Old Earth Ministries Online Dinosaur Curriculum

Free online curriculum for homeschools and private schools

From Old Earth Ministries (We Believe in an Old Earth...and God!)

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Lesson 72 - Plesiosaurs

A plesiosaur (Greek meaning 'near' or 'close to' a 'lizard') was a type of carnivorous aquatic (mostly marine) reptile. After their discovery, plesiosaurs were somewhat fancifully said to have resembled "a snake threaded through the shell of a turtle", although they had no shell. The common name "plesiosaur" is applied both to the "true" plesiosaurs (Superfamily Plesiosauroidea), which include both long-necked (elasmosaurs) and short-necked (polycotylid) forms, and to the larger taxonomic rank of Plesiosauria, which includes the pliosaurs. The pliosaurs were the short-necked, large-headed plesiosaurians that were the apex predators for much of the Mesozoic.

Plesiosaurs (sensu Plesiosauroidea) appeared at the end of the Triassic Period and throve until the K-T extinction, at the end of the Cretaceous Period. While they were Mesozoic diapsid reptiles that lived at the same time as dinosaurs, they were not dinosaurs. Gastroliths are frequently found associated with plesiosaurs.


Quick Facts


Length:  10 feet for smallest species, up to 65 feet for largest species

Date Range:  210 - 65 Ma, Triassic - Cretaceous Period



Reconstructed skeleton of Thalassomedon hanningtoni, an elasmosaurid   (Picture Source)


The first plesiosaur skeletons were found in England by Mary Anning, in the early 19th century, and were amongst the first fossil vertebrates to be described by science. Many museums all over the globe have plesiosaur specimens.

Plesiosaurs had a broad body and a short tail. They retained their ancestral two pairs of
Dolichorhynchops, a short-necked, long-jawed plesiosaur, National Museum of Natural History, Washington D.C.  (Picture Source
 limbs, which evolved into large flippers.

It has been determined by teeth records that several sea-dwelling reptiles, including Plesiosaurs, had a warm-blooded metabolism similar to that of mammals. They had the ability to generate endothermic heat to survive in colder habitats.


Plesiosaurs evolved from earlier, similar forms such as pistosaurs or very early, longer-necked pliosaurs. There are a number of families of plesiosaurs, which retain the same general appearance and are distinguished by various specific details. These include the Plesiosauridae, unspecialized types which are limited to the Early Jurassic period; Cryptoclididae, (e.g. Cryptoclidus), with a medium-long neck and somewhat stocky build; Elasmosauridae, with very long, inflexible necks and tiny heads; and the Cimoliasauridae, a poorly known group of small Cretaceous forms. According to traditional classifications, all plesiosaurs have a small head and long neck but, in recent classifications, one short-necked and large-headed Cretaceous group, the Polycotylidae, are included under the Plesiosauroidea, rather than under the traditional Pliosauroidea. Size of different plesiosaurs varied significantly, with an estimated length of Trinacromerum being three meters and Mauisaurus growing to twenty meters.


Unlike their pliosaurian cousins, plesiosaurs (with the exception of the Polycotylidae) were
Restoration of a Plesiosaurus dolichodeirus pair, one catching a fish  (Picture Source
 probably 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 four-flippered swimming adaptation may have given them exceptional maneuverability, so that they could swiftly rotate their bodies as an aid to catching prey.

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. Even if they had been able to bend their necks upward to that degree (which they could not), gravity would have tipped their body forward and kept most of the heavy neck in the water.  Most famous of these poses is the ones that portray the mythical Loch Ness Monster.

The series Walking with Dinosaurs shows the plesiosaur Cryptoclidus hauling out on land like a sea lion.


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