720 via @physorg_com

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