Thalattosauroidea is a superfamily of thalattosaur

Thalattosaur

Thalattosaurs are a group of prehistoric marine reptiles which lived during the mid-late Triassic Period. Some species of thalattosaur grew to over 4 meters in length, including a long, flattened tail used in underwater propulsion...

s, a Triassic

Triassic

The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

 group of marine reptile

Marine reptile

Marine reptiles are reptiles which have become secondarily adapted for an aquatic or semi-aquatic life in a marine environment.The earliest marine reptiles arose in the Permian period during the Paleozoic era...

s. It was named in 1904 by paleontologist John Campbell Merriam to include the genus Thalattosaurus

Thalattosaurus

Thalattosaurus is an extinct genus of marine reptile in the family Thalattosauridae that lived during the Triassic period. It includes two known species.Thalattosaurus was a shellfish-eating reptile around 2 meters in length....

from California

California

California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

. Thalattosauroids are one of two groups of Thalattosauria, the other being Askeptosauroidea

Askeptosauroidea

Askeptosauroidea is a superfamily of thalattosaurs, a Triassic group of marine reptiles. Askeptosauroidea is one of two major subgroups of Thalattosauria, the other being Thalattosauroidea...

. Thalattosauroids make up the "traditional" thalattosaurs with large downturned snouts, short necks, and long, paddle-like tails.

Classification

Thalattosauria includes North American genera such as Thalattosaurus and Nectosaurus

Nectosaurus

Nectosaurus is a genus of marine diapsid reptile which lived during the Late Triassic of what is now California. The type species is N. halinus, described by John C. Merriam in 1905. A 2002 analysis of Nectosaurus classifies it as a thalattosaurian, one of a group of marine reptiles which lived...

as well as recently described Chinese

China

Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

 forms such as Xinpusaurus. A 1999 study of thalattosaurs, which established much of the currently accepted phylogeny of the group, referred to Thalattosauroidea as Thalattosauria, while calling the larger group Thalattosauriformes. More recent phylogenetic studies have came upon the same conclusions, but refer to the group as Thalattosauroidea in order to contrast it with another superfamily of thalattosaurs, Askeptosauroidea. Below is a cladogram

Cladogram

A cladogram is a diagram used in cladistics which shows ancestral relations between organisms, to represent the evolutionary tree of life. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational...

 from Wu et al. (2009) showing the phylogenetic relationships of Thalattosauroidea: