Plesiosauria: Definition, Habitat & Facts

Instructor: Lauren Posey

Lauren has taught intermediate reading in an English Language Institute, and she has her Master's degree in Linguistics.

Plesiosauria were a group of large marine reptiles. In this lesson, we'll look at their habitat, and some interesting facts that scientists have learned from looking at fossils.

What are Plesiosauria?

When you think about reptiles, what are the characteristics that come to mind? How about flippers? These may not be common to all reptiles, but they are typical of marine reptiles, both extinct ones and those living today. A modern day example would be sea turtles, and a group of extinct marine reptiles are the plesiosauria. This group includes a number of different species, all of which were marine reptiles.

Even the smallest plesiosauria were much bigger than sea turtles today. Plesiosauria ranged from more than 12 feet long to over 60 feet. They were present on Earth for about 155 million years, from the late Triassic through the late Cretaceous. The exact dates are debated to a certain extent, but fossils date from about 220 million years ago through about 65 million years ago.


Plesiosauria were completely aquatic, which means they spent their whole lives in water, specifically in the ocean. It is difficult to look for fossils in a lot of our oceans today, so we can only guess at their true range. Fossils have mostly been found in shallow and coastal waters, but that could be simply because it is easier and more likely to find fossils in these areas. Since they could grow so large, it makes sense to assume that they roamed the deeper oceans as well, just as large marine animals do today.

There have been fossils found off the coasts of Europe, and throughout many areas of the Pacific Ocean. This includes coasts of Asia, Australia, and North America. Clearly, plesiosauria were very widespread, even given the limited areas in which we can search for fossils.

It is also important to remember that the continents looked very different at the beginning of the plesiosaur's time on Earth. Where we find the fossils today does not necessarily reflect where they actually lived, since what was once deep ocean may now be shallow coastline, and vice versa. What we do know for certain is that they were widespread ocean dwellers.

The continents looked very different during the Triassic Period
Triassic map

Interesting Facts

Plesiosauria died out long before humans came around, so all we know of them comes from fossils. As a result, scientists have to guess at what their behavior might have been like. However, fossils can give us a good idea of what they looked like, and some starting point for their behaviors.

Physical Traits

Plesiosauria had flippers and short tails. The species were divided between two groups: pliosaurs and plesiosauroids. Pliosaurs had long heads and snouts, but short necks. Plesiosauroids, on the other hand, had small heads on very long necks. Plesiosaurus, one of the more famous plesiosauria, was a plesiosauroid. In addition, later plesiosauria tended to be much bigger than earlier ones.

Plesiosaurus is a well-known species of plesiosaur

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