Were There Dinosaurs in the Ocean?

Mesozoic oceans were full of large reptiles, but ichthyosaurs, plesiosaurs and mosasaurs were not dinosaurs. Non-avian dinosaurs arose as a land-living branch of archosaurs. Some fed in water and were capable swimmers, yet no non-avian dinosaur is known to have made the complete transition to life in the open sea seen in an ichthyosaur.

“Marine dinosaur” can be a useful casual phrase if the animal’s actual relationships are made clear. Appearance and geological age do not define a group: a dolphin is not a fish because it has flippers, and a mosasaur is not a dinosaur because it lived during the Cretaceous.

What scientists mean by dinosaur

Dinosauria is an archosaur branch defined by common ancestry and a suite of skeletal features. It includes theropods, sauropodomorphs and ornithischians. Birds are nested within theropods, so marine dinosaurs do exist in the strict evolutionary sense today: penguins spend much of their lives at sea and use their wings as flippers.

Ichthyosaurs were a separate group of marine reptiles with streamlined bodies. Plesiosaurs belonged to Sauropterygia and swam with four flippers. Mosasaurs were marine squamates, closer to the lizard-and-snake branch than to dinosaurs. The marine reptile catalogue keeps these distinct lineages together by habitat while preserving their different ancestry.

Ichthyosaurs: streamlined reptiles

Early ichthyosauromorphs appeared soon after the Permian mass extinction. Over a relatively short span of geological time, they evolved flippers, a shortened trunk and a powerful tail. In later forms, the upper lobe of the tail fin lacked a long bony support. Its outline is reconstructed from rare impressions of soft tissue, so not every detail is equally direct.

Ichthyosaurs did not need to return to land to lay eggs. Fossils of pregnant females and embryos provide evidence for live birth. A newborn commonly emerged tail-first, reducing the chance of inhaling water before it was fully delivered. This is strong evidence of a fully marine life history, for which there is no equivalent in non-avian dinosaurs.

Plesiosaurs and four underwater wings

Both pairs of plesiosaur limbs became large flippers. Hydrodynamic models indicate that they generated lift like underwater wings. The front pair may have supplied most of the thrust, while the rear pair helped with manoeuvring and added force during a coordinated stroke.

Body proportions varied widely. Long-necked forms carried a small head at the end of a chain of neck vertebrae. Pliosaurs had shorter necks and enormous, heavier skulls. The name plesiosaur is not a label for every long-necked marine reptile, just as “dinosaur” does not mean every Mesozoic animal.

Plesiosaurs also gave birth to live young. One famous specimen preserves a large embryo inside the body, suggesting a small number of relatively developed offspring. The idea that these animals hauled themselves onto beaches using four inflexible flippers is not supported by the limb anatomy.

Mosasaurs: later rulers of Cretaceous seas

Mosasaurs appeared much later than ichthyosaurs and plesiosaurs. Their long bodies were driven by a tail with a vertical fluke. Soft-tissue outlines and the bend in the last tail vertebrae support this reconstruction. The limbs became flippers but retained the internal arrangement of a hand and foot.

Large mosasaurs had powerful jaws and an extra row of teeth on the roof of the mouth. Stomach contents, bite marks and damaged bones document prey that included fish, ammonites, marine turtles, birds and other reptiles. This does not make every mosasaur a generalist super-predator: tooth shape and body size differed among genera.

Marine crocodylomorphs and turtles

Several crocodylomorph lineages entered marine environments independently. Jurassic metriorhynchids lost the heavy dorsal armour of many relatives, evolved flipper-like limbs and developed a tail fin. Their ancestry was on the crocodile side of Archosauria, not among dinosaurs, despite their strikingly marine appearance. The ancient crocodylomorph catalogue includes land-running and marine members of this wider group.

Marine turtles also appeared in the Cretaceous. Large forms such as Archelon had enormous front flippers and a lightened shell. Their relationship to living turtles is much closer than to any dinosaur branch.

Was Spinosaurus a marine dinosaur?

Spinosaurus was a genuine theropod with several adaptations associated with water. These include dense bones, long jaws, shortened hind limbs and a tall, flexible tail. A model of the tail produced more thrust in water than the tails of land-living theropods.

Swimming ability, however, is not the same as permanent life in the open ocean. Spinosaurus remains come from river and delta deposits in North Africa. Researchers debate how deeply it could dive, how stable its body was in water and how effectively it could pursue prey underwater. Hydrodynamic models reach different results depending on how unpreserved soft tissues are reconstructed.

The cautious description is a large, semiaquatic dinosaur associated with rivers and coastlines. There is no evidence that it gave birth in water, migrated across oceans or had fully flipper-like limbs.

How marine reptiles breathed

None of these groups had gills. Ichthyosaurs, plesiosaurs, mosasaurs and marine crocodylomorphs breathed air and had to surface. Ribcage and nostril anatomy helps researchers assess respiration, but the intervals between breaths are unknown for most extinct forms.

Live birth allowed fully marine species to avoid returning to land. A newborn still had to begin breathing immediately. The separate guide to how marine reptiles breathed explains this physiological constraint in more detail.

Why marine reptiles reached enormous sizes

Water supports the body and removes some of the constraints that a land-dwelling giant places on its limbs. Large size can also help retain heat and travel efficiently over long distances. But a larger body needs more food and still encounters water resistance.

The longest known marine reptiles were ichthyosaurs. The record is based on scattered jaw bones compared with better-preserved relatives. Estimates therefore have ranges rather than a precise total length. A profile of the largest marine reptiles compares Ichthyotitan, Shonisaurus, mosasaurs and pliosaurs without treating them as dinosaurs.