Tylosaurus

*Tylosaurus proriger*

***Tylosaurus*** was one of the largest hunters of the Cretaceous seas. *Tylosaurus* used its snout to locate prey, which, once inside this mosasaur's menacing jaws, was swallowed whole. When the sea monster opened wide for the final gulp, two extra rows of teeth on the roof of its mouth allowed crippled captives no escape. According to a new study, it hunted like an Orca ramming the prey from below before killing it.

## 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 this name soon proved to be preoccupied. 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 skeleton 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 new (2007) National Geographic IMAX movie - "Sea Monsters", and a book by the same name (Everhart, 2007).

Note that the "early photograph" of a *Tylosaurus* skull (above) 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 this 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.

## Species

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.); *Tylosaurus nepaeolicus* (Cope, 1874), from the Santonian of North America (Kansas); *Tylosaurus haumuriensis* (Hector, 1874; =Taniwhasaurus oweni), from the lower to middle Campanian of New Zealand; *Tylosaurus kansasensis* Everhart, 2005, from the late Coniacian of Kansas.

A closely related genus, *Hainosaurus* ("Haine lizard", named after the Haine River in Belgium) is known from the Creatceous of North America and Europe. Both *Tylosaurus* and *Hainosaurus* are grouped together into the subfamily Tylosaurinae and are referred to informally as "tylosaurines" or "tylosaurs." Bell placed the tylosaurines together with the plioplatecarpine mosasaurs (*Platecarpus*, *Plioplatecarpus*, etc.) in an informal monophyletic grouping which he called the "Russellosaurinae."

## 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

*Tylosaurus* length varies between 9-14 meters and it's mass varies between 4-11 tonnes, making it one of the largest of the marine reptiles. The size variation is quite extensive, mainly because most Tylosaurus specimens are around 9 meters long. But we also have several very large specimens like "Sophie" or "Bunker", which are both over 12 meters long and weigh over 9 tonnes. The largest known specimen "Bonker" (FHSM VP-2496) is assumed to even surpass 14 meters in length and 20 tonnes in weight, making Tylosaurus the largest mosasaur and one of the biggest marine reptiles of all time. Like all mosasaurs, a long and muscular, vertically flattened tail powered *Tylosaurus* through the water, allowing it to ambush its prey with rapid bursts of acceleration. Paddle-like limbs helped steer the slim body covered in lizard-like scales through the water.

## Classification

As a mosasaur, Tylosaurus is classified within the family Mosasauridae in the superfamily Mosasauroidea. The genus is the type genus of its own subfamily, the Tylosaurinae. Other members of this group include Taniwhasaurus and possibly Kaikaifilu, and the subfamily is defined by a shared feature of an elongated conical rostrum that does not bear teeth.[5] The closest relatives of the Tylosaurinae include the Plioplatecarpinae and the primitive subfamilies Tethysaurinae and Yaguarasaurinae; together they are members of one of three possible major lineages of mosasaurs (the others being the Mosasaurinae subfamily and Halisauromorpha group) that was first recognized in 1993. This clade was named the Russellosaurina by Polcyn and Bell in 2005.[83][84][85

Tylosaurus was among the earliest known derived mosasaurs. The oldest fossil attributed to the genus is of a skull (SGM-M1) recovered from Upper Turonian deposits of Ojinaga Formation in Chihuahua, Mexico,[2][5] which may date around 90 million years old at earliest based on correlations with index fossils.[1] This skull has been identified as an indeterminate species with significant affinities with T. kansasensis, which was previously considered the most primitive species. A tooth from a Late Maastrichtian deposit in Nasiłów, Poland dating close to the Cretaceous–Paleogene boundary has been attributed to Hainosaurus sp.[3][7] With the incorporation of Hainosaurus as a synonym of Tylosaurus, this also makes the genus one of the last mosasaurs. Currently, eight species of Tylosaurus are recognized by scientists as taxonomically valid. They are as follow: T. proriger, T. nepaeolicus, T. bernardi, T. gaudryi, T. ivoensis, T. iembeensis, T. pembinensis, and T. saskatchewanensis. The validity of two additional taxa remain unsettled; there is still debate whether T. kansasensis is synonymous with T. nepaeolicus, and T. borealis has yet to be described in a formal publication.

In 2020, Madzia and Cau performed a Bayesian analysis to better understand the evolutionary influence on early mosasaurs by contemporaneous pliosaurs and polycotylids by examining the rates of evolution in mosasauroids like Tylosaurus (specifically T. proriger, T. nepaeolicus, and T. bernardi). A Bayesian analysis in the study's implementation can approximate numerically-defined rates of morphological evolution and ages of divergence of clades. The Tylosaurinae was approximated to have diverged from the Plioplatecarpinae around 93 million years ago; the divergence was characterized by the highest rate of evolution among all mosasaurid lineages. This trend of rapid evolution coincided with the extinction of the pliosaurs and a decrease in polycotylid diversity. The study noted converging traits between Tylosaurus, pliosaurs, and some polycotylids in tooth morphology and body size. However, there was no evidence to suggest that Tylosaurus or its precursors evolved as a result of out-competing and/or driving to extinction the pliosaurs and polycotylids. Instead, Madiza and Cau proposed that Tylosaurus may have taken advantage of the extinction of the pliosaurs and decline of polycotylids to quickly fill the ecological void they left behind. The Bayesian analysis also approximated a divergence of T. nepaeolicus from the rest of the genus around 86.88 million years ago and a divergence between T. proriger and T. bernardi around 83.16 million years ago. The analysis also generated a paraphyletic status of the genus, approximating Taniwhasaurus to have diverged from Tylosaurus around 84.65 million years ago, but this result is not consistent with previous phylogenetic analyses.

In the Western Interior Seaway, two species—T. nepaeolicus and T. proriger—represent a chronospecies, in which they make up a single lineage that continuously evolves without branching off in a process known as anagenesis. This is evident by how the two species do not stratigraphically overlap, are sister species, share minor and intermediate morphological differences such as a gradual change in the development of the quadrate bone, and lived in the same locations. The means by which this lineage evolved has been hypothesized to be through one of two evolutionary mechanisms related to changes in ontogeny. First, Jiménez-Huidobro, Simões, and Caldwell proposed in 2016 that T. proriger evolved as a paedomorph of T. nepaeolicus, in which the descendant arose as a result of morphological changes through the retention of juvenile features of the ancestor in adulthood. This was based on the presence of a frontal crest and convex borders of the parietal bone of the skull shared in both juvenile T. nepaeolicus and all T. proriger but lost in adult T. nepaeolicus.[76][78] However, an ontogenetic study by Zietlow (2020) found that it was unclear whether this observation was a result of paedomorphosis, although this uncertainty may have been due that the sample size of mature T. nepaeolicus was too low to determine statistical significance. Second, the same study proposed an alternative hypothesis of peramorphosis, in which T. proriger evolved by developing traits found in mature T. nepaeolicus during immaturity. Based on results from a cladistical ontogram developed using data from 74 Tylosaurus specimens, the study identified a multitude of traits that were present in all T. proriger and mature T. nepaeolicus but absent in juvenile T. nepaeolicus: the skull size and depth are large, the length of the elongated rostrum exceeds 5% of the total skull length, the quadrate suprastapedial processes are thick, the overall quadrate shape converges, and the posteroventral process is fan-like.

## Paleobiology

Konishi and colleagues in 2018 assigned specimen FHSM VP-14845, a small juvenile with an estimated skull length of 30 centimeters (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 cm (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.

Preserved stomach contents indicate a diet heavy on fish, but seabirds, sharks, plesiosaurs, and other mosasaurs also failed to escape *Tylosaurus* lethal grip. Though not a dinosaur, *Tylosaurus* lived alongside them and went extinct at around the same time. Many *Tylosaurus* remains have been found in Kansas, which was once covered by a large ocean called the Western Interior Seaway.

## Dinosaur Field Guide Description

Tylosaurus ("knob lizard") has Al large, strong chest and arms, big hands, a long body with weak legs, and a strong tail for swimming. It has a ram-shaped snout and big conical teeth for catching prey. It is a mosasaur, or Meuse River lizard, a true marine lizard. The mosasaurs are related to the modern monitor lizards (like the Komodo dragon), Gila monsters, and snakes. Mosasaurs, however, lived their whole lives in the water. As in ichthyosaurs, their young developed inside the mother and were born live into the water. Mosasaurs were probably not as fast as the earlier ichthyosaurs had been, and instead ambushed their prey the way many sharks do today. Some mosasaurs had flatter teeth than Tylosaurus, which they used for cracking open shellfish.

### Fun Facts

A fossil of a giant sea turtle has been found with a flipper bitten off-probably by a Tylosaurus.

### Trivia

Mosasaurs were the first group of extinct reptiles known to science.

## Appearance in other media

### *Jurassic Park*

* A *Tylosaurus* appears in Jurassic Park: The Game where it consumes Dr Laura Sorkins and attempt to eat the rest of the survivors. The cloned *Tylosaurus* had a row spikes running down its neck to its upper back like outdated depictions of mosasaurs from the late 1800s. It also lacked a tail fluke, with another row of spikes covering the tail. The clone's skull was unlike *Tylosaurus* being as short as the mosasaur *Platecarpus* with the jaws not being completely straight. The skin color of the cloned *Tylosaurus* was a shade of blue with black striping and a yellowish white underbelly. It was originally the unnamed marine reptile in the game being only referred to as "Mosasaur". In Laura Sorkin's research journal, she describes the creature and labels it "Mosasaur". She also writes: "*Upwards of 50 feet long, depending on genus.*" From this text it is clear that she doesn't know its genus, indicating that she calls the creature "Mosasaur", she refers to the family to which it belongs; which is Mosasaurs (Mosasauridae). The identity of Jurassic Park's Mosasaur was ultimately revealed to be *Tylosaurus* in the InGen Field Guide.

* *Tylosaurus* is number 127 of the Carnivore Threes that can be created in *Jurassic Park III: Park Builder*. As a carnivore, it should be placed in its own enclosure, considering that if it gets placed with another animal, it will kill it. In-game it looks like a plesiosaur, probably because the game makers didn't have large enough a budget to make its own model.

* *Tylosaurus* appears in the underwater park update for the game Jurassic Park: Builder.