The marine squamates (reptiles) from the Cenomanian-Turonian of the Tethys: a noteworthy radiation
Résumé
The Cenomanian-Turonian interval is a key period in the evolution of squamates. They display a spectacular radiation in the marine realm, in particular on the northern and southern margins of the Mediterranean Tethys, though some taxa reached the Interior Sea of North America. Snakes probably originated in the Mediterranean Tethys from this assemblage, but that is the issue of a current debate among paleontologists. In the span of time comprising the Cenomanian-Turonian stages four major groups diversified in the Mediterranean zone: the "hind-limbed snakes", the dolichosaurs, the "aigialosaurs" and the mosasaurs (the last two grouped as the Mosasauroidea). About thirty species are known in the fossil record. "Hind-limbed" snakes, their phylogenetic position still under discussion, attain a length of 1.5 m and are all pachyostotic. They are known only from the Cenomanian of the northern (western Europe and Adriatic region) and southern (North Africa and Middle East) margins of the Mediterranean Tethys (Rage & Escuillié, 2003). Dolichosaurs are snake-like lizards with long necks but still have short limbs, and range up to one meter in length. They are known in Europe and the Middle East mainly from the Cenomanian, but their range in North America extends from the Cenomanian-Turonian to the Santonian. Mosasauroids include basal forms, collectively labelled as "aigialosaurs", probably a paraphyletic group, and a more specialized one, the mosasaurids. "Aigialosaurs", aquatic lizards measuring up to 1.5 m long, are known during the Cenomanian-Turonian, mainly in Europe. Mosasaurids, large predators of the open sea (over 2 m in length), are known from the Middle Turonian to the end of the Maastrichtian. Most of the Turonian forms are from Africa and the Americas. With the exception of the mosasaurids, this important radiation of squamates in the marine realm is restricted in time (Cenomanian-Turonian) and space (almost entirely to the northern and southern margins of the Mediterranean Tethys). It should be noted that most of the known taxa are monotypic. Only the snake Simoliophis and the dolichosaurs Coniasaurus and Pontosaurus are represented by several species and have a relatively broad geographic distribution. Among the groups discussed above, only the mosasaurids subsist during the end of the Cretaceous and show a strong evolutive radiation during the Santonian-Maastrichtian, when they are represented by very numerous cosmopolitan taxa that display very diverse ecological types. It is probable that: 1) certain major geological and biological events that characterize this Cenomanian-Turonian pivotal period, could have permitted this radiation (Jacobs et alii, 2005a, b); 2) the Mediterranean portion of the Tethys seems to have played an important role in both the radiation and dispersion of these animals (Rage & Escuillié, 2003); and 3) factors inherent to each of the four groups (size, bone structure, ecological and palaeobiogeographical preferences, etc) could have had an effect on and insured the success of one group -the mosasaurids- with respect to the others of these groups, whereas the radiation of snakes succeeded only in continental environments.
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