Champsosaurus

Champsosaurus was the first member of the Choristodera to be described. Champsosaurus was named by Edward Drinker Cope in 1876, from isolated vertebrae found in Late Cretaceous strata of the Judith River Formation on the banks of the Judith River in Fergus County, Montana. Cope designated C. annectens as the type species rather than the first named C. profundus due to the larger number of vertebrae he attributed to the species. C. annectens was based on 9 isolated vertebral centra (AMNH FR 5696) that were not figured in the paper of which two are now lost. The conclusion that C. annectens was undiagnostic was supported by William Parks in 1933. Brown in 1905 named two species of Champsosaurus. One was C. ambulator, named from the specimen AMNH 983, a fragmentary skeleton with a partial skull found in the Hell Creek Formation of Montana. The other was C. laramiensis, named from AMNH 982, a nearly complete skeleton and skull, also found in the Hell Creek Formation. Parks in 1933 named the species C. natator from an incomplete skeleton with a fragmentary skull (TMP 81.47.1) found in the Belly River Group in the Red Deer River valley in Alberta. In 1972, Bruce Erickson named the species C. gigas from SMM P71.2.1, a partial skeleton and skull found in the Sentinel Butte Formation, Golden Valley County, North Dakota. Erickson subsequently in 1981 named the species C. tenuis from SMM P79.14.1, a partial skeleton and skull found in the Bullion Creek Formation, North Dakota. In 1998 K. Q. Gao and Richard Carr Fox described the species C. lindoei from UALVP 931, a nearly complete skeleton with skull and jaws from the Dinosaur Park Formation in Alberta. The publication also thoroughly reviewed Champsosaurus, rediagnosing most species except for C. ambulator and C. laramiensis. Fossils of Champsosaurus 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 late Paleocene. Remains tentatively referred to Champsosaurus are known from the high Canadian Arctic, dating to the Coniacian–Turonian, a time of extreme warmth. == Taxonomy ==

Sixteen species of Champsosaurus have been named, of which seven are presently considered valid. The type species Champsosaurus annectens Cope, 1876 is considered to be dubious. The only named European species C. dolloi Sigogneau-Russell 1979 was considered to be too fragmentary to warrant a new species by Gao and Fox in 1998. ==Description==

Most species grew to about 1.50 m (5 ft) long, though Champsosaurus gigas, the largest species, reached 3–3.5 m (10–12 ft) in length. The body is flat and streamlined, with heavy gastralia (rib-like bones situated in the belly). Teeth 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 teeth. Like other choristoderes, Champsosaurus possessed palatal teeth (teeth present on the bones of the roof of the mouth), with longitudinal rows present on the pterygoid, palatine and vomer, alongside a small row on the flange of the pterygoid. The palatine teeth of Champsosaurus are located on raised platforms of bone, though the wideness of the platforms, the sharpness and orientation of teeth vary between species. The orientation of the teeth varies in the jaw, with the posterior teeth being orientated backward. The palatal teeth, likely in combination with a fleshy tongue probably aided in gripping and swallowing prey. Skin Skin impressions of Champsosaurus have been reported. They consist of small (0.6-0.1 mm) pustulate and rhomboid scales, with the largest scales being located on the lateral sides of the body, decreasing in size dorsally, no osteoderms were present. == Classification ==

Champsosaurus belongs to the Neochoristodera, a clade within Choristodera, the members of which are characterised by elongated snouts and expanded temporal arches. The group first appeared during the Early Cretaceous in Asia, and are suggested to have evolved in the regional absence of aquatic crocodyliformes. Phylogeny of Choristodera after Dong and colleagues (2020). }} == Paleobiology ==

Champsosaurus is thought to have been highly specialised for aquatic life. Erickson 1985 proposed that the position of the nostrils at the front of the snout allowed Champsosaurus to spend large amounts of time at the bottom of water bodies, with the head being angled upwards to allow the snout to act like a snorkel when the animal needed to breathe. Previously, two species of Champsosaurus were identified from the Tullock Formation in Montana. However, these differences are now thought to be sexually dimorphic, with presumed females possessing robust limb bones. Non-deformation related fusion of the sacral vertebrae is also observed in specimens with robust limb bones. These are hypothesised to be related to breeding behaviour, with the more robust limb bones and fused sacrals of the females allowing them to move themselves onto land to lay eggs. ==See also==