Palaeophis ("ancient serpent") is an extinct genus of sea snakes. Species in this genus lived from the Cretaceous period to the Eocene epoch, from about 70.6 to about 33.9 million years ago.
These species varied in size. Palaeophis casei is the smallest known, at 1.3 meters in length, while Palaeophis colossaeus, known from a single vertebra, is the largest within the estimated size limits for this genus. The latter reached over 9 meters in length and was one of the largest snakes known. However, most species in the genus were not as large. Palaeophis maghrebianus is also a species of Palaeophis discovered in Morocco and North Africa.
Palaeophis species were specialized aquatic animals, as their fossils appear primarily in marine strata, though a few remains have also been found in estuaries. Studies on Palaeophis vertebrae show a high degree of vascularization, suggesting they had a considerably faster metabolic rate and growth rate than modern snakes. This may indicate that Palaeophis, much like other marine reptiles such as Mosasaurs, could have been evolving toward endothermy.
