Tetrapodophis

Tetrapodophis (Greek meaning "four-footed snake") is an extinct genus of lizard from the Early Cretaceous (Aptian) aged Crato Formation of Brazil. It has an elongate snake-like body, with four limbs. Tetrapodophis has been considered by some authors to be one of the oldest members of Ophidia (snakes and their extinct relatives). However, this classification has been disputed by some other authors, who identify Tetrapodophis as a dolichosaurid much less closely related to snakes.

Tetrapodophis Temporal range: Early Cretaceous Aptian PreꞒ Ꞓ O S D C P T J K Pg N
Fossil in Bürgermeister-Müller-Museum
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Order:	Squamata
Family:	†Dolichosauridae
Genus:	†Tetrapodophis Martill et al., 2015
Type species
†Tetrapodophis amplectus Martill et al., 2015

 

Tetrapodophis possesses small yet well-developed fore- and hindlimbs like a lizard and a long body similar to a snake, around 19.5 cm (7.7 in) in length. Nevertheless, it shares many characteristics with modern snakes, including an elongate body, short tail, broad belly scales, a skull with a short snout and long braincase, curved jaws, and sharp hooked teeth. BMMS BK 2-2 contains bones of another animal in its gut, indicating that Tetrapodophis was carnivorous like most snakes. Other features such as short neural spines suggest that Tetrapodophis was adapted to burrowing, lending support to the hypothesis that snakes evolved in terrestrial environments (another hypothesis posits that they evolved in aquatic environments). The high number of vertebrae (upwards of 150) in Tetrapodophis and snakes is not seen in other burrowing reptiles with elongate bodies and reduced or absent limbs, meaning that it is most likely not an adaptation for a serpentine form of movement. Dolichosaurids are thought to be phylogenetically close to snakes as members of Ophidiomorpha, which explains some of the morphological similarities.

The type species, Tetrapodophis amplectus, was named in 2015 on the basis of a complete skeleton (BMMS BK 2-2) preserved on a limestone slab in the Bürgermeister Müller Museum in Solnhofen, Germany, which was labeled as "unknown fossil" until its importance was recognized by paleontologist David Martill.

A phylogenetic analysis published alongside the original 2015 description of Tetrapodophis places it as a close relative of other lizards, but outside the crown group Serpentes, meaning that Tetrapodophis branched off before the most recent common ancestor of all living snakes. Below is a cladogram from that analysis:

The interpretation of Tetrapodophis amplectus as an early fossorial snake was challenged by Caldwell et al. (2016), who considered it more likely to be a dolichosaurid squamate related to mosasaurs. A position supported by later analysis in 2018 and in 2021.

Cladogram after Caldwell et. al. 2021:

However, a study in 2023 again supported its position as a stem-snake, finding it to be unrelated to mosasaurs.