Plesiosauroidea

Plesiosauroidea is a clade of extinct, carnivorous marine reptiles.

These animals lived in the oceans during the Jurassic and Cretaceous periods. They appeared during the Early Jurassic, specifically the late Sinemurian stage. They thrived until the K-Pg extinction event ended the Cretaceous Period. While they lived alongside dinosaurs, they were not part of the dinosaur group. Instead, they were Mesozoic diapsid reptiles. They are most famous for having the longest neck-to-body ratio of any reptile.

Some early observers described them as looking like a snake threaded through a turtle shell. However, these reptiles did not actually possess a shell.

The physical structure of a plesiosauroid was very specialized for life in the sea. They possessed a broad body and a relatively short tail. They kept two pairs of limbs from their ancestors, but these evolved into large flippers. These four flippers likely provided exceptional maneuverability. This allowed them to rotate their bodies quickly to catch prey. Most plesiosauroids were probably slow swimmers. They likely cruised just below the water's surface. They used their long, flexible necks to position their heads to snap up fish or cephalopods.

It was not actually possible for them to lift their heads above the water in a swan-like pose. Because of gravity, lifting a heavy neck would have tipped their bodies forward into the water.

There were several distinct families within the Plesiosauroidea. The Plesiosauridae were unspecialized types that lived during the Early Jurassic. The Cryptoclididae, such as *Cryptoclidus*, had a somewhat stocky build and medium-long necks. The Elasmosauridae were known for having very long, flexible necks and tiny heads.

A less understood group called the Cimoliasauridae included small forms from the Cretaceous. In recent scientific classifications, the Polycotylidae are also included in this clade. Unlike most others, Polycotylidae had large heads and short necks. Scientists have also identified a more exclusive group called Cryptoclidia. This group was defined in 2010 by researchers Hilary Ketchum and Roger Benson. It includes the last common ancestor of *Cryptoclidus eurymerus* and *Polycotylus latipinnis* and all its descendants.

Our understanding of these creatures comes from a long history of fossil discoveries. In the early 19th century, Mary Anning found the first complete plesiosauroid skeletons in England. These were among the first fossil vertebrates described by science.

In 1844, the Scottish geologist Hugh Miller found remains in the Great Estuarine Group of western Scotland. Many other fossils have been found by amateurs and professionals alike. In 2002, a person fishing from the shore in Somerset found a fine specimen. This is known as the Collard specimen. Another skeleton was found that same year by an amateur paleontologist in the cliffs of Filey, Yorkshire.

The size and biology of these reptiles varied greatly. For instance, *Trinacromerum* had an estimated length of three meters. In contrast, *Mauisaurus* could grow as large as twenty meters. Tooth records suggest that several sea-dwelling reptiles, including plesiosauroids, had a warm-blooded metabolism. This is called endothermic heat. This ability allowed them to generate internal heat to survive in colder habitats. Researchers also frequently find gastroliths, which are stones swallowed by animals, associated with plesiosaur remains.

One of the most surprising discoveries involves how they reproduced. For a long time, people thought they crawled onto land to lay eggs like sea turtles. However, anatomy suggests they could not move well on land. In 2011, researchers described a fossil found on a Kansas ranch in 1987. This fossil was a pregnant *Polycotylus latipinus*. It proved that these predatory reptiles gave birth to single, large, live offspring. This is very different from other marine reptiles that typically produce many small babies.

Plesiosauroids represent a significant chapter in the history of marine life. They show how reptiles adapted to life entirely in the ocean. Their evolution from earlier forms, such as pistosaurs, demonstrates the changing nature of prehistoric ecosystems. By studying their specialized limbs and necks, scientists learn how animals occupy specific roles in the sea. Their long history from the Early Jurassic to the end of the Cretaceous shows how successful this body plan was for millions of years.