Mesozoic marine reptiles

Jules Verne

In 1867, Jules Verne published his famous novel Journey to the Center of the Earth, whose protagonists discover an underground ocean at great depth, which they baptize as the Liddenbrock Sea, and witness the fight to the death of two great marine animals: a plesiosaur and an ichthyosaur, supposed descendants of ancestors that sank with the Liddenbrock Sea a hundred million years before and survived down there, protected by the Earth's crust, while their surface congeners became extinct as a result of the impact of the meteorite or comet that put an end to most dinosaurs, although that was not known in Verne’s time. But what were those plesiosaurs and those ichthyosaurs that populated the seas during the Mesozoic, formerly called the Secondary Era?

At the dawn of the age of dinosaurs, the oceans of the Earth were joined in a single sea, Panthalassa, which surrounded the supercontinent Pangaea. In that global ocean, at the beginning of the Triassic period, ichthyosaurs appeared. Since the Jurassic, and during the rest of the Mesozoic, Pangaea gradually fragmented, giving rise to new seas.

Ichthyosaurs, whose shape resembled fish and dolphins, reached 18 meters in length. They conquered the waters until the plesiosaurs appeared at the end of the Triassic and proliferated during the Jurassic. In the discovery of the fossils of both groups played an important role the Englishwoman Mary Anning, the first female paleontologist. She discovered the first ichthyosaur skeleton that was correctly identified (the previous ones had been misclassified); she also found the first two almost complete plesiosaur skeletons; and the first pterosaurs found outside Germany.

Neither ichthyosaurs nor plesiosaurs were dinosaurs, although they are sometimes called that way, especially in less rigorous publications. Both groups belong, like dinosaurs, to the class of Sauropsids, which in modern classifications, which follow cladistic methods, is one of the two clades into which the ancient group of reptiles has been divided:

• Synapsids: a group of ancient reptiles that appeared during the Paleozoic. This group also includes mammals, which are their descendants.
• Sauropsids: to this group belong all present reptiles, as well as numerous extinct groups. The best known are ichthyoptera (ichthyosaurs), sauropterygia (plesiosaurs), pterosaurs (flying reptiles), and dinosaurs. Birds, which descend from dinosaurs, are also integrated in this group. Among all these groups, those closest to dinosaurs are crocodiles and pterosaurs; plesiosaurs are related to turtles; and ichthyosaurs are the most primitive, far from the other groups of sauropsids.

This is what one of the characters from the Journey to the Center of the Earth says about the two animals that undertake a deadly fight in the Liddenbrock Sea:

Voyage to the Center
of the Earth

The first of these monsters has the snout of a porpoise, the head of a lizard, crocodile teeth, that's what has tricked us. It’s the most fearsome of the antediluvian reptiles: the ichthyosaur!... The other one is a snake hidden in the shell of a turtle, the fearsome enemy of the first, the plesiosaur!

Note that Verne is correct when he related plesiosaurs with turtles, although the comparison he uses dates from 1832, when Professor Buckland, presenting the recently discovered plesiosaurs, compared them to a sea serpent run through a turtle. This shows that first impressions are not always wrong, since cladistics classifies today plesiosaurs in a group (Pantestudines) that also includes turtles.

Loch Ness monster

A curious detail: plesiosaurs take part in the science (or pseudoscience) called cryptozoology, which argues that there are still quite a few unknown animals on our planet. One of them is the famous yeti of the Himalayas. Another is the Loch Ness monster, which has sometimes been identified with a plesiosaur that would have survived the catastrophe marking the boundary between the Cretaceous and Tertiary periods.

The same post in Spanish

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