Snakes are renowned for their ability to engulf extremely large prey, and t
heir highly flexible skulls and extremely wide gape are among the most stri
king adaptations found in vertebrates(1-5). However, the evolutionary trans
ition from the relatively inflexible lizard skull to the highly mobile snak
e skull remains poorly understood, as they appear to be fundamentally diffe
rent and no obvious intermediate stages have been identified(4,5). Here we
present evidence that mosasaurs-large, extinct marine lizards related to sn
akes-represent a crucial intermediate stage. Mosasaurs, uniquely among liza
rds, possessed long, snake-like palatal teeth for holding prey. Also, altho
ugh they retained the rigid upper jaws typical of lizards, they possessed h
ighly flexible lower jaws that were not only morphologically similar to tho
se of snakes, but also functionally similar. The highly flexible lower jaw
is thus inferred to have,evolved before the highly flexible upper jaw-in th
e macrophagous common ancestor of mosasaurs and snakes-for accommodating la
rge prey. The mobile upper jaw evolved later-in snakes-for dragging prey in
to the oesophagus. Snakes also have more rigid braincases than lizards, and
the partially fused meso- and metakinetic joints of mosasaurs are transiti
onal between the loose joints of lizards and the rigid joints of snakes. Th
us, intermediate morphologies in snake skull evolution should perhaps be so
ught not in small burrowing lizards, as commonly assumed, but in large mari
ne forms.