From examination of unearthed fossils, it is thought that the mesosaur, a distant relative of the dinosaurs that lived 300 million years ago in what is now Brazil, was the first reptile to go back into the water, where its amphibian ancestors had originally lived. A discovery of a fossil of a pregnant keichousaur in Guizhou, China in 2002, however, gave natural scientists their first knowledge about how these animals reproduced.
從已出土的化石標本推測，屬於恐龍遠親、最早從陸地「下海」的海棲爬行動物，是距今 3 億年前的巴西「中龍」，但直到 2002 年，考古學家在中國貴州發現了懷孕的「胡氏貴州龍」化石，自然學界對它們的生殖狀況才有了初步瞭解。