The fact that all snakes confirmed or inferred to be limbed are of Cenomanian age is striking. It is true, too, that the rare fossils found in older beds (Upper Albian), are all in the form of poorly preserved isolated vertebrae so that it is not possible to determine whether or not they had legs. Whatever may be the case, it must be stated that legged snakes and those supposed to have had them have vertebrae of a very special morphology that distinguishes them from all other snakes. But this type of vertebra has been found only in the Cenomanian. Consequently, one may assume that legged snakes occur only in strata assigned this stage.
In addition, these snakes have a very restricted geographic distribution. All occur in the "Mediterranean" area of the Tethys or in its immediate vicinity: the north, east and south margins of the existing Mediterranean and its extension as far as the transitional area between the Aquitaine and Paris basins (Fig. 6 ).
If hindlimbed snakes really belong to a basal group (sister group to all other snakes) as we uphold, and as did Caldwell & Lee (1997) and Scanlon & Lee (2000), then their range that is very restricted in both time and space deserves attention. As a consequence, snakes would have originated in an aquatic, marine environment (Nopcsa, 1923; Caldwell, 1999; Rage, 2000), although Lee et alii (1999) did not reject the possibility that legged snakes were only secondarily aquatic. If snakes really originated in a marine environment, the ''Mediterranean'' part of the Tethys might be regarded as the cradle of the whole group. As far as their very limited stratigraphic range (Cenomanian only) is concerned, no explanation can be brought forward, although a taphonomic bias cannot be definitely ruled out.