At 13.2m from snout to tail, Tylosaurus rex is one of, if not the largest mosasaur currently known to science. This ancient marine reptile terrorised the seas during the Late Cretaceous (90 to 80 million years ago), just a few million years before its equally ferocious, two-legged namesake did the same thing on land.
What is Tylosaurus rex?
Not to be confused with the iconic theropod dinosaur Tyrannosaurus rex, Tylosaurus rex is a marine reptile that belongs to a large-bodied group known as mosasaurs.
These aquatic predators belong to the same order as lizards and snakes (Squamata), though they’re about as different from these scaly, largely terrestrial pipsqueaks as we are from whales.
Mosasaurs were the most dominant marine predators of the Late Cretaceous (94 to 66 million years ago). They appeared at the same time ichthyosaurs (another group of ferocious marine reptiles that lived during the ‘Age of the Dinosaurs’) became extinct, and shortly before similar-looking marine reptiles known as pliosaurs also disappeared.
It’s thought mosasaurs such as Tylosaurus rex evolved to fill the vacant niches left by ichthyosaurs and pliosaurs - with no other large-bodied marine reptiles to compete with, they prospered and spawned many different lineages.
There are four main groups of mosasaurs. Despite the shared ancestry of these groups and the fact that they can all trace their roots back to the same common ancestor, it’s thought they evolved large size, shark-shaped body plans, and flippers independently.
How was Tylosaurus rex discovered?
While Tylosaurus rex was described quite recently - in May 2026 - the fossils used to inform its official description were actually gathered over several decades from sites in northern Texas, US.
These fossils include ‘Bunker’, a massive, exceptionally well-preserved specimen discovered way back in 1911 and now housed at the University of Kansas, ‘Sophie’, a smaller specimen currently on display at the Yale Peabody Museum, and ‘The Black Knight’, another giant specimen on display at the Perot Museum.
These specimens were originally identified as another species of Tylosaurus known as Tylosaurus proriger. However, a recent study re-examined these fossils and concluded they were substantially larger than other known remains of Tylosaurus proriger, warranting the designation of a new species.
The name, Tylosaurus rex, means ‘King of the Knob Lizards’ and was chosen on the basis that specimens such as ‘Bunker’ are amongst the largest of their kind ever found.
Tylo, or ‘knob’, refers to the distinct, elongated protrusion of bone on the end of its snout, which in Tylosaurus rex contributed to a skull that measured up to 1.63m in length - roughly the height of an average adult woman.
What did Tylosaurus rex look like?
Tylosaurus rex looked like a strange cross between a shark, a whale, and a lizard.
However, unlike sharks and whales, Tylosaurus rex (and most other species of mosasaurs) lacked a dorsal fin. It also had four huge flippers, as opposed to a whale’s two and a shark’s relatively narrow pair of pectoral fins.
While most mosasaurs were between three to nine metres long - comparable in size to modern large sharks - Tylosaurus rex measured 13.2m from snout to tail. This puts it in a size bracket occupied today by right whales, sperm whales, and humpback whales.
At this size, Tylosaurus rex was likely the largest predator of its time, larger than Ptychodus, a 10m-long shark that used rounded teeth to crush shell-dwelling animals, and Xiphanctus, a 6m-long predatory fish with fangs.
When did Tylosaurus rex live?
Based on the age of the fossils Tylosaurus rex has been identified from, it’s thought to have lived between 90 and 80 million years ago, which is roughly 11 million years before its terrestrial namesake appeared.
This makes it one of the earliest known members of the tylosaurine group, though not the very first. More ‘primitive’ tylosaurines, such as Tylosaurus nepaeolicus and Tylosaurus kansasensis, are known, but even these species already display many of the features seen in later members of the group. As a result, the evolutionary origins of tylosaurines is shrouded in mystery.
So far, palaeontologists haven’t found fossils of early tylosaurines that clearly document how the group evolved its distinctive adaptations, such as the bony ‘knob’ at the end of their snouts.
Where did Tylosaurus rex live?
Most fossils belonging to Tylosaurus rex have been unearthed in northern Texas, which during the Late Cretaceous lay near the southwestern edge of the Western Interior Seaway - a massive, relatively shallow inland sea that split North America in half for more than 30 million years.
The Western Interior Seaway was home to an impressive diversity of giants, including other mosasaurs such as Prognathodon and Mosasaurus - the genus from which the entire group takes its name.
These waters were also inhabited by Archelon, a car-sized turtle that had a crude hook for a beak, as well as several species of giant shark. Along its coastal margins lurked Deinosuchus, a 12m-long crocodile that’s thought to have preyed on anything that ventured too close to the water’s edge, including dinosaurs.
What did Tylosaurus rex eat?
Thanks to the preserved stomach contents of several Tylosaurus specimens, we know these giant marine reptiles were apex predators that dined on a variety of different prey, from fish, sharks, and turtles, to other mosasaurs. The remains of seabirds have also been found in the fossilised guts of Tylosaurus, suggesting they’d sometimes hunt at the water’s surface.
A particularly well-preserved specimen of Tylosaurus proriger recovered from the Pierre Shale of South Dakota was found to have the remains of an unidentified, 2m-long mosasaur, a Hesperornis diving bird, a Bananogmius fish, and a shark all within its stomach. While not a Tylosaurus rex, it’s likely the two species ate similar prey - and had similarly large appetites.
Tylosaurus rex may have also been a cannibal, eating other tylosaurines whenever opportunities presented themselves. It’s clear, based on fossil evidence, Tylosaurus rex at least fought others of its own kind. The specimen known as ‘The Black Knight’ is missing part of its snout and has a broken lower jaw, injuries researchers say are consistent with an attack from another Tylosaurus rex.
Is Tylosaurus rex the biggest mosasaur ever?
Until the recent description of Tylosaurus rex, Mosasaurus hoffmannii reigned as the undisputed king of the mosasaurs, in terms of size.
However, size estimates of Mosasaurus hoffmannii vary, ranging from 12 to 17 metres in length, and are obscured by a poorer fossil record. Tylosaurus rex, in contrast, is known from several almost complete specimens and has therefore been able to be precisely measured, returning lengths of 13.2m.
Based on these measurements of Tylosaurus rex and size estimates of Mosasaurus hoffmannii, it’s tough to say which was bigger - it’s likely they were roughly the same size.
While a lot of Mosasaurus hoffmannii remains have been found across Europe, some remains have also been found in the US, suggesting they may have lived alongside Tylosaurus rex.
It’s thought these two giants may have been able to live in the same areas thanks to a process known as niche partitioning - where competing species use the environment differently in a way that allows them to coexist. That said, meetings were likely vicious and there’s at least one instance in the fossil record of a Tylosaurus attacking a Mosasaurus.
Why did Tylosaurus rex become extinct?
Tylosaurus rex likely disappeared 80 million years ago, making way for more derived tylosaurines. It’s not clear exactly why this happened, but small changes in climate or increased competition with other mosasaurs may have been enough to drive Tylosaurus rex into irreversible decline.
As a genus, however, Tylosaurus prospered until 66 million years ago, ultimately facing extinction at the same time as all other known mosasaurs, as well as several other groups of marine reptiles. These ancient predators were wiped out by the same asteroid that also claimed the lives of the non-avian dinosaurs.
The so-called ‘K/Pg Mass Extinction’ was arguably worse in the ocean than it was on land. As soot and dust from the impact blocked out the sun, temperatures plummeted, severely disrupting the oceanic food chain and leading to the collapse of food webs that Tylosaurus and other large-bodied marine reptiles relied on.
Around 93% of marine reptiles were wiped out, while the ammonites - amongst the most iconic and abundant prey of these ancient predators - were driven to total extinction, disappearing from the ocean forever.
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Top image: Courtesy of Alderon Games - Path of Titans
