Champsosaurus is an extinct genus of diapsid reptiles belonging to the order Choristodera, that existed in the Late Cretaceous and Paleogene periods. It consists of seven species: C. albertensis, C. ambulator, C. gigas, C. laramiensis, C. lindoei, C. natator, and C. tenuis. The name Champsosaurus is thought to come from χαμψαι [champsai] (said in an Ancient Greek source to be an Egyptian word for "crocodiles"), and σαύρος [sauros] (Greek for "lizard"). The largest species, C. gigas, grew to 3-3.5 m (10–12 ft) in length.
Temporal range: Late Cretaceous–Eocene
|C. natator skeleton|
Its fossils have been found in North America (Alberta, Saskatchewan, Montana, New Mexico, Texas, Colorado, and Wyoming) and Europe (Belgium and France), dating from the Upper Cretaceous to the mid Eocene.
It grew to about 1.50 m (5 ft) long, though C. gigas, the largest species, reached 3-3.5 m (10–12 ft) in length.Champsosaurus vaguely resembled a gharial and, like gharials, it was primarily aquatic, catching fish with its long, tooth-lined jaws. It probably swam with lateral body movements, pinning its limbs against its body to increase its streamline, just like crocodiles and the marine iguana. Behind the eyes, Champsosaurus's skull was very wide, where powerful jaw muscles were attached. Due to specializations in hip and limb anatomy, only females could come ashore to lay eggs, while males primarily lived in water.
A skeleton of Champsosaurus reveals a flattened skull in the dorsoventral region, implicating adaptions to aquatic life. The skeleton featured a well-developed, intact secondary palate. Additionally, Champsosaurus is thought to have had a relatively rigid ribcage given the presence of a complete post-cranial skeleton. Internal dermal bones (gastralia) function with this ribcage to regulate the volume of the pleural cavity, effectively controlling the animal's center of gravity for underwater motion. 
Champsosaurus, like many of its fellow neochoristoderes, features teeth with striated enamel of the tooth crown with enamel infolding at the base. Anterior teeth are typically sharper and more slender than posterior segments.
Previously, two species of Champsosaurae were identified from the Tullock formation in Montana, USA. Because of their nesting behavior on land, it is believed that female champsosaurs were better adapted to terrestrial life, and this is seen in limb morphology between males and females. Sacral fusion is featured in discovered specimens, with of without deformations alike. The existence of a non-deformational development of sacral fusion, coupled with the presence of more robust limb bones in some discovered individuals is thought to be a result of specific variation, in which said limbs belong to females. 
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- Lambert, et al. (2001).
- Yoshihiro Katsura, Fusion of sacrals and anatomy in Champsosaurus (Diapsida, Choristodera), doi:10.1080/08912960701374659
- ERICKSON, B. R. (1985). ASPECTS OF SOME ANATOMICAL STRUCTURES OF CHAMPSOSAURUS (REPTILIA: EOSUCHIA). JOURNAL OF VERTEBRATE PALEONTOLOGY, 5(2), 111-127.
- KATSURA, Y. (2004). SEXUAL DIMORPHISM IN CHAMPSOSAURUS (DIAPSIDA, CHORISTODERA). LETHAIA, 37(3), 245-253.
- KATSURA, Y. (2007). FUSION OF SACRALS AND ANATOMY IN CHAMPSOSAURUS (DIAPSIDA, CHORISTODERA). HISTORICAL BIOLOGY, 19(3), 263-271.
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