Tylosaurus

***Tylosaurus*** (meaning "protruding lizard") was a mosasaur, a large, predatory marine reptile closely related to modern monitor lizards and to snakes, from the Late Cretaceous.

## Description

A distinguishing characteristic of *Tylosaurus* is the elongated conical rostrum that protrudes from its snout, from which the genus is named. Unlike typical mosasaurs, *Tylosaurus* did not have teeth up to the end of the snout nor on the bony protuberance that is the rostrum, and scientists believe that this feature was primarily used for combative purposes such as ramming. This is supported with a uniquely broad and somewhat rectangular internarial bar (the extension of the premaxilla on the top of the skull that held together the nasal and upper jaws) that provided high cranial stability and resistance to stress forces. In addition, the development of the elongated rostrum in infant *Tylosaurus* rules out the alternative explanation that it was developed as a function of sexual behavior. *Tylosaurus* also had 24 to 26 teeth in the upper jaw, 20 to 22 teeth on the palate, 26 teeth on the lower jaw, 29 to 30 vertebrate between the skull and hip, 6 to 7 vertebrae in the hip, 33 to 34 vertebrae in the tail with chevrons, and a further 56 to 58 vertebrae making up the tip of the tail.

### Size

*Tylosaurus* was one of the largest mosasaurs of all time. The largest known specimen, a skeleton of *T. proriger* from the University of Kansas Natural History Museum nicknamed "Bunker" (KUVP 5033), has been estimated to measure between 12–15.8 meters (39–52 ft) long. Some isolated fragments have been estimated to belong to individuals measuring 14 meters (46 ft) or more. The genus exhibits Cope's rule, in which its body size has been observed to generally increase over geologic time. In North America, the earliest representatives of *Tylosaurus* during the Turonian and Coniacian (90-86 mya), which included early *T. nepaeolicus* and its precursors, typically measured 5–7 meters (16–23 ft) long and weighed between 200–500 kilograms (440–1,100 lb). During the Santonian (86-83 mya), *T. nepaeolicus* and newly-appearing *T. proriger* were 8–9 meters (26–30 ft) long and weighed around 1,100 kilograms (2,400 lb). By the Early Campanian, *T. proriger* attained lengths of 13–14 meters (43–46 ft).

Commenting on the maximum sizes of large mosasaurs, paleontologist Mike Everhart, a leading expert on mosasaurs, speculated that it would be possible for some extremely old *Tylosaurus* individuals to reach 20 meters (66 ft) in absolute maximum length. However, this is with awareness that there is no fossil evidence suggesting such sizes and that the odds of preserving such a rare individual are "far too great."

In 2014, the Guinness World Records awarded the Canadian Fossil Discovery Centre a record for "Largest mosasaur on display," specifically, a 13.05 m (42.8 ft) long skeleton of *T. pembinensis* nicknamed "Bruce." However, the skeleton was assembled for display prior to a 2010 reassessment of the species that found its original number of vertebrae to be exaggerated, implying that the actual size of the animal was likely smaller.

### Skin and coloration

Fossil evidence of the skin of *Tylosaurus* in the form of scales has been described since the late 1870s. These scales were small and diamond-shaped and were arranged in oblique rows, comparable to that found in modern rattlesnakes and other related reptiles. However, the scales in the mosasaur were much smaller in proportion to the whole body. An individual measuring 5 meters (16 ft) in total body length had dermal scales measuring 3.3 by 2.5 millimeters (0.130 in × 0.098 in), and in each square inch (2.54 cm) of the mosasaur's underside an average of ninety scales were present. Each scale was keeled in a form resembling that of a shark's denticles, suggesting that they functioned in reducing underwater drag.

Microscopic analysis of 86 million-year-old fossil scales of a *T. nepaeolicus* in a 2014 study detected high traces of the pigment eumelanin, indicating that the species possessed a dark coloration similar to the leatherback sea turtle in life. This may have also been complemented with countershading, which is present in many aquatic animals, but the distribution of dark and light pigments in the species remains unknown. In addition, the presence of keeled scales in the genus would have reduced reflection on the skin. A dark-colored form would have provided several evolutionary advantages for *Tylosaurus*. Dark coloration increases absorption of heat, allowing the animal to maintain elevated body temperatures in colder environments. Possession of this trait during infancy would in turn facilitate fast growth rates. Additionally, unreflective dark coloring and countershading would have provided the mosasaur with increased camouflage. Several speculative functions have also been proposed, such as increased tolerance to solar UV radiation, strengthened integuments, and increased aggressiveness (certain genes that code for high melanin may also have side influences promoting this trait).

## *Tylosaurus* in The Amazing Dinosaur Adventure

*Tylosaurus* from the opening narration to *Journey Through the Mists*