An Ultimate Guide to Tylosaurus: The Lump Lizard

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Name MeaningKnob LizardLength12–15.8 meters (39–52 feet)
PronunciationTie-lo-sore-usWeight6.7 tons (14,700 lbs) 
EraMesozoicLate CretaceousLocationUSA (North America) 
ClassificationSquamata, Pythonomorpha, & Mosasauroidea

Tylosaurus Pictures

Tylosaurus flippers and tail
Tylosaurus flippers and tail | CoreyFord via Getty Images

The Tylosaurus

Gage Beasley Prehistoric's Tylosaurus Concept
Gage Beasley Prehistoric’s Tylosaurus Concept

Tylosaurus was a marine reptile that lived during the Cretaceous Period about 85 million years ago. 

It was a mosasaur, a distant relative of modern snakes and monitor lizards. 

This group of marine reptiles replaced older groups like the ichthyosaurs, pliosaurs, and plesiosaurs as the apex predators of the marine ecosystem towards the end of the Cretaceous Period. 

Tylosaurus was the top predator of the Western Interior Seaway, a shallow sea that covered parts of North America during the Late Cretaceous. 

Gage Beasley's Prehistoric Shirt Collection
Gage Beasley’s Prehistoric Shirt Collection
Gage Beasley's Prehistoric Plush Collection
Gage Beasley’s Prehistoric Plush Collection

It was a vicious predator that attacked prey by ramming them with its knob-like snout. 

The name Tylosaurus, which translates as “knob lizard,” references this efficient weapon on the reptile’s long snout. 

Fossils of the Tylosaurus were discovered in the 1800s and were part of the North American Bone Wars, the long feud between paleontologists Edward Cope and Othniel Marsh. 

Both scientists discovered fossils of this marine reptile at different times and assigned different names and classifications to it. 

The skull of MCZ 4374, the holotype of T. proriger, in Cope (1870)
The skull of MCZ 4374, the holotype of T. proriger, in Cope (1870) | Edward Drinker Cope via Synopsis of the extinct Batrachia and Reptilia of North America

Different species were also assigned to the Tylosaurus genus, many of which were later discovered to be invalid. 

Over the years, the discovery of additional fossil materials has helped clarify the identity of this marine reptile and properly classify it. 

In this article, we’ll discuss some of the facts scientists have been able to learn about this fearsome reptile and one of the deadliest marine hunters of all time. 

Physical Characteristics

Tylosaurus dinosaur 3D illustration
Tylosaurus dinosaur 3D illustration | Photo: Warpaintcobra via Getty Images

Tylosaurus ruled the seas during the Late Cretaceous, along with other marine reptiles like the Kronosaurus and Mosasaurus, but it was the biggest mosasaur around. 

The general body plan of this marine reptile is similar to that of other mosasaurs. 

It had a large and muscular streamlined body.  

The head of Tylosaurus was large, with a long snout filled with sharp, conical teeth, while the tail was long and vertically flattened.

Skull of T. gaudryi (MNHN 1896–15)
Skull of T. gaudryi (MNHN 1896–15) | FunkMonk via Wikipedia (CC BY-SA 3.0)

Like other marine reptiles of the Mesozoic, Tylosaurus still had well-developed flippers that aided in maneuvering through the water. 

But the tail was the Tylosaurus’ main mechanism for getting through the water. 

The Tylosaurus was arguably the largest mosasaur of all time.

The largest species in the genus (Tylosaurus proriger) grew to an estimated length of about 12 to 15.8 meters (39–52 feet) and weighed about 6.7 metric tons.

Gage Beasley Prehistoric's Tylosaurus Size Comparison Chart
Gage Beasley Prehistoric’s Tylosaurus Size Comparison Chart

However, it is worth noting that not all Tylosaurus species were this big. 

The Tylosaurus’ body was covered by small, diamond-shaped scales arranged similar to that of modern rattlesnakes and other related reptiles. 

The scales were small in proportion to the reptile’s entire body, each measuring about 3.3 by 2.5 millimeters (0.130 inches ×0.098 inches) for a 5 meters (16 feet) long individual. 

The scales were keeled, which would have helped to reduce drag as the Tylosaurus swims in the water. 

Preserved skin of Tylosaurus | Ghedoghedo via Wikipedia (CC BY-SA 4.0)

Scientists have also found preserved skin specimens of this reptile, and studies show that it contains traces of the pigment eumelanin. 

This suggests that the species likely had a dark coloration similar to that of leatherback sea turtles. 

This may have been counter-shaded, as seen in many aquatic animals, but the distribution of these light and dark pigments in the Tylosaurus isn’t known.  

Habitat and Distribution

Tylosaurus fossils have been discovered in various locations across North America.

This marine reptile primarily lived in the warm, shallow waters of the Western Interior Seaway, a vast inland sea that stretched from present-day Canada down to the Gulf of Mexico. 

This sea divided the North American continent into eastern and western landmasses during the Late Cretaceous Period.

Fossils of the Tylosaurus are most commonly discovered in the central United States, especially in Kansas and surrounding states. 

This suggests that the mosasaur’s range was mainly in this region. 

Western Interior Seaway | Photo via Plos One [Paleogeography of North America during the late Campanian Stage of the Late Cretaceous (∼75 Ma)]

However, it was present in other parts of the continent as well. ‬

The Cretaceous Period, when the Tylosaurus was alive, was generally characterized by a warm greenhouse climate. 

The global temperatures were higher than present conditions, resulting in extensive polar ice melting and causing a significant rise in sea levels. 

The high sea level gave rise to inland or epicontinental seas like the Western Interior Seaway in various locations all over the world. 

Behavior and Diet

Tylosaurus | MR1805 via Getty Images

As a marine reptile, Tylosaurus was an incredibly efficient and powerful swimmer.

However, unlike the marine reptiles that lived before it, like the pliosaurs and plesiosaurs, Tylosaurus and other mosasaurs didn’t rely on their flippers for locomotion.

Instead, it used its tail for propulsion through the water. 

The long and muscular tail had at least 80 vertebrae, each with a tall neural spine. 

The tail was laterally compressed but deep, providing a high surface area that could push against the water to propel the Tylosaurus’ body forward effectively. 

The rest of the Tylosaurus’ body was also designed to allow efficient swimming. 

Although it wasn’t as streamlined as a fish’s body, the Tylosaurus was still built to be as hydrodynamically efficient as possible. 

It also had a long sloping skull that further reduced drag when swimming through the water. 

Tylosaurus | MR1805 via Getty Images

The Tylosaurus’ flippers were proportionally smaller than those of the pliosaurs and plesiosaurs but were still useful for steering in the water. 

The smaller size reduced drag, further boosting the speed and agility of this predator. 

In addition to being fast, Tylosaurus was the biggest and deadliest hunter of the Late Cretaceous seas. 

It was big enough to capture and kill pretty much any creature in its habitat. 

This reptile had massive jaws lined on each side with double rows of long cone-shaped teeth.

Interestingly, the Tylosaurus’ massive jaws were not the main attack weapon. 

The front of the jaw had a long rostrum that was practically toothless. 

The Turonian-aged skull of T. sp. aff. kansasensis (SGM-M1) is the oldest known fossil of Tylosaurus. | Levi bernardo via Wikipedia (CC BY-SA 3.0)

However, this snout was also reinforced and built for impact. 

Scientists think this reptile used its rostrum to attack prey by ramming into them at high speed. 

While this type of attack may not kill the prey right away, it was effective enough to disorient it so the Tylosaurus could easily feed on it. 

The menacing jaws of the Tylosaurus were designed to open wide so the sea monster could swallow its prey whole. 

The roof of its mouth had two additional rows of teeth that helped to further hold and cripple prey until the Tylosaurus swallowed it. 

Isolated tooth of T. ivoensis | Ichthyovenator via Wikipedia (CC BY-SA 4.0)

Some fossils of this marine reptile have been recovered with preserved gut content. 

This has made it possible to determine what the Tylosaurus ate when it was alive. 

It had a diet heavy on fish which were quite abundant in the warm shallow sea where it lived. 

However, this carnivore had a varied appetite that included sharks, seabirds, and other marine reptiles, including plesiosaurs and mosasaurs. 

The margins of the seas where this reptile lived had different dinosaur species, and there’s evidence to suggest that the Tylosaurus at least scavenged on dinosaur remains washed out to sea. 

Life Cycle

Tylosaurus in the stormy ocean | MR1805 via Getty Images

One of the most enigmatic aspects of the Tylosaurus’ life (and that of other mosasaurs) is how they reproduced. 

They most likely engaged in sexual reproduction, but it isn’t clear if they laid eggs or gave birth to live young. 

Earlier marine reptiles could come ashore to lay eggs. 

But as the Mesozoic Era progressed, the marine reptiles became more aquatic, and most of them were no longer capable of coming on land to lay eggs. 

Since it’s unlikely that they laid eggs in the open sea, and no egg nests belonging to a member of this group have been found so far, scientists have proposed alternative forms of birth for the mosasaurs, including oviparity and ovoviviparity. 

Mosasaur Sea Lizard Fossil on black background. | barbaraaaa via Getty Images

Although relatively rare, at least two fossils of baby mosasaurs have been identified in the fossil record. 

The area where these fossils were found indicates that they were most likely born in the open sea. 

Scientists have also found marine reptiles like the Ichthyosaurs in the process of giving birth to live young. 

While they represent a completely different group of marine reptiles, the fact that they gave birth this way suggests that the Tylosaurus and other mosasaurs may have birthed live young this way as well. 

Juvenile Tylosaurus had a similar appearance as adult specimens. 

Two Tylosaurus | MR1805 via Getty Images

The elongated snout was present at a very young age, which means it is not a structure they developed at maturity.

Tylosaurus juveniles most likely exhibited rapid growth during their early life stages. 

As hatchlings, they would have been small and vulnerable, living in the open seas. 

But they probably grew quickly and attained full adult size within a relatively short time.

Evolution and History

Tylosaurus pembinensis, aka “Bruce”, from Campanian of Canada. | Dmitry Bogdanov via Wikipedia (CC BY 3.0)

Tylosaurus was a mosasaur, a group of marine reptiles related to modern snakes and monitor lizards.

The ancestors of the mosasaurs were likely a group of aquatic lizards known as the aigialosaurs

These, in turn, evolved from terrestrial lizards that made the transition back to the marine environment during the Early Cretaceous Period. 

The early mosasaurs began adapting more to life in the oceans during the Late Cretaceous Period. 

This required them to ditch their plesiopedal limbs for flippers and gradually develop a streamlined body.

Mounted skeleton of T. bernardi (IRSNB R23) | Ghedoghedo via Wikipedia (CC BY-SA 4.0)

In the last 20 million years of the Cretaceous Period, mosasaurs like the Tylosaurus evolved to become the apex predator of the marine ecosystem. 

This has been attributed to the extinction of the ichthyosaurs and pliosaurs, which were the dominant predators before them.

Tylosaurus also evolved into bigger sizes throughout the Cretaceous. 

Earlier species in the genus that were alive about 90 million years ago were typically about five to seven meters long. 

By the time they went extinct at the end of the Cretaceous Period, the biggest Tylosaurus was up to twice that size.

Interactions With Other Species

1899 Knight restoration showing Tylosaurus inaccurately moving by undulating its body from side to side | Charles R. Knight via Flickr

The Western Interior Seaway where the Tylosaurus lived was a vast body of water that was home to diverse species of fish that formed the bulk of this reptile’s diet.  

But evidence from its gut content shows that Tylosaurus individuals also ate turtles, birds, sharks, and other plesiosaurs. 

Invertebrates such as giant squid and ammonites were probably on the menu, too.

It was an agile reptile that hunted prey by ambushing them and chasing them in the open water. 

Big sharks and other marine reptiles represented the biggest competition to the Tylosaurus when it was alive. 

These animals occupied a similar ecological niche and may have competed for prey and other resources. 

Fish become prey to a pair of Tylosaurus | CoreyFord via Getty Images

Being the biggest carnivore in its ecosystem meant the Tylosaurus was easily the apex predator, and it occasionally hunted these other marine predators too.  

Scientists think the Tylosaurus was highly territorial and aggressive. 

There’s at least one record of a fatal encounter between two Tylosaurus individuals, which ended in the death of one of them. 

The five meters (16 feet) long reptile was found with numerous injuries that appeared to have been caused by combat with a larger Tylosaurus

The Late Cretaceous ecosystem could not have supported a large number of these predators indefinitely, which means encounters like this (over territory and food) would have been quite common. 

Cultural Significance

Tylosaurus proriger (KUVP 5033) mounted skeleton in the Rocky Mountain Dinosaur Resource Center in Woodland Park, Colorado | MCDinosaurhunter via Wikipedia (CC BY-SA 3.0)

At the time of its discovery in the late 1800s, Tylosaurus was the third mosasaur genus to be identified and described in North America.  

The discovery came at a significant time in paleontological history known as the Bone Wars. 

This was a period of major rivalry between two of the leading paleontologists in North America, struggling to be the first to find and name a new species of prehistoric animal

Consequently, the first few years of the Tylosaurus discovery were plagued by controversies over its name and possible identity. 

However, the controversial nature of this prehistoric animal also boosted its popularity.

Earlier depictions were also inaccurate, and many species classified in the genus were invalid. 

Over the course of the 19th and 20th centuries, scientists gained an increased understanding of the Tylosaurus and mosasaurs in general. 

Reconstructed skeleton of T. saskatchewanensis | Will Beckett via Flickr (CC BY 2.0)

The discovery of more complete skeletons of this reptile has helped to clarify some misconceptions about its appearance. 

Generally, Tylosaurus and other 19th-century fossils played a significant role in the early development of the field of paleontology while also contributing to our understanding of prehistoric life. 

As an apex predator, the discovery of this genus has also helped with the reconstruction of the Late Cretaceous ecosystem where it lived. 

The abundance of well-preserved fossils and their gut content made it possible to accurately determine this reptile’s diet and its possible relationship with other animals in its ecosystem. 

Given its massive size and status as one of the largest predators of the Late Cretaceous Period, Tylosaurus is fairly popular with the general public. 

It frequently appears in documentaries about prehistoric times and is also commonly featured in museum exhibits. 

In 2014, a Tylosaurus fossil on display at the Canadian Fossil Discovery Centre won the Guinness World Record for the “largest mosasaur on display.”


Tylosaurus was a marine reptile that lived in North America during the Late Cretaceous Period. 

A relative of modern snakes and monitor lizards,  this marine reptile grew to become one of the top predators inhabiting the Western Interior Seaway. 

It was an active swimmer that gilded through the sea, preying on fish, seabirds, turtles, and other marine animals. 

Tylosaurus was an aggressive and ferocious predator that may have preyed on other reptiles, too, while scavenging on the remains of dinosaurs and other land animals washed up into the sea.

One of the deadliest hunters of the ancient seas eventually died off at the end of the Cretaceous Period, marking an end to an era of beastly sea creatures of its kind. 


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