Tetrapodophis – new four legged very basal, very tiny snake

A new paper by Martill, Tischlinger and Longrich (2015) brings us a really tiny, new Early Cretaceous snake, Tetrapodophis amplectus (Fig. 1, BMMS BK 2-2, ), with four limbs and all of its fingers and toes. The authors suggest this basal snake and thus all snakes evolved from burrowing rather than marine ancestors in accord with the  Longrich, Bullar and Gauthier (2012) assessment of another tiny snake, Coniophis, which is known from only a few skull parts. (Also see below.)

Unfortunately Tetradopodophis (so far based on skull traits only) nests in the large reptile tree between Adriosaurus + Pontosaurus and DinilysiaPachyrhachis + Boa, so an aquatic origin is recovered from the cladogram despite the extremely tiny size of Tetrapodophis (skull length about 1 cm, total length about 16 cm). Martill et al. used mosasaurs and several incomplete taxa (Eophis, Diablophis, Portugalophis and Parviraptor, none included in the large reptile tree) for outgroups and nested Tetrapodophis as a sister to Coniophis and basal to Najash, Dinilysia and all other snakes. The authors note, “As the only known four-legged snake, Tetrapodophis sheds light on the evolution of snakes from lizards. Tetrapodophis lacks aquatic adaptations (such as pachyostosis or a long, laterally compressed tail) and instead exhibits features of fossorial snakes and lizards: a short rostrum and elongation of the postorbital skull, a long trunk and short tail, short neural spines, and highly reduced limbs.”

I wonder if Tetrapodophis is a hatchling? Or does it represent yet another example of phylogenetic miniaturization at the origin of a major clade? It is similar in size to Jucaraseps, a more primitive lizard with snake affinities. Tetrapodophis may be a late surviving (Early Cretaceous) very basal snake with likely origins in the Middle Jurassic. DGS (digital graphic segregation) was helpful in pulling out details (Fig. 1) overlooked or ignored by the original authors.

Figure1. The skull of Tetrapodophis in situ and colorized (middle) as originally interpreted (below) and reconstructed using DGS (above). I have not seen the fossil, but examination of the photograph using DGS permits more details to be identified. This image will be tested for validity Monday. Only the major bones were identified here. The skull is about 1 cm in length.

Figure1. The skull of Tetrapodophis in situ and colorized (middle) as originally interpreted (below) and reconstructed using DGS (above). I have not seen the fossil, but examination of the photograph using DGS permits more details to be identified. This image will be tested for validity Monday. Only the major bones were identified here. The skull is about 1 cm in length.

The preparator did an excellent job on such a tiny (16 cm) specimen, unless it split naturally into part and counterpart. The specimen was in a private collection for decades before getting its museum number.

Like non-snakes, Tetrapodophis retained a postorbital, squamosal and lacrimal. A broken jugal was also found. Palatal fangs were present along with a deep coronoid process. There is a mass at the back of the throat that makes it difficult to identify the posterior palatal bones. The authors report, BMMS BK 2-2 is distinguished from all other snakes by the following combination of characters: 160 precaudal and 112 caudal vertebrae, short neural spines, four limbs, metapodials short, penultimate phalanges hyper elongate and curved, phalangeal formula 2?-3-3-3-3? (manus) 2-3-3-3-3 (pes).”

Although DGS was able to pull lots of details out of this specimen, don’t expect the DGS detractors to applaud this example, although It would be nice to get a tip of the hat for this one. It’s a pretty striking example and only took an hour or two to do.

Figure 2. Tiny Tetrapodophis at full scale if your monitor produces 72 dpi images (standard on many monitors).

Figure 2. Tiny Tetrapodophis at full scale if your monitor produces 72 dpi images (standard on many monitors).

This is a major find and congratulations are due to the authors. More on this specimen in future blog posts.

References Longrich NR, Bullar B-A S and Gauthier JA 2012. A transitional snake from the Late Cretaceous period of North America. Nature 488, 205-208. Martill DM, Tischlinger H and Longrich NR 2015. A four-legged snake from the Early Cretaceous of Gondwana. Science 349 (6246): 416-419. DOI: 10.1126/science.aaa9208

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