Prehistoric Animals


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Sea snakes : plesiosaurs and pliosaurs

The plesiosaurs and the pliosaurs are distinguished from all the other reptiles that inhabited the planet in the Mesozoic era: everyone agrees that the Tyrannosaurus completely disappeared a long time ago while a small minority of people firmly believe that some species of sea snake have survived up to this point. However, this group of fanatics does not include imminent biologists and paleontologists, and it is largely quibbles.

Aquatic reptiles

Plesiosaur Elasmosaurus
Plesiosaure Elasmosaurus

The plesiosaurs were large aquatic reptiles recognizable by their long necks and four fins which paddled the oceans, lakes, rivers and swamps of the Jurassic and Cretaceous period. To add to the confusion, the term plesiosaur also encompasses pliosaurs ("Pliocene lizard" - they lived tens of millions of years earlier) that possessed a more hydrodynamic body, a larger head, and a shorter neck. Even the largest plesiosaurs (such as Elasmosaurus, which was 40 feet long) were relatively soft animals while the larger pliosaurs (such as Liopleurodon) were as formidable as the Great White Shark.

Evolution of plesiosaurs and pliosaurs

Although these sea snakes had adopted an aquatic lifestyle, they nevertheless remained lizards that frequently had to resurface in order to breathe. This implies that the ancestors of these aquatic reptiles were almost certainly terrestrial archosaurs of the Lower Triassic period. Some experts think that the first marine ancestors of the plesiosaurs were the nothosaurs, represented by Nothosaurus of the lower Triassic.

As evolution had desired for many other animal species, the Upper Jurassic and Cretaceous plesiosaurs and pliosaurs tended to be much larger than their lower Jurassic cousins. One of the first known plesiosaurs, Thalassiodracon, was only 6 feet long; it was a run-off compared to Upper Cretaceous plesiosaurs such as Mauisaurus which measured 55 feet long. Similarly, the lower Jurassic Pliosaurus Rhomaleosaurus was only 20 feet in size with respect to its Upper Jurassic counterpart Liopleurodon, which was 40 feet long and weighed around 25 tons. However, the pliosaurs were not all as massive as Liopleurodon: for example Dolichorhynchops was a 17-foot long Cretaceous noodle that fed on soft squid rather than robust prehistoric fish.

Behavior of plesiosaurs and pliosaurs

These sea serpents differed not only physically but also in behavioral terms. For a long time, paleontologists were perplexed by the extreme neck length of some plesiosaurs, speculating that these reptiles kept their heads well out of the water (such as storks) and plunged the latter only to spear fish. It turns out, however, that the head and neck of the plesiosaurs were neither strong enough nor flexible enough to be used in this way, although it would have been an impressive device for underwater fishing.

Although they had a smooth body, the plesiosaurs were far from being the fastest aquatic reptiles of the Mesozoic era (in a race against the clock, most of the plesiosaurs would have been defeated by most ichthyosaurs, these first "lizardfish" that evolved a better hydrodynamic form). What ultimately caused the disappearance of upper Cretaceous plesiosaurs was the evolution of faster and more suitable fishes, as well as the appearance of more agile and vicious aquatic reptiles such as mosasaurs.

In general, pliosaurs from the Upper Jurassic and Cretaceous period were larger, stronger, and meaner than their long-neck relatives. Some genera such as Kronosaurus and Cryptoclidus were similar in size to modern gray whales but were equipped with many sharp teeth rather than plankton wilts. While the majority of plesiosaurs remained on fish, pliosaurs (such as prehistoric sharks) swallowed almost anything that ventured near them, ranging from fish to squid and even other aquatic reptiles.

Plesiosaurs and pliosaurs fossils

Plesiosaur fossil
Plesiosaur fossil

One of the strange things about these sea snakes is that the distribution of the oceans on the surface of the Earth 100 million years ago was radically different from what it is today. As a result, new fossils of aquatic reptiles are unceasingly found in places as unusual as the American West and the Midwest: a large proportion of these areas were once covered by the Western Inland Sea.

Unlike those of terrestrial dinosaurs, fossils of plesiosaurs and pliosaurs are often found in a single articulated piece (which is undoubtedly attributable to the protective qualities of silt at the bottom of the ocean). These remains have baffled naturalists for as long as the 18th century: a fossil in particular of long-necked plesiosaurs prompted an unknown paleontologist to joke with sarcasm that it looked like a snake that snuck through the carapace of a turtle.

Another plesiosaur fossil is one of the most famous dust collectors in the history of paleontology. In 1868, the famous fossil scientist Edward Drinker Cope reassemble a skeleton of Elasmosaurus with its head put at the wrong end (this was the first time that paleontologists encountered such an aquatic reptile). This mistake was seized by his great rival Othniel C. Marsh triggering what is now known as "The Fossil War".

Are sea snakes like plesiosaurs and pliosaurs still extant today?

Even before a living Coelacanth - a species of prehistoric fish thought to have disappeared for tens of millions of years - was found off the shores of Africa in 1938, people known as cryptozoologists had emitted the far-fetched hypothesis that sea snakes such as plesiosaurs and pliosaurs had not all died out at the same time as the dinosaurs 65 mya. Although it would have been very difficult for a surviving terrestrial dinosaur to hide its existence, the ocean is extremely vast and deep and offers multitude of habitats for a plesiosaur colony.

The first plesiosaur that comes to mind is the mythical Loch Ness monster, Nessie, whose photographs look like Elasmosaurus. There are, however, two problems with the theory that the Loch Ness monster is really a plesiosaur: the first is that these aquatic reptiles need to surface in order to breathe which would probably attract some attention. The second, a little more esoteric, is that the neck of the plesiosaurs simply was not strong enough to allow them to take the majestic pose of the Loch Ness monster.

Of course, as we say, the absence of proof is no proof of absence. Very large areas of the ocean remain unexplored, and it is plausible (though unlikely) that one day a living plesiosaur will be caught in fishing nets. It should not be hoped, however, that it be in Scotland, on the edge of a famous lake.