At 13.2m from snout to tail,\u00a0Tylosaurus rex\u00a0is one of, if not\u00a0the\u00a0largest 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.<\/p>\n
What is\u00a0Tylosaurus rex?<\/p>\n
Not to be confused with the iconic theropod dinosaur\u00a0Tyrannosaurus rex<\/a>,\u00a0Tylosaurus rex\u00a0is a marine reptile that belongs to a large-bodied group known as mosasaurs.\u00a0<\/p>\n These aquatic predators belong to the same order as lizards and snakes (Squamata), though they\u2019re about as different from these scaly, largely terrestrial pipsqueaks as we are from whales.<\/p>\n 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 \u2018Age of the Dinosaurs\u2019) became extinct, and shortly before similar-looking marine reptiles known as pliosaurs also disappeared.<\/p>\n It\u2019s thought mosasaurs such as\u00a0Tylosaurus rex\u00a0evolved 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.<\/p>\n 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\u2019s thought they evolved large size, shark-shaped body plans, and flippers independently.\u00a0<\/p>\n How was\u00a0Tylosaurus rex\u00a0discovered?<\/p>\n While\u00a0Tylosaurus rex\u00a0was described quite recently –\u00a0\u00a0in May 2026 – the fossils used to inform its official description were actually gathered over several decades from sites in northern Texas, US.<\/p>\n These fossils include \u2018Bunker\u2019, a massive, exceptionally well-preserved specimen discovered way back in 1911 and now housed at the University of Kansas, \u2018Sophie\u2019, a smaller specimen currently on display at the Yale Peabody Museum, and \u2018The Black Knight\u2019, another giant specimen on display at the Perot Museum.<\/p>\n These specimens were originally identified as another species of\u00a0Tylosaurus\u00a0known as\u00a0Tylosaurus proriger. However, a recent study re-examined these fossils and concluded they were substantially larger than other known remains of\u00a0Tylosaurus proriger, warranting the designation of a new species.<\/p>\n The name,\u00a0Tylosaurus rex, means \u2018King of the Knob Lizards\u2019 and was chosen on the basis that specimens such as \u2018Bunker\u2019 are amongst the largest of their kind ever found.<\/p>\n Tylo, or \u2018knob\u2019, refers to the distinct, elongated protrusion of bone on the end of its snout, which in\u00a0Tylosaurus rex\u00a0contributed to a skull that measured up to 1.63m in length – roughly the height of an average adult woman.<\/p>\n What did\u00a0Tylosaurus rex\u00a0look like?<\/p>\n Tylosaurus rex\u00a0looked like a strange cross between a shark, a whale, and a lizard.\u00a0<\/p>\n However, unlike sharks and whales,\u00a0Tylosaurus rex\u00a0(and most other species of mosasaurs) lacked a dorsal fin. It also had four huge flippers, as opposed to a whale\u2019s two and a shark\u2019s relatively narrow pair of pectoral fins.<\/p>\n While most mosasaurs were between three to nine metres long – comparable in size to modern large sharks –\u00a0Tylosaurus rex\u00a0measured 13.2m from snout to tail. This puts it in a size bracket occupied today by\u00a0right whales<\/a>,\u00a0sperm whales<\/a>, and\u00a0humpback whales<\/a>.<\/p>\n At this size,\u00a0Tylosaurus rex\u00a0was likely the\u00a0largest predator of its time<\/a>, larger than\u00a0Ptychodus, a 10m-long shark that used rounded teeth to crush shell-dwelling animals, and\u00a0Xiphanctus, a 6m-long predatory fish with fangs.<\/p>\n When did\u00a0Tylosaurus rex\u00a0live?<\/p>\n Based on the age of the fossils\u00a0Tylosaurus rex\u00a0has been identified from, it\u2019s thought to have lived between 90 and 80 million years ago, which is roughly 11 million years before its terrestrial namesake appeared.<\/p>\n This makes it one of the earliest known members of the tylosaurine group, though not the very first. More \u2018primitive\u2019 tylosaurines, such as\u00a0Tylosaurus nepaeolicus\u00a0and\u00a0Tylosaurus 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.<\/p>\n So far, palaeontologists haven\u2019t found fossils of early tylosaurines that clearly document how the group evolved its distinctive adaptations, such as the bony \u2018knob\u2019 at the end of their snouts.<\/p>\n Where did\u00a0Tylosaurus rex\u00a0live?<\/p>\n Most fossils belonging to\u00a0Tylosaurus rex\u00a0have 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.<\/p>\n The Western Interior Seaway was home to an impressive diversity of giants, including other mosasaurs such as\u00a0Prognathodon\u00a0and\u00a0Mosasaurus\u00a0– the genus from which the entire group takes its name.\u00a0<\/p>\n