1. Department of Zoology, University of Queensland, St Lucia, Brisbane, Queensland 4072, Australia
2. Museum of Geology, South Dakato School of Mines and Technology, Rapid City, South Dakota 57701, USA
3. Paleobiology, Canadian Museum of Nature, PO Box 3443, Station D, Ottawa, Ontario K1P 6P4, Canada

Correspondence to: Correspondence should be addressed to M.L. (e-mail: Email: mlee@zoology.uq.edu.au).

Abstract

Snakes are renowned for their ability to engulf extremely large prey, and their highly flexible skulls and extremely wide gape are among the most striking adaptations found in vertebrates^{1, 2, 3, 4, 5}. However, the evolutionary transition from the relatively inflexible lizard skull to the highly mobile snake skull remains poorly understood, as they appear to be fundamentally different and no obvious intermediate stages have been identified⁴,⁵. Here we present evidence that mosasaurs — large, extinct marine lizards related to snakes — represent a crucial intermediate stage. Mosasaurs, uniquely among lizards, possessed long, snake-like palatal teeth for holding prey. Also, although they retained the rigid upper jaws typical of lizards, they possessed highly flexible lower jaws that were not only morphologically similar to those of snakes, but also functionally similar. The highly flexible lower jaw is thus inferred to have evolved before the highly flexible upper jaw — in the macrophagous common ancestor of mosasaurs and snakes — for accommodating large prey. The mobile upper jaw evolved later — in snakes — for dragging prey into the oesophagus. Snakes also have more rigid braincases than lizards, and the partially fused meso- and metakinetic joints of mosasaurs are transitional between the loose joints of lizards and the rigid joints of snakes. Thus, intermediate morphologies in snake skull evolution should perhaps be sought not in small burrowing lizards, as commonly assumed, but in large marine forms.