What Lived in the Seas During the Age of Dinosaurs?
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What lived in the seas during the age of dinosaurs was every bit as impressive as what walked on land, and almost none of it was a dinosaur. The Mesozoic oceans belonged to marine reptiles: air breathing animals descended from land ancestors that returned to the water and became superbly adapted to it.
Reptiles, but not 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
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
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
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.
What else was down there
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
Mary Anning and the first discoveries
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 takeaway
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.