Welcome to Hell’s Aquarium - A scene depicting fauna from the Coniacian-Campanian-aged Niobrara Formation/Smokey Hill Chalk of Kansas, from right to left: Xiphactinus audax, Enchodus petrosus, Clidastes propython, Cretoxyrhina mantelli, Hesperornis regalis, Pteranodon longiceps, Dolicorhynchops osborni, Elasmosaurus platyurus, Pentanogmius evolutus, Pachyrhizodus caninius, Squalicorax falcatus, and Tylosaurus proriger.

Over the past 20 million years since the Campanian, the Western Interior Seaway has receded, and by the Maastrichtian its saltwater inhabitants such as the Mosasaurs are now adapting to estuarine life and are invading the freshwater streams and rivers which dot the Hell Creek Formation…

Based on the discovery of Mosasaur teeth found at a Hell Creek Formation site in South Dakota last month…

Paleo-profiles: Tylosaurus

Tylosaurus, whose name means “knobbed lizard”, is a long-lived genus of several long-snouted mosasaurs which ruled the shallow late Cretaceous seas from the Cenomanian to the Maastrichtian stages of the Late Cretaceous, around 92-66 million years ago. The many species of Tylosaurus varied greatly in size, ranging from the small 7-meter-long Tylosaurus nepaeolicus to the giant 13-meter-long or school bus-sized Tylosaurus proriger (shown here), which had a skull length of 1.4-1.5 meters long and preyed upon the large fish, plesiosaurs, turtles and smaller mosasaurs of the Western Interior Seaway alongside the local species of Mosasaurus, and some of them have also been found in the tropical seas surrounding the European archipelago alongside Mosasaurus hoffmani. Moreover, some species of this mosasaur have also evolved unique adaptations for thriving in colder waters to the north of their cosmopolitan range. While the larger species of Tylosaurus were formidable apex predators which would have likely used their snouts to ram into the flanks of their large prey in order to stun them, the smaller species might have more often fed on smaller fish or turtles, and fossil evidence of bite marks found on the snouts of some skulls from this mosasaur have also shown that large individuals of Tylosaurus were highly aggressive or even cannibalistic towards smaller members of their own kind and could have engaged in infraspecific combat, possibly with adult males fighting each other over hunting grounds or mates. Until the release of the 2015 film Jurassic World cemented Mosasaurus to mainstream popularity, this genus was the archetypal mosasaur in educational or non-educational works featuring dinosaurs and other prehistoric life, appearing in a wide variety of media ranging from a book featuring Tarzan and pulp-fiction novels to documentaries such as Sea Monsters: A Walking with Dinosaurs Trilogy and Sea Monsters: A Prehistoric Adventure, albeit without the fork tail that it possessed in life, both of which feature Tylosaurus proriger swimming about in its Niobrara Formation environment alongside Hesperornis, Xiphactinus, an Elasmosaurid with Elasmosaurus in the former and Styxosaurus in the latter, a Protostegid turtle with Archelon in the former and Protostega in the latter, and a few small mosasaurs and sharks. One particular T.proriger specimen found from this area in 1917 by Sternberg and his sons also has the remains of a small plesiosaur preserved in its stomach.

Spectember 2025 Day 30: Eobasilosaurus giganteus

What would mosasaurs look like if they have evolved to become baleen whale-like filter-feeders?Already towards the last 20 million years of the Cretaceous, from the Turonian to Maastrichtian stages, these aquatic cousins of monitor lizards were reaching the peak of their evolutionary diversity following the extinctions of the icthyosaurs and giant pliosaurs. Some of them such as Globidens and Platecarpus were specialized ammonite or fish eaters, some such as Mosasaurus hoffmani and Tylosaurus proriger were giant apex predators, and others such as Megapterygius and Plotosaurus developed large flippers and even small dorsal fins and were highly specialized, fast-moving pelagic or open-ocean hunters. But had the Cretaceous-Palaeogene extinction event never occurred, the shapes and sizes achieved by the mosasaurs might have been even greater, and some of them could become the largest marine animals the world has ever known.

Eobasilosaurus giganteus is a species of gigantic tylosaurine mosasaur which inhabits the shallow tropical ocean waters of an alternate earth in which the asteroid impact never occurred and the non-avian dinosaurs never went extinct, as well as the seas which surround Sekaia’s island continent of Aetherosia. Having evolved from a Tylosaurus-like ancestor around 66-56 million years ago, Eobasilosaurus represents a peak in mosasaur evolution in that it is a filter feeding hunter of small to-medium-sized fish and marine reptiles which occupies a similar niche to the late Triassic icthyosaur Icthyotitan, and its long, unhinged jaws and small but sharp teeth support a wide system of baleen rakes that can be opened or shut to trap in hundreds of fish in a single gulp. Like all mosasaurs, Eobasilosaurus is oviparious, giving birth to about 4 to 6 offspring, and the young stay very close to their mother until they reach the age of 10 and wander off to forage on their own. The species is also highly territorial, with adult males patrolling vast swathes of open ocean hunting or mating grounds and fighting off intruders with their toothy jaws and slaps of their large flippers and tails. With a head and flipper length ranging from 4.5-5.5 meters and a total body length of 34-35 meters from the tip of the snout to the end of the tail, Eobasilosaurus giganteus is not only the largest marine reptile of the Cretaceous, but also the largest Mesozoic reptile ever to have existed.

Quick pencil sketch I’ve made of the giant yet criminally-underrated mosasaur species Prognathodon curri from the late Campanian of Israel

Tylosaurus proriger with company

The giant shark Cretoxyrhina devours a Clidastes 83 million years ago in what is now the Niobrara Chalk formation of Kansas

A huge Tylosaurus proroger attacks a Pteranodon longiceps from below the ocean’s surface 81 million years ago in the middle of the Western Interior Seaway, in what is now the Niobrara Formation of Kansas

Clidadtes Pair on the Hunt

A pair of Clidastes propython (Female at the bottom, male at the top) hunt for shoals of feeder fish in the depths of what is now the Niobrara Chalk Formation. “Clidastes” means “Locked Vertebrae” in Greek, and it was one of the first mosasaurs to show full adaptations for an ocean-going lifestyle. One of the smallest and most primitive of North American mosasaurs (Only the more primitive Dallasaurus is even smaller) the 2-4 meter long C.propython can be distinguished from other mosasaurs for its elongated torso. Clidastes was also prey for larger, fiercer predators such as Cretoxyrhina, Mosasaurus, large species of Squalicorax, Tylosaurus, and Xiphactinus.

Dakotan Globe Tooth

Attack of the Meuse Lizard

In a warm, shallow tropical sea representing what is now the Maastricht Formation in the Low Countries, a huge Mosasaurus hoffmani emerges to catch one of a few mackerel sharks attracted to the scent of a smaller mosasaur that has been injured hundreds of meters away. Parapuzosia ammonites drift to the left, along with a pair of Pachyrhizodus sp. and Pycnodontids at the bottom. An Allopleurodon sea turtle also appears in the far distance at the lower right corner.