陸地脊椎動物過去曾多次返回大海。現代海豹和鯨由於是哺乳動物，因而是胎生的，它們的性別由基因型決定。但曾經在海洋裡生活的很多爬行動物如滄龍、魚龍和蛇頸龍的情況是什麼樣的呢？性別也許是由環境決定的或基因型決定的，它們也許是胎生，也許是卵生。
根據複雜的系統發生分析，Organ等人提出，過去到海裡去生活的爬行動物不僅是胎生的（這一點是從化石記錄知道的），而且它們的性別也是由基因型決定的。這樣，它們就不需要返回陸地去生育後代（羊膜卵在水中會死亡），它們也便能夠發生形態轉變，變成高度進化的魚類形式。(生物谷Bioon.com)
生物谷推薦原始出處：
Nature 461, 389-392 (17 September 2009) | doi:10.1038/nature08350
Genotypic sex determination enabled adaptive radiations of extinct marine reptiles
Chris L. Organ1, Daniel E. Janes1, Andrew Meade2 & Mark Pagel2
1 Museum of Comparative Zoology, Harvard University, 26 Oxford Street, Cambridge, Massachusetts 02138, USA
2 School of Biological Sciences, University of Reading, Reading RG6 6AJ, UK
Adaptive radiations often follow the evolution of key traits, such as the origin of the amniotic egg and the subsequent radiation of terrestrial vertebrates. The mechanism by which a species determines the sex of its offspring has been linked to critical ecological and life-history traits1, 2, 3 but not to major adaptive radiations, in part because sex-determining mechanisms do not fossilize. Here we establish a previously unknown coevolutionary relationship in 94 amniote species between sex-determining mechanism and whether a species bears live young or lays eggs. We use that relationship to predict the sex-determining mechanism in three independent lineages of extinct Mesozoic marine reptiles (mosasaurs, sauropterygians and ichthyosaurs), each of which is known from fossils to have evolved live birth4, 5, 6, 7. Our results indicate that each lineage evolved genotypic sex determination before acquiring live birth. This enabled their pelagic radiations, where the relatively stable temperatures of the open ocean constrain temperature-dependent sex determination in amniote species. Freed from the need to move and nest on land4, 5, 8, extreme physical adaptations to a pelagic lifestyle evolved in each group, such as the fluked tails, dorsal fins and wing-shaped limbs of ichthyosaurs. With the inclusion of ichthyosaurs, mosasaurs and sauropterygians, genotypic sex determination is present in all known fully pelagic amniote groups (sea snakes, sirenians and cetaceans), suggesting that this mode of sex determination and the subsequent evolution of live birth are key traits required for marine adaptive radiations in amniote lineages.