March 7th

Sea Monsters

Yesterday we began an exploration of non-dinosaurian Mesozoic marauders. While the pterosaurs were soaring through the prehistoric skies, a great diversity of marine reptiles were swimming through the ancient oceans. These real-life sea monsters came in a bewildering number of shapes and sizes. We’ll provide just a glimpse into the main types of aquatic reptiles that dominated the waters of the world while dinosaurs ruled the Earth’s terrestrial ecosystems.


Three decades before dinosaurs were identified as a distinct group of animals, the ‘fish lizards’ were being uncovered from Lyme Regis in England by Mary Anning. The site became known as the Jurassic Coast for the spectacular fossils discovered there.

Senckenberg Museum

Ichthyosaurs resemble modern dolphins, but ichthyosaurs are reptiles, not mammals, and beat their tails from side to side instead of up and down. The two groups do share similar body plans. They both are descended from land-dwelling ancestors who made their way back into the oceans. This phenomenon is known as convergent evolution–where two different groups of animals adopt similar body plans and lifestyles because of similar ecological demands.

The ichthyosaurs survived for nearly all of the Mesozoic–some diversifying into giants, others becoming specialized for deep sea swimming with large paddles and oversized eyes for low-light visibility. The reign of the fish lizards lasted from nearly 250 million years ago until around 90 million years ago, when their ecological niche was filled by another sea monster family.



During the last 20 million years of the Cretaceous, mosasaurs rose to prominence. They perished at the same time as the dinosaurs, some 66 million years ago, but they were formidable hunters and, like their ichthyosaur predecessors, diversified into many niches.

Mental Floss

Until Jurassic World, many people were unfamiliar with mosasaurs, but the movie created an instant visual touchpoint for these monitor lizard-like titans. Some grew to 50 feet long.



Some 200 million years ago, long-necked, four-paddled sea monsters began cruising the world beneath the waves.


They persisted until the end-Cretaceous mass extinction event, spreading into a variety of forms–the smallest around 10 feet long and the largest in excess of 45 feet. ‘Nessie’ from Loch Ness would be a Plesiosaurus.



Pliosaurs are very close cousins of the plesiosaurs, having shorter next and more robust skulls–some craniums were up to 10 feet long. They grew enormous–to lengths of 50 feet.


They were fearsome in oceans filled with frightening predators. From 250 million years ago right until the end of the Cretaceous and the demise of the dinosaurs, pliosaurs stuck to their predatory habits.


The reign of the sea monsters is over but they live on in our imaginations. There are many more weird and wonderful oceanic creatures, now extinct, that we don’t have time to feature here, so share photos of your favorite prehistoric marine reptiles with us on Facebook and Twitter using the hashtag #TDIDinos.