|Average Size||20-50 cm|
|Average weight||Estimated 4-5 kg|
|Diet||Carnivorous; likely fish|
|Species||Keichousaurus hui Keichousaurus yuananensis|
Keichousaurus is a genus of very small nothosauroid marine reptiles. Fossil remains have been found in Mongolia and China, and date to the Norian stage of the Late Triassic period. Keichousaurus is the smallest known fossil marine reptile found to date.
The genus Keichousaurus consists of two species: K. hui and K. yuananensis. Size variations and slight morphological differences separate the three species; however, some authors have proposed that such variations represent different stages of growth, rather than separate species.
While it is the smallest marine reptile known, Keichousaurus is highly variable in terms of size. The smallest specimens, which likely represent juveniles, are around 20 centimetres in length; the larger specimens being well over twice the size of the smallest.
Fossil Keichousaurus are often preserved in exquisite detail, with delicate features such as fingers preserved. Based on these, it appears that Keichousaurus' possessed webbed feet rather than flippers; suggesting that it may have been among the less aquatic nothosaur species. Some have suggested that Keichousaurus inhabited sheltered lagoons and may have climbed out of the water in similar fashion to modern sea lions, while others have proposed that it inhabited mangrove swamps and freshwater lakes.
Fossils have been found of female Keichosaurus hui with fetuses within the lower portion of the thoracic cavity. Their position implies that they are not victims of cannibalism and that K. hui, at least, was an ovoviviparous species. Ovoviviparity was likely also present in the other two Keichousaurus species.
The presence of individuals at different stages of growth within the same fossil beds appears to indicate social behaviour, and perhaps parental investment by the adults.