Plesiosaurs
Plesiosaurs (plee-zee-oh-SAWRS), Meaning: Near Lizard, were carnivorous aquatic (mostly marine) reptiles. The common name ‘plesiosaur’ is applied both to the ‘true’ plesiosaurs (Suborder Plesiosauroidea) and to the larger taxonomic rank of Plesiosauria, which includes both long-necked (elasmosaurs) and short-necked (polycotylid) forms.
What Did Plesiosaurs Look Like?
Plesiosaurs with short necks and large heads, are more properly called pliosaurs. There were many species of plesiosaurs and not all of them were as large as Liopleurodon, Kronosaurus or Elasmosaurus.
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Plesiosaurs first appeared at the very start of the Jurassic Period and thrived until the K-T extinction, at the end of the Cretaceous Period. While they were Mesozoic reptiles that lived at the same time as dinosaurs, however, they were not dinosaurs.
How Big Were Plesiosaurs?
Plesiosaurs ranged in size from 8 – 46 feet long (2.5 – 14 metres). They had 4 flippers, sharp teeth in strong jaws and short, pointed tails. Plesiosaurs may have evolved from the Nothosaurs or Pistosaurus, a mid-Triassic reptile. They had a broad body from which their long neck extended.
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Plesiosaurs may have laid eggs in nests that they dug into the sand, much as modern-day sea turtles do.
Plesiosaurs (with the exception of the Polycotylidae) were probably relatively slow swimmers. It is likely that they cruised slowly below the surface of the water, using their long flexible neck to move their head into position to snap up unwary fish or cephalopods.
Their unique, four-flippered swimming adaptation may have given them exceptional flexibility, so that they could swiftly rotate their bodies as an aid to catching their prey.
Some Plesiosaurs have been found with small stones in their stomachs – these may have been used to help grind up their food, or as ballast, to help them dive.
Contrary to many reconstructions of plesiosaurs, it would have been impossible for them to lift their head and long neck above the surface, in the ‘swan-like’ pose that is often shown.
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Even if they had been able to bend their necks upward, to that degree (they could not), gravity would have tipped their body forward and kept most of the heavy neck in the water.
Types of Plesiosaur
Plesiosauroids
Had long, snake-like necks, tiny heads and wide bodies. They ate small sea creatures, probably using their long necks like a snake to catch their prey.
They included: Plesiosaurus – 7.6 feet (2.3 metres) long – with a long neck, 4 wide, paddle-shaped flippers, and a tapered body.
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Cryptoclidus
Cryptoclidus was 13 feet (4 metres) long with curved, interlocking teeth and large flippers. Remains of this Plesiosaur have been found in England. This animal would have lived during the late Jurassic period.
Muraenosaurus
Muraenosaurus was 20 feet (6 metres) long – with a very long neck, and a wide body. Its remains have been found in England and France, and would have lived during the late Jurassic period.
Woolungosaurus
Woolungosaurus was 26-33 feet (8-10 metres) long – with a very long neck. This aniamals reamins have been found in Queensland, Australia. This Plesiosaur lived during the early Cretaceous period, about 110 million years ago.
Elasmosaurus
Elasmosaurus was 46 feet (14 metres) long with an extremely long neck that made up to half of its total body length.
It had and had 71 vertebrae, 28 of which were in its neck. It had 4 very long paddle-like flippers, and a short, pointed tail. Remains of this animal have been found in Japan and Kansas, USA. Elasmosaurus would have lived during the late Cretaceous period.
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Thalassomedon
Thalassomedon was around 40 feet or 12 metres long and had a very long neck, which contained around 63 vertebrae.
Thalassomedon remains have been found in Colorado, USA, and would have lived during the late Cretaceous period.
Thalassomedon name translates to “sea lord”, and was named in 1943 by Welles, an American palaeontologist.
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Pliosauroids
These had large heads with very strong jaws, short necks and resembled modern-day whales. They ate larger sea creatures. They included:
Macroplata
Macroplata is estimated to have been up to 15 feet (4.5 metres) in length. Macroplata had long, toothed jaws and a long neck which had around 29 vertebrae.
Remains of this animals have been found in England, and would have lived during the early Jurassic period.
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Peloneustes
Peloneustes is estimated to have been up to 10 feet or 3 metres in length. Peloneustes had a large head, a streamlined body.
The neck was a fair size too and would have had roughly 20 vertebrae Fossilised stomachs of Peloneustes have been found which contained suckers from squid like created such as cephalopods.
Peloneustes remains have been found in England and eastern Europe, and would have lived during the late Jurassic period.
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Kronosaurus
Kronosaurus (Kroe-noe-sore-uss) was one of the sea reptiles known as pliosaurs, a member of the plesiosaur group, but in the Pliosauridae family (including Macroplata, Peloneustes and Liopleurodon). Kronosaurus lived in the Aptian – Albian Stages during the early Cretaceous period, some 120 million years ago.
Kronosaurus was one of the fiercest predators in the seas of the Mesozoic Era. It was named after the leader of the Greek Titans, Kronos. It’s important to note that Kronosaurus was not a dinosaur but lived at the time of dinosaurs.
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How big was Kronosaurus?
Kronosaurus measured 43 feet (13 metres) in length. Its head measured 9 feet (2.7 metres) in length alone, about a quarter of the entire length of its body.
Kronosaurus had huge head with huge powerful jaws with rows of sharp teeth measuring 10 inches (25 centimetres) long.
Kronosaurus was heavily built, more streamlined, faster and more ferocious than other plesiosaurs. Even though it was a member of the Pliosauridae Family, it had a distinctive feature of having a much shorter and thicker neck.
Kronosaurus had 4 strong limbs which had evolved into flippers which helped it to swim powerfully through the waters. Each flipper was located at each corner of its elongated body. Its tail was quite short and pointed and probably helped Kronosaurus to steer itself through the water.
Kronosaurus lived in the open oceans but could breathe air. Like modern day sea turtles, these sea reptiles probably laid their eggs in sand on the beach.
Large Teeth
It had rounded teeth at the back of the jaws which could crush shells and cephalopods (squid and octopi). They were more suited to catching fewer larger meals than other plesiosaurs.
What did Kronosaurus eat?
Kronosaurus was a fierce carnivore and was an effective predator on many other marine reptiles, turtles, large fish, ammonites and mollusks, basically anything that got in its way much like modern sharks do.
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Some Plesiosaurs have been found with small stones in their stomachs which may have been used to help grind up their food, or as ballast, to help them dive. Furthermore Fossilized plesiosaurs and turtles have been found in the stomach cavity of Kronosaurus.
Who discovered Kronosaurus?
Kronosaurus fossils were first discovered in Queensland, Australia in 1889. Fossils have also been found in Colombia, South America. Kronosaurus was finally named and described by Longman in 1924.
Elasmosaurus
Elasmosaurus (eh-LAZZ-mo-SAWR-us) lived during the late Cretaceous period and went extinct during the K-T mass extinction, 65 million years ago. Elasmosaurus was a reptile and not a dinosaur. They are distinguished by having two holes in the rear upper part of their skulls and two holes behind the eyes, like all Diapsids.
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How big was Elasmosaurus?
Elasmosaurus measured around 46 feet (14 metres) in length and weighed over 2.2 tons (2,000 kilograms), making it the longest ever plesiosaur species. It had a large body and 4 flippers for limbs.
Elasmosaurus had plate-like bones in its pelvic girdle and an extremely long neck. More than half of its length was its neck, which had more than 70 vertebrae, more than any other animal.
It had a small head with sharp teeth and most likely ate small bony fish such as belemnites which were similar to squid, lepidotes and ammonites’ molluscs. It swallowed small stones to aid its digestion.
Elasmosaurus lived in the open oceans and breathed air. Elasmosaurus swam slowly using its 4 paddle-like flippers in a manner similar to that of modern turtles. It may have been able to move around on sandy shores, perhaps to lay its eggs.
Who discovered Elasmosaurus
Elasmosaurus was named by paleontologist E. D. Cope in 1868 from a fossil was found in Wyoming, USA. Other Elasmosaurus fossils have also been found in North America.
FURTHER READING & STUDY
- Samuel L. Tutin (2018) The completeness of the fossil record of plesiosaurs, marine reptiles from the Mesozoic.
- Luke E. Muscutt (2017) The four-flipper swimming method of plesiosaurs enabled efficient and effective locomotion.
- Valentin Fischer (2020) The macroevolutionary landscape of short-necked plesiosaurians.