A transitional snake from the Late Cretaceous period of North America

Abstract

Snakes are a remarkably diverse and successful group today, but their evolutionary origins are obscure. The discovery of snakes with legs has shed light on the transition from lizards to snakes, but no snake has been described with four limbs. We describe a four-limbed snake from the Early Cretaceous (Aptian) Crato Formation of Brazil. The new snake has a serpentiform body plan with an elongate trunk, short tail, and large ventral scales suggesting characteristic serpentine locomotion, but retains small, prehensile limbs. Furthermore, the body proportions and reduced neural spines indicate a fossorial lifestyle, suggesting that snakes evolved from burrowing rather than marine ancestors. The structure of the hind limbs suggests that they may have functioned either to grasp prey or as claspers during mating. Hooked teeth, an intramandibular joint, a flexible spine capable of constricting prey, and the presence of vertebrate remains in the guts indicate that this species preyed on vertebrates, and that snakes made the transition to carnivory early in their history. Together with a diverse fauna of basal snakes from the Cretaceous of South America, Africa, and India, the new snake shows that crown Serpentes originated in Gondwana.

The oldest known snakes from the Middle Jurassic-Lower Cretaceous provide insights on snake evolution

“…Right maxilla (LACM 4684/140572) small, preserves 10-11 tooth spaces with broken crowns; ascending process low and long with small notch in posterior margin of apex; supradental shelf broken, thin anteriorly and thickened posteriorly; lateral surface smooth with four nutrient foramina; alveoli/interdental ridges with incised medial walls forming nutrient notch; tooth crowns conical, recurved, sharp; holotype right dentary missing symphysis and postdentary articulation region; straight in dorsal view with medial turn anteriorly and 11 tooth spaces; well-developed subdental lamina forming medial border of subdental gutter; lateral wall with seven mental foramina; dentary teeth attached to three-sided alveoli; teeth circular in cross-section, conical and strongly recurved; neural spine tall, synapophyses massive and vertical, and condyle elevated above bottom of centrum; neural canals show 'trefoil' organization often present in snakes 3,8 ; condyle round, with weakly constricted neck and synapophysis with posteriorly expanded parapophyseal and diapophyseal facets; centrum narrow posteriorly, wide anteriorly; inferior margin of centrum without haemal keel; paralymphatic fossae present; no notochordal canal; small zygosphene-zygantrum articulations. Etymology.…”

“…However, snakes possess a number of detailed cranial and postcranial anatomies that are definitively characteristic of the group, distinguishing them from other squamates, and that are present in these new Middle Jurassic-Lower Cretaceous forms. The unique features of snake cranial, dental and axial skeletal elements make it possible to identify snakes in the fossil record from isolated or even fragmentary elements 2,3,20 .…”

“…At the alpha taxonomic level, based on comparative anatomy, Parviraptor, Diablophis, Eophis and Portugalophis are confidently identified as snakes [1][2][3]20,[22][23][24] . However, to address questions of phylogeny, we conducted two separate analyses using two vastly different data matrices.…”

“…To test the possibility that these new snakes might fall outside of the snake clade in an overall analysis of squamate phylogeny, we included them in a recent large matrix of all squamates 25 . We also examined the more specific characters related to snakes, by including them in a data matrix originally developed to test snake ingroup relationships 3,7,8,[26][27][28] (Supplementary Methods; Supplementary Dataset 1). Both phylogenetic analyses were run using the software programs PAUP 29 and TNT 30 , the latter was also used to calculate bootstrap values of statistical support.…”

“…In our parsimony analyses of snake ingroup phylogeny, we submitted a taxon-character matrix of 27 terminal taxa and 237 characters, derived in large part from a recent study 3 that in turn had adapted characters and taxa from several other works 8,11,24,26,27,31 (Supplementary Methods; Supplementary Dataset 1). Our characterization of the Upper Cretaceous snake, Coniophis precedens 3 is more conservative than that given in a recent report, and is restricted to the vertebral assemblage, a single maxillary fragment (the other maxillary fragments are identified here as lizards, not snakes) and a fragmentary right dentary 3,24 .…”