What Lived in the Seas During the Age of Dinosaurs?

They were not. Ichthyosaurs, plesiosaurs and mosasaurs were all reptiles, but each belonged to a separate group outside the dinosaurs. Dinosaurs are defined by particular hip and limb features that these swimming animals do not share.

All of them breathed air, which means they had to surface regularly. That single constraint shaped everything about how they lived, hunted and reproduced, and it is what makes them comparable to modern whales and dolphins rather than to fish.

Ichthyosaurs looked remarkably like dolphins, with streamlined bodies, a dorsal fin and a crescent shaped tail. This is a classic case of convergent evolution, where unrelated animals arrive at the same solution to the same problem.

Some had extremely large eyes, among the biggest of any known animal, suggesting they hunted deep or at night. Fossils have been found containing embryos, showing they gave birth to live young rather than laying eggs on land, and at least one specimen preserves a birth in progress.

Plesiosaurs are the long necked animals often used in lake monster illustrations. They had four large flippers and swam by flapping them, more like a sea turtle than a fish.

The group split into two broad body plans. Some had very long necks and small heads, suited to snapping at fish from a distance. Others, the pliosaurs, had short necks and enormous heads, and were among the most powerful predators in the ocean.

Mosasaurs appeared late, in the Cretaceous, and quickly became the dominant marine predators. They were closely related to modern monitor lizards and snakes rather than to the earlier marine reptiles.

They swam with a powerful side to side motion of the tail and reached lengths of over ten metres in the largest species. Like the dinosaurs on land, they disappeared in the extinction event at the end of the Cretaceous.

The Mesozoic seas were full of life beyond the big reptiles:

• •Ammonites, coiled shelled relatives of squid, found everywhere as fossils
• •Belemnites, squid like animals with an internal bullet shaped guard
• •Sharks, already ancient and already successful
• •Early bony fish and vast numbers of plankton

Many of these animals were first brought to scientific attention by Mary Anning, a working class fossil collector in Lyme Regis on the south coast of England, in the early nineteenth century.

She found the first correctly identified ichthyosaur skeleton, the first complete plesiosaur and an early pterosaur, and her observations informed the leading geologists of the day. As a woman without means she could not join the scientific societies that discussed her finds, and her specimens were frequently published by others. Her contribution has been substantially reassessed in recent decades.

The big three marine reptile groups were not alone. Sea turtles were already present in the Cretaceous, and one of them, Archelon, reached around four metres in length, far larger than any living turtle. In the Jurassic, relatives of crocodiles called metriorhynchids took to the open sea. They lost the heavy bony armour of their land relatives, developed paddle-like limbs and grew a tail fin, becoming streamlined hunters rather than ambush predators lurking at the water's edge.

There is one twist in the claim that nothing in the sea was a dinosaur. Birds are living dinosaurs, descended from small feathered theropods, and in the Late Cretaceous a flightless bird called Hesperornis dived for fish in shallow seas covering parts of North America. It had tiny wings, powerful legs set far back on its body for swimming, and small teeth in its jaws. So while the great marine reptiles were not dinosaurs, a few dinosaurs in the form of seabirds did make a living in the Mesozoic oceans.

Fossils can preserve meals. Some ichthyosaur skeletons contain masses of tiny hooks from the arms of squid-like animals, the hard parts that survived digestion, along with fish scales. A few plesiosaur and mosasaur specimens hold the bones of fish, turtles or smaller reptiles in the region of the stomach. Many plesiosaurs also contain clusters of polished stones. Whether these gastroliths helped grind food or acted as ballast for diving is still argued over, and some researchers have suggested the amount found is too small to make much difference to buoyancy.

Fossil droppings tell a similar story. In the 1820s the geologist William Buckland recognised that curious stones found among ichthyosaur bones on the Dorset coast were fossilised faeces, and he named them coprolites. Mary Anning had noticed such stones inside skeletons too. Cut open, they contain fish scales and bones, giving a direct record of diet. More recent chemical studies of the oxygen in fossil teeth suggest that ichthyosaurs and plesiosaurs kept body temperatures well above that of the surrounding sea, which fits their life as fast, active hunters in open water.

For much of the Late Cretaceous, sea levels were far higher than they are today, and shallow seas spread across the continents. One of them, the Western Interior Seaway, ran from the Arctic to the Gulf of Mexico and split North America into two separate landmasses. Its warm waters were home to mosasaurs, plesiosaurs, giant fish and sea turtles, and rocks laid down on its floor, especially the chalk beds of Kansas, have produced some of the richest collections of marine reptile fossils anywhere in the world.

Chalk is itself a product of these seas. It is made mostly of coccoliths, microscopic plates of calcium carbonate shed by single-celled algae, which settled on the seabed in vast numbers. The period is named after it, since Cretaceous comes from creta, the Latin word for chalk, and the White Cliffs of Dover are built from the same material. In the eighteenth century, quarrymen working soft limestone near Maastricht in the Netherlands found a huge toothed skull. It was later named Mosasaurus, the lizard of the Meuse, and the last stage of the Cretaceous, the Maastrichtian, takes its name from the city.

Plesiosaurs have been reconstructed wrongly more than once. In 1868 the American palaeontologist Edward Drinker Cope described Elasmosaurus, a plesiosaur with an extraordinarily long neck of more than seventy vertebrae, and in his first reconstruction he put the skull on the end of the tail. Joseph Leidy pointed out the mistake, and Cope tried to buy up the copies of his publication. The embarrassment is often said to have helped sour his relations with his rival Othniel Charles Marsh, feeding the bitter competition later called the Bone Wars.

A more common error still appears in pictures today: the plesiosaur holding its head high out of the water like a swan. Studies of how the neck vertebrae fit together suggest the neck was fairly stiff, bending more easily from side to side than upwards, and lifting a long, heavy neck out of the water would have been extremely difficult. Fossils have also settled how they reproduced. A specimen of Polycotylus from Kansas, described in 2011, contains a single large unborn young, showing that plesiosaurs gave birth to live offspring, probably one large baby at a time, rather than crawling ashore to lay eggs.

The seas of the dinosaur age were ruled by ichthyosaurs, plesiosaurs and mosasaurs, air breathing marine reptiles that were not dinosaurs, alongside ammonites and sharks in a rich and completely alien ocean.