What Is a Mosasaur? Lizards That Took Over the Sea and Then Lost It

Mosasaurs belong to the squamates, meaning the group containing lizards and snakes, and their closest living relatives are generally reckoned to be monitor lizards, with some analyses placing snakes nearby. That makes them entirely separate from ichthyosaurs and plesiosaurs, which were already present or had already declined. The body is elongated with a long deep tail, and the limbs are modified into paddles, with the digits elongated and enclosed in flesh. Analysis of specimens preserving soft tissue outlines established that the tail carried a downward-pointing fin lobe, comparable to a shark's in function though inverted in shape, which revised earlier reconstructions showing a simple tapering tail. Skin impressions show small overlapping scales with a keeled surface, which would have reduced drag. Their jaws carry a second row of teeth on the palate, which is a squamate feature, and a flexible joint within the lower jaw allowing the mouth to widen, which is the same arrangement snakes use to swallow large prey.

The group is well sampled and several findings are unusually direct:

• •Rapid growth to large size, with the largest species exceeding ten metres and reaching adult size quickly according to bone histology
• •Live birth, supported by specimens containing embryos and by the absence of any adaptation for coming ashore
• •A range of feeding specialisations, with some species having crushing teeth for shelled prey and others sharp recurved teeth for fish and cephalopods
• •Stomach contents including fish, ammonites, birds and other mosasaurs, which establishes both diet and cannibalism
• •Bite marks on ammonite shells attributed to mosasaurs, which have been debated since some marks proposed as bites were later reinterpreted as limpet damage
• •Global distribution including polar regions, which indicates tolerance of a wide temperature range and possibly some internal heat regulation

The first recognised specimen was found in a chalk quarry near Maastricht in the 1760s and became the subject of a dispute that carried it into the history of ideas. Georges Cuvier examined it around 1800 and concluded that it belonged to a giant marine lizard unlike anything alive, which was among the arguments that established extinction as a fact rather than a possibility, at a time when the idea that species could disappear entirely was resisted. The specimen itself was seized by French forces during the revolutionary wars and taken to Paris, where it remains, which is an early instance of a dispute over scientific material removed during conflict. The animal was named after the river Meuse, and the Maastrichtian, the final stage of the Cretaceous, is named after the same town, so the group is tied to the period whose end destroyed it by an accident of where the first specimen turned up.

The group appears in the fossil record around ninety million years ago and diversifies rapidly, which coincides closely with the disappearance of ichthyosaurs and the decline of several plesiosaur lineages, and the relationship between those events has been argued about. Whether mosasaurs displaced the earlier groups competitively or simply expanded into space already vacated by environmental change is not settled, and the timing evidence has been read both ways. What is clear is that they became the dominant marine predators for the final twenty million years of the Cretaceous and that they disappeared completely at the boundary. Their extinction alongside the non-avian dinosaurs, the ammonites and much else is consistent with a rapid catastrophic cause, and the marine ecosystem that followed was reassembled with sharks and later with marine mammals occupying the roles. Nothing in the squamate lineage returned to the sea at that scale, and modern marine lizards are limited to one small iguana.

These were lizards closest to monitors, unrelated to the other marine reptiles, with paddle limbs, a downward tail fin established from soft tissue, palatal teeth and a flexible jaw joint inherited from their squamate ancestry. They gave birth at sea and ate fish, ammonites, birds and each other. The first specimen helped Cuvier establish that extinction had occurred, and was taken to Paris as spoils of war.