|"Bunker" T. proriger (KUVP 5033) mounted skeleton in the Rocky Mountain Dinosaur Resource Center in Woodland Park, Colorado|
Tylosaurus (Greek τυλος/tylos "protuberance, knob" + Greek σαυρος/sauros "lizard") was a mosasaur, a large, predatory marine lizard closely related to modern monitor lizards and to snakes, from the Late Cretaceous.
A distinguishing characteristic of Tylosaurus is its elongated, cylindrical premaxilla (snout) from which it takes its name. Unlike other mosasaurs, Tylosaurus did not have teeth all the way forward on its premaxilla, as the bony protuberance was free of teeth. Tylosaurus also have 24-26 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 56-58 vertebrae making up the tip of the tail.
Early restorations showing Tylosaurus with a dorsal crest were based on misidentified tracheal cartilage, but when the error was discovered, depicting mosasaurs with such crests was already a trend. Its skin had many small scales.
Along with plesiosaurs, sharks, fish, and other mosasaurs, Tylosaurus was a dominant predator of the Western Interior Seaway during the Late Cretaceous. The genus was among the largest of the mosasaurs (along with Mosasaurus hoffmannii), with the possibly conspecific Hainosaurus bernardi reaching lengths up to 12.2 m (40 ft), and T. pembinensis reaching comparable sizes. T. proriger, the largest species of Tylosaurus, reached lengths of about 14 m (46 ft).
Discovery and naming
Like many other mosasaurs, the early history of this taxon is complex and involves the infamous rivalry between two early American paleontologists, Edward Drinker Cope and Othniel Charles Marsh. Originally, the name "Macrosaurus" proriger was proposed by Cope for a fragmentary skull and thirteen vertebrae collected from near Monument Rocks in western Kansas in 1868. It was placed in the collections of the Harvard Museum of Comparative Zoology. Only a year later, Cope redescribed the same material in greater detail and referred it, instead, to the English mosasaur taxon Liodon. Then, in 1872, Marsh named a more complete specimen as a new genus, Rhinosaurus ("nose lizard"), but soon discovered that this name had already been used for a different animal. Cope suggested that Rhinosaurus be replaced by yet another new name, Rhamposaurus which also proved to be preoccupied. Marsh finally erected Tylosaurus later in 1872, to include the original Harvard material as well as additional, more complete specimens which had also been collected from Kansas. A giant specimen of T. proriger, recovered in 1911 by C. D. Bunker near Wallace, Kansas is one of the largest skeletons of Tylosaurus ever found. It is currently on display at the University of Kansas Museum of Natural history.
In 1918, Charles H. Sternberg found a Tylosaurus, with the remains of a plesiosaur in its stomach. The specimen is currently mounted in the United States National Museum (Smithsonian) and the plesiosaur remains are stored in the collections. Although these important specimens were briefly reported by C. H. Sternberg (1922), the information was lost to science until 2001. This specimen was rediscovered and described by Everhart (2004a). It is the basis for the story line in the 2007 National Geographic IMAX movie Sea Monsters, and a book by the same name (Everhart, 2007).
A photograph of a Tylosaurus skull was taken by George F. Sternberg about 1926 after he collected and prepared the specimen. It was discovered in the Smoky Hill Chalk of Logan County, Kansas. Sternberg offered the specimen to the Smithsonian and included a photograph in his letter to Charles Gilmore. Copies of the original photos are in the archives of the Sternberg Museum of Natural History (FHSM). The specimen is FHSM VP-3, the exhibit specimen in the same museum.
Though many species of Tylosaurus have been named over the years, only a few are now recognized by scientists as taxonomically valid. They are as follows: Tylosaurus proriger (Cope, 1869), from the Santonian and lower to middle Campanian of North America (Kansas, Alabama, Nebraska, etc.) and Tylosaurus nepaeolicus (Cope, 1874 ), from the Santonian of North America (Kansas). Tylosaurus kansasensis, named by Everhart in 2005 from the late Coniacian of Kansas, has been shown to be based on juvenile specimens of T. nepaeolicus. It is likely that T. proriger evolved as a paedomorphic variety of T. nepaeolicus, retaining juvenile features into adulthood and attaining much larger adult size.
A closely related genus, Hainosaurus, is known from the Cretaceous of North America and Europe. Both Tylosaurus and Hainosaurus are members of the group Tylosaurinae  and are referred to informally as "tylosaurines" or "tylosaurs." Research published in 2016, however, indicates that Hainosaurus is likely congeneric with Tylosaurus. Bell  placed the tylosaurines together with the plioplatecarpine mosasaurs (Platecarpus, Plioplatecarpus, etc.) in an informal monophyletic grouping which he called the "Russellosaurinae."
Konishi and colleagues in 2018 assigned specimen FHSM VP-14845, a small juvenile with an estimated skull length of 30 centimetres (12 in), to Tylosaurus based on the proportions of the braincase and the arrangement of the teeth in the snout and on the palate. However, the specimen lacks the characteristic snout projection of other Tylosaurus, which is present in juveniles of T. nepaeolicus and T. proriger with skull lengths of 40–60 centimetres (16–24 in). This suggests that Tylosaurus acquired the snout projection rapidly at an early stage in life, and also suggests that it did not develop due to sexual selection. Konishi and colleagues suggested a function in ramming prey, as employed by the modern orca.
Stomach contents associated with specimens of Tylosaurus proriger indicate that this mosasaur had a varied diet, including fish, sharks, smaller mosasaurs, plesiosaurs, and flightless diving birds such as Hesperornis. Like its modern monitor lizard relatives, it would have both eaten any prey it could find and scavenged for an easier meal. In some paleoenvironments, Tylosaurus seems to have preferred shallow, nearshore waters (as with the Eutaw Formation and Mooreville Chalk Formation of Alabama), while favoring deeper water farther out from shore in other environments (as with the Niobrara Chalk of the western U.S.). This indicates that was an adaptable predator.
The Talkeetna Mountains Hadrosaur was a hadrosaurid of indeterminate classification whose carcass appeared to have been deposited in a bathyal or outer shelf environment that later became Alaska's Matanuska Formation. Every element of its skeleton not found in a concretion bore many closely spaced ovualar conical depressions ranging in diameter from 2.12–5.81 mm (0.083–0.229 in) and 1.64–3.62 mm (0.065–0.143 in) deep. These depressions are probably bite marks. The depressions are not symmetrical enough for gastropod drill marks and are not shaped like sponge borings. None of the preserved fish fossils of the Matanuska Formation fit the size or geometry of the borings. The size and spacing and shape by contrast, however, closely resembles the teeth of Tylosaurus proriger. The apparent tooth marks are unlikely to have occurred before the carcass was washed out to sea since that would have punctured the body, preventing the buildup of bloating gases that allowed the carcass to drift out to sea in the first place. The distribution of bite marks corresponds inversely to the presence of flesh in the animal. For instance, lower limb bones sustained the most damage because there was the least amount of flesh shielding the bones at those locations. The concretions formed as the flesh chemically reacted to the seafloor on the largest parts of the animal where the scavenging mosasaur would be unable to fully wrap its jaws around the carcass. Bones pulled free from the carcass were buried in the mud and preserved in mudstone.
- Paulina Jiménez-Huidobro; Michael W. Caldwell; Ilaria Paparella; Timon S. Bullard (2018). "A new species of tylosaurine mosasaur from the upper Campanian Bearpaw Formation of Saskatchewan, Canada". Journal of Systematic Palaeontology. Online edition. doi:10.1080/14772019.2018.1471744.
- Paulina Jimenez-huidobro and Michael W. Caldwell (2016). "Reassessment and reassignment of the early Maastrichtian mosasaur Hainosaurus bernardi Dollo, 1885, to Tylosaurus Marsh, 1872". Journal of Vertebrate Paleontology. Online edition: e1096275. doi:10.1080/02724634.2016.1096275.
- Konishi, Takuya; Jiménez-Huidobro, Paulina; Caldwell, Michael W. (2018). "The Smallest-Known Neonate Individual of Tylosaurus (Mosasauridae, Tylosaurinae) Sheds New Light on the Tylosaurine Rostrum and Heterochrony". Journal of Vertebrate Paleontology. doi:10.1080/02724634.2018.1510835.
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