Thin sections through the dentary of two specimens of the Late Cretaceous mosasaur Clidastes propython document the upright (vertical) position of developing replacement teeth. Snakes are the only squamates with replacement teeth that develop in a nearly horizontal, posteriorly inclined position. The thin sections also corroborate the finding in 2003 by M. L. Caldwell, L. A. Bundy, and D. O. Lamourex that interdental ridges fully separate successive teeth at their base in mosasaurs, at least on the dentary. This feature is also seen in alethinophidian snakes among squamates, but it remains unknown for more basal members of the mosasauroid clade, and it is absent in scolecophidian snakes.