Plesiosaur

The Plesiosauria or plesiosaurs were an order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia. They first appeared in the latest Triassic Period, possibly in the Rhaetian stage, about 203 million years ago. Plesiosaurs became common during the Jurassic Period and disappeared due to the Cretaceous–Paleogene extinction event about 66 million years ago. They lived in oceans all around the world, and some might have also been in freshwater.

Plesiosaurs had a broad, flat body and a short tail. Their limbs turned into four long flippers, which they used to move through the water. These strong flippers had muscles connected to wide bony plates on their shoulder and hip areas. Plesiosaurs breathed air and gave birth to live young.

There were two main types of plesiosaurs. One type had very long necks and small heads. These moved slowly and ate small sea animals. The other type had short necks and large heads. These were fast hunters and could catch large prey. Some grew up to seventeen meters long! Scientists now think these traits might have been mixed among different species.

Unlike other ancient marine reptiles such as the Ichthyosauria and Mosasauria, the different groups of plesiosaurs are not part of the Dinosauria clade. Plesiosaurs were among the first fossil reptiles found, and over a hundred valid species have been identified.

History of discovery

Main article: Timeline of plesiosaur research

Bones of plesiosaurs were some of the first fossils of extinct reptiles that scientists found. At first, scientists often thought these bones belonged to fish. One of the earliest known plesiosaur fossils was described in 1719 by William Stukeley. This partial skeleton was found in a quarry in Lincolnshire, England. Today, it is displayed in the Natural History Museum.

In the 1800s, many more plesiosaur fossils were found, especially in England. Scientists began to realize that plesiosaurs were a special group of sea reptiles. In 1821, William Conybeare and Henry Thomas De la Beche named the genus Plesiosaurus. Fossils found by Mary Anning in Lyme Regis showed scientists how strange these animals looked, with long necks and big bodies. As more fossils were found around the world, scientists learned about the different kinds of plesiosaurs and how they lived.

Evolution

The Plesiosauria came from the Sauropterygia, a group of reptiles that went back to live in the sea. During the Upper Triassic, they split into two groups. One group kept joints that worked well, while the other, the Pistosauria, became better suited for ocean life. Their limbs changed into flippers, and they had babies that could live right away.

From the earliest Jurassic period, plesiosaurs became very common. They were split into two kinds: those with long necks and those with short necks. Some grew up to ten meters long, while others, like the large Pliosauridae, reached up to seventeen meters. During the Early Cretaceous, the Elasmosauridae appeared, famous for their very long necks. All plesiosaurs went extinct at the end of the Cretaceous period, about 66 million years ago.

Description

Plesiosaurs were different sizes, from about 1.5 meters (5 feet) to over 15 meters (49 feet) long. Some were very big sea hunters, about the same size as large sharks, ichthyosaurs, and early toothed whales. Their bodies were wide and flat, with short tails. They had four large flippers for swimming, moving up and down in the water. Unlike fish-like swimmers such as ichthyosaurs, plesiosaurs used all four flippers to move, with their tail helping to steer.

Plesiosaurs had a special body with strong muscles attached to bony plates on their undersides. Their limbs were large and flat, turned into flippers with many small bones for extra length. They had long necks and small heads or short necks and large heads, depending on the species. Their teeth were different shapes, from thin and pointy to round, for catching animals in the ocean.

Paleobiology

Plesiosaurs had different ways of eating depending on their neck length. Long-necked plesiosaurs, known as "plesiosauromorphs," probably hunted fish and squid. They used their large eyes to spot their food. They may have also eaten creatures that lived on the ocean floor. Short-necked plesiosaurs, called "pliosauromorphs," were strong hunters. They could chase and catch large animals.

Scientists have found stones in plesiosaur stomachs. These stones are called gastroliths. They might have helped with digestion or kept the plesiosaurs floating in the water.

Distribution

Plesiosaur fossils have been found on every continent, including Antarctica. These ancient sea reptiles lived in the oceans when the dinosaurs were around, from about 203 million years ago to 66 million years ago.

Main article: List of plesiosaur-bearing stratigraphic units

The list of rock layers with plesiosaur fossils shows how far and wide these creatures lived long ago.

In contemporary culture

Main article: Loch Ness Monster

See also: Sea monster

Plesiosaurs are sometimes confused with dinosaurs in movies and books, but they are not dinosaurs. Some people think that old stories of sea monsters and recent sightings of strange creatures in lakes might be because plesiosaurs are still alive today. Scientists do not believe this because there is no proof.

The famous Loch Ness Monster is often described to look like a plesiosaur, but this is unlikely. For example, the lake where the monster is said to live is too small and too young to support such large animals. Because of this, many people think the sightings are caused by things like waves, floating objects, or tricks of the light. Still, many people link plesiosaurs with the legend of the Loch Ness Monster.