Plesiosaurs were a group of large marine reptiles that lived during the Mesozoic era, specifically the Jurassic and Cretaceous periods. They were characterized by their unique body structure, which includes a small head, a long neck, a broad turtle-like body, and two pairs of large, paddle-like limbs used for swimming.

This family of creatures has been given many very similar names which can easily cause confusion. So it’s worth taking a moment to better understand the difference between these names.

Plesiosaurs are classified in the order Plesiosauria. The term "Plesiosaur" is often used as a catch-all term for these creatures. Within Plesiosauria, there are two primary families: the Plesiosauridae and the Pliosauridae.

• The Plesiosauridae, often referred to as "true plesiosaurs," is the family that contains the genus Plesiosaurus among others—this is where this collection is classified. The Plesiosaurus was the first of this group to be discovered and are well-known for their long necks and small heads. They were likely slower swimmers that used their lengthy necks to catch fish and other small marine animals. There is only one valid species within Plesiosaurus, P. dolichodeirus, which was discovered by Mary Anning.
• On the other hand, the Pliosauridae, commonly known as pliosaurs, had a different body shape. Pliosaurs generally had shorter necks and larger heads, and some species, like Liopleurodon, had skulls up to 2 meters long. Pliosaurs are believed to have been fast swimmers and top predators, capable of taking on larger prey.

Although these two families had different physical characteristics and hunting strategies, both were part of the larger group known as plesiosaurs and played a critical role in the marine ecosystems of their time. Despite their extinction at the end of the Cretaceous period, plesiosaurs remain iconic figures in our understanding of prehistoric marine life.

As a carnivorous marine reptile, it likely fed on fish and other marine creatures. Plesiosaur teeth varied in shape and orientation depending on the species and their feeding habits. However, many plesiosaurs had teeth that were conical, sharp, and slightly curved, which helped them efficiently catch and hold onto their prey. In the teeth pointed slightly outward or laterally, which would have facilitated a better grip on slippery prey like fish and squid.

They had a unique respiratory system that included both lungs and a trachea. This allowed them to efficiently extract oxygen from the air while surfacing between dives. Plesiosaurs are believed to have given birth to live young instead of laying eggs. This adaptation allowed them to be fully adapted to life in the ocean and avoid the need to return to land for reproduction.

Plesiosaurs went extinct during the Cretaceous-Paleogene (K-Pg) mass extinction event around 66 million years ago, along with many other marine reptiles and non-avian dinosaurs.