The word 'Dinosaur' refers to animals that lived on and roamed the land so, technically speaking, no dinosaurs actually lived in the sea. But the oceans were still inhabited by enormous beasts.

 

Archelon lived about 75 - 65 million years ago. It was a slow mover and found most of its food drifting near the sea surface. It had little need to dive deep except when hibernating on the seabed. It was an omnivorous grazer, sweeping up drifting fish, jellyfish and dead carrion as well as plants. Its sharp, powerful beak could break open shelled animals such as ammonites. Like modern turtles, it laid eggs by burying them in sandy beaches under cover of darkness. Its nearest living relative is the world's largest turtle, the leatherback.

 

Basilosaurus was the first whale to become truly gigantic in size. Their large bulk meant they needed a great deal of food to keep them going and it is thought that most Basilosaurus spent their days swimming slowly through the shallows, watching for potential prey. Basilosaurus was not a particularly fussy eater. Fish, sharks, squid, turtles and other marine mammals were all a staple part of its diet and Basilosaurus was well-equipped to find and tackle prey with its keen eyesight and hearing.

 

Cymbospondylus was an early member of the Icthyosaur group, which looked slightly like modern dolphins. Cymbospondylus had no dorsal fin and its tail was long like an eel's.

Its long tail made it a powerful swimmer, it patrolled in deep offshore waters looking for prey.

Cymbospondylus had a skull 1m long with short, sharp teeth good for grabbing quite large reptiles but it favoured fish and cephalopods such as ammonites.

Cymbospondylus appears to have given birth to live young as it had no way to lay eggs.

 

 

 

Archelon	Basilosaurus	Cymbospondylus

 

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Jurassic seas were home to voracious sharks such as Hybodus and the nasty crocodilian Metriorhynchus but they would not have stood a chance against the fearsome Liopleurodon: the largest predator of all time.

This enormous carnivore belonged to a family of marine reptiles called pliosaurs, and we know from its nostrils that it had an extremely good sense of smell. This undoubtedly made it an extremely efficient predator.

But while a dip in a Jurassic sea should cast fear into the hearts of even the bravest soul, the succeeding period in Earth history would see the appearance of even more terrifying sea creatures.

 

The Cretaceous ocean ranks as the most dangerous sea of all time due to the sheer number and ferocity of its marine predators.

You just have to look at Hesperornis. This bird spent much of its time on rocky ledges above the water. But it was frequently picked off by small mosasaurs like Halisaurus, who waited in the shallow caves beneath these ledges for a Hesperornis to dive in. When it did, Halisaurus would grab the bird in its short, sharp teeth.

But these mosasaurs were dwarfed by their huge relatives, the Giant Mosasaurs. These gargantuan fiends reached up to 17 metres in length and could take on pretty much everything in the sea.