Tylosaurus, Globidens, Plotosaurus and the Mosasaurs

Tylosaurus kansasensis with large flippers.
Tylosaurus proriger with mid-sized flippers.
Tylosaurus dyspelor with small flippers, all about 30 feet (9.1m) long.

Tylosaurus proriger (Marsh 1872, Late Cretaceous) was a giant mosasaur, a clade that traditionally and currently nests with small Aigialosaurus. Mosasaurs were sea-going giant varanoid lizards that gave live birth and had extra phalanges on the medial digits as the hands and feet were transformed into paddles. Large teeth appear on the pterygoids, but these are convergent with those found on terrestrial snakes like Pachyrhachis. The nasals were fused together and fused to the premaxilla ascending process.

Boxed: The shoulder girdle, left paddle and cartilaginous sternal cartilages of Tylosaurus in dorsal view. These broad ribs and sternum anchored powerful pectoral muscles to the front paddles.

Tethysaurus nopscai (Bardet, Pereda-Superbiola and Jalil 2003) was a small 3 m (9.8 ft) mosasaur from the Early Turonian (Late Cretaceous. It was initially considered a basal mosasaur, but now considered a derived form. Here it nests with Tylosaurus (above), not far from Aigialosaurus. These taxa were derived from a sister to Estesia, a prehistoric varanoid.

Plotosaurus benisoni (originally Kolposaurus, preoccupied; Camp 1942, 1951; Late Cretaceous; 9m) is a large mosasaur (Lepidosauria, Squamata, Scleroglossa) with relatively small flippers and a large tail fin. It enters the LRT as a sister to Tylosaurus. The nasals, tiny in Tylosaurus, are absent in Plotosaurus.

Globidens dakotensis (Gilmore 1912, Russell 1975, Late Cretaceous) nests with Plotosaurus in the LRT. Note truncated epipterygoids, the contact between the prefrontal and fused postfrontal + postorbital. The vomer is taphonomically absent. The jugal dips below the orbit. The teeth are sphere-shaped. The pterygoids do not reach the quadrates. The lacrimal is fused to the maxilla.

Bardet N, Pereda-Suberbiola X and Jalil NE 2003. A new mosasauroid (Squamata) from the Late Cretaceous (Turonian) of Morocco. Comptes Rendus Palevol. 2(8):607-616.

Camp CL 1942. California Mosasaurs. Memoirs of the University of California 13:1-68.

Camp CL 1951. Plotosaurus, a new generic name for Kolposaurus Camp, preoccupied. Journal of Paleontology 25:822.

Gilmore CW 1912. A new mosasauroid reptile from the Cretaceous of Alabama, Proceedings U.S. National Museum, 40(1870): 489-484, 3 fig., pl. 39-40

Marsh OC 1872. Note on Rhinosaurus. American Journal of Science 4 (20):147.

Russell D 1975. A new species of Globidens from South Dakota, and a review of globidentine mosasaurs. Fieldiana Geology. 33 (13): 235–256.

wiki/Tylosaurus
wiki/Tethysaurus
wiki/Plotosaurus
wiki/Globidens