Plesiosaurs were a group of marine reptiles that dominated ancient oceans for millions of years. Their distinctive forms and adaptations have long intrigued scientists and the public. These ancient creatures were colossal predators that shaped prehistoric marine ecosystems.
An Ancient Ocean Predator
Plesiosaurs were diverse extinct marine reptiles from the order Plesiosauria. They appeared in the late Triassic Period, around 203 million years ago, and thrived through the Jurassic and Cretaceous periods. Their existence ended with the Cretaceous-Paleogene extinction event about 66 million years ago, which also marked the end of non-avian dinosaurs.
These air-breathing reptiles had a broad, flat body and a short tail. They were characterized by four large, paddle-like limbs, evolved from ancestral terrestrial limbs. Some species had short necks and large heads, while others, known as plesiosauromorphs, had long, slender necks and small heads. The earliest genus, Plesiosaurus, was named in 1821, and over a hundred species have since been described.
The Scale of Giants
Plesiosaurs varied greatly in size. Smaller forms were around 5 feet (1.5 meters) long. Some of the largest pliosaurs, a type of plesiosaur with short necks and large heads, grew up to 56 feet (17 meters) long and weighed up to 10 tons. For example, Kronosaurus, an Early Cretaceous species from Australia, had an estimated skull length of 7 feet 3 inches to 9 feet 4 inches (2.21–2.85 meters).
Among the longest plesiosaurs were the Elasmosauridae, which appeared later in the Early Cretaceous. These long-necked forms reached lengths of up to 50 feet (15 meters), with their necks containing many vertebrae, sometimes as many as 76. An Antarctic specimen, identified as Aristonectes sp., is considered among the largest known long-necked plesiosaurs, measuring over 35 feet long and weighing between 11 to 14 tons. These dimensions rival the size of many modern large marine animals, such as orcas or great white sharks.
Life in the Mesozoic Seas
Plesiosaurs navigated aquatic environments using their four large flippers, which generated propulsion through an up-and-down “flying” motion, similar to modern sea turtles. Their short tails likely provided directional control. The function of their long necks is debated, with theories suggesting they intercepted fast-moving fish or were used for bottom-feeding.
Their diet varied by morphology. Long-necked plesiosauromorphs, with small, needle-like teeth, fed on smaller prey like fish and soft-bodied cephalopods such as belemnites and ammonites. Fossilized stomach contents, including cephalopod shells, support this. Short-necked, large-headed pliosaurs, with large, conical teeth, were apex predators that hunted larger prey. While most plesiosaurs lived in marine environments, recent fossil discoveries in ancient riverbeds in Morocco’s Sahara Desert indicate some species also lived in freshwater.
From Bones to Legends
Our understanding of plesiosaurs comes from the fossil record, with skeletal remains providing insights into their anatomy and lifestyle. Mary Anning, an early 19th-century fossil hunter, made some of the first documented plesiosaur discoveries in England, including a nearly complete Plesiosaurus skeleton. Later finds in North America, particularly in the Cretaceous Western Interior Seaway, contributed to early paleontological research. A nearly complete Plesiopterys wildi fossil recently found in Germany provided new details on the diversification of these marine reptiles in the Early Jurassic.
Despite their extinction 66 million years ago, plesiosaurs capture public imagination, largely due to their association with cryptids like the Loch Ness Monster. The popular image of “Nessie” often depicts a creature resembling a plesiosaur, with a long neck and small head. However, scientific consensus indicates plesiosaurs, like other non-avian dinosaurs, died out at the end of the Cretaceous Period. Loch Ness itself is a geologically young freshwater body, formed only about 10,000 years ago, making the survival of a large, ancient marine reptile there highly improbable.