Viviparidade em lagartos marinhos do Cretáceo

terça-feira, outubro 27, 2009

Live birth in Cretaceous marine lizards (mosasauroids)

Michael W. Caldwell1* and Michael S. Y. Lee2,,3
+ Author Affiliations

1Department of Earth and Atmospheric Sciences, and Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E3

2Department of Zoology, The University of Queensland, Brisbane 4072,Australia

3The South Australian Museum, North Terrace, Adelaide 5000,Australia


Although live-bearing (viviparity) has evolved around 100 times within reptiles, evidence of it is almost never preserved in the fossil record. Here, we report viviparity in mosasauroids, a group of Cretaceous marine lizards. This is the only known fossil record of live-bearing in squamates (lizards and snakes), and might represent the oldest occurrence of the trait in this diverse group; it is also the only known fossil record of viviparity in reptiles other than ichthyosaurs. An exceptionally preserved gravid female of the aigialosaur Carsosaurus (a primitive mosasauroid) contains at least four advanced embryos distributed along the posterior two-thirds of the long trunk region (dorsal vertebrae 9-21). Their orientation suggests that they were born tail-first (the nostrils emerging last) to reduce the possibility of drowning, an adaptation shared with other highly aquatic amniotes such as cetaceans, sirenians and ichthyosaurs; the orientation of the embryos also suggests that they were not gut contents because swallowed prey are usually consumed head-first. One embryo is located within the pelvis, raising the possibility that the adult died during parturition. Viviparity in early medium-sized amphibious aigialosaurs may have freed them from the need to return to land to deposit eggs, and permitted the subsequent evolution of gigantic totally marine mosasaurs.


Mosasauroids Viviparity Aigialosaurs Squamates


↵* Author for correspondence