Mosasaurus and Prehistoric Sea Creatures

The Mesozoic oceans were home to three distinct groups of large marine reptiles that evolved independently from land-dwelling ancestors. The first group, ichthyosaurs, appeared around 250 million years ago and became so perfectly adapted to ocean life that they evolved a dolphin-shaped body, giving birth to live young underwater. The largest ichthyosaur, Shonisaurus, reached 70 feet long and is one of the biggest animals ever to have lived.

Plesiosaurs and pliosaurs formed a second group that survived until the asteroid impact 66 million years ago. Long-necked plesiosaurs like Elasmosaurus and Styxosaurus used their flexible necks to dart at fish from unpredictable angles. Short-necked pliosaurs like Kronosaurus and Liopleurodon were built like enormous crocodiles with four powerful flippers, capable of crushing mosasaurs and other large prey in a single bite.

Mosasaurs were the last group to evolve, arriving in the Late Cretaceous around 98 million years ago after ichthyosaurs had already disappeared. They were relatives of monitor lizards and rapidly diversified into dozens of species. The largest, Mosasaurus hoffmannii, could reach 57 feet and had a second set of teeth on the roof of its mouth to help swallow prey whole. By the end of the Cretaceous, mosasaurs had spread to every ocean on Earth.

Elasmosaurus is probably the most recognizable plesiosaur because of its impossibly long neck. When paleontologist Edward Cope first assembled its skeleton in 1869, he put the head on the tail end by mistake, not realizing how long the neck actually was. When rival paleontologist Othniel Marsh pointed out the error, it sparked one of the most famous scientific feuds in history, called the Bone Wars, which shaped American paleontology for decades.

Mosasaurus was discovered in 1764 in a limestone quarry near Maastricht in the Netherlands, making it one of the first large prehistoric reptiles ever found by science. For years nobody could figure out what it was, with scientists debating whether it was a crocodile, a whale, or something entirely new. Georges Cuvier finally identified it as a giant lizard in 1808, which helped establish the scientific idea that species could go extinct entirely, something many scientists had previously rejected.

Liopleurodon became famous worldwide after a BBC Walking with Dinosaurs documentary in 1999 depicted it as a 150-foot monster. In reality, Liopleurodon probably reached about 21 to 33 feet, still making it one of the largest predators of the Jurassic seas. It had a skull over 5 feet long with enormous nostrils that could detect prey from hundreds of feet away by detecting chemical gradients in the water.

Marine reptile fossils are found on every continent, including Antarctica, because the Mesozoic seas were global. Some of the richest sites for plesiosaur fossils are in the chalk deposits of Kansas and South Dakota, which were once the floor of the Western Interior Seaway, a shallow inland sea that split North America in two. The Pierre Shale formation has produced dozens of spectacular plesiosaur skeletons, including the first Styxosaurus specimens ever found.

Ichthyosaur fossils are particularly common in the Holzmaden Shale of Germany, where the fine-grained sediment preserved not only bones but also the outlines of soft tissue, showing scientists that ichthyosaurs had dorsal fins and tail flukes just like dolphins. The Aust Cliff in England has produced important early ichthyosaur specimens, and the Nevada desert, once covered by the Panthalassic Ocean, has yielded gigantic shonisaur ichthyosaurs nearly 70 feet long.

Mosasaur fossils are found worldwide because these animals spread across every ocean before the mass extinction. The Phosphatic Chalk of Morocco is especially rich in mosasaur specimens, and new species are still being discovered there regularly. In 2022, paleontologists in Morocco described a new mosasaur species with distinctive paddle-like fins, providing new evidence about how mosasaur limbs evolved for ocean life. Belgium, the Netherlands, and the American Great Plains chalk formations also rank among the most productive mosasaur sites on Earth.