Plesiosauria were marine reptiles that dominated the oceans throughout the Mesozoic Era, from the Late Triassic to the end of the Cretaceous period. These air-breathing predators were not dinosaurs, but a diverse group characterized by their unique four-flipper body plan. The size of plesiosaurs varied dramatically across different species and epochs. Their dimensions ranged from species no larger than a small seal to massive apex predators that rivaled the largest modern whales in mass.
The Distinct Body Structure and Size Measurement
The body architecture of plesiosaurs presents challenges for estimating their size. These reptiles possessed a broad, flattened torso, a short tail, and four large, paddle-like flippers, a design that allowed them to “fly” through the water. The order is divided into two main body types: the long-necked, small-headed Plesiosauroids and the short-necked, large-headed Pliosauroids.
The most significant variable in determining overall length is the neck, which could contain an extraordinary number of vertebrae. Because complete skeletons are rare, size estimation relies on scaling known dimensions from fragmentary fossils to a complete body plan. Key metrics used as proxies for total size include the length of the skull or the dimensions of the dorsal vertebrae within the trunk.
By studying the relationship between various skeletal elements in more complete specimens, researchers have developed scaling equations to estimate total length and body mass from partial remains. The trunk length and the dimensions of the dorsal vertebrae, which form the main body cavity, have been identified as robust indicators for estimating the overall volume and weight. This methodology helps to account for the dramatic variation in neck length between species, providing a more consistent basis for size comparison.
Typical Dimensions of Plesiosaur Species
The average size of a plesiosaur is best represented by the more common, medium-sized species that thrived across the Jurassic and Cretaceous periods. Plesiosaurus dolichodeirus, an early species, reached an average total length of approximately 3.5 to 4.5 meters (11 to 15 feet).
These mid-sized plesiosaurs were active hunters, feeding on fish and cephalopods with their relatively small heads and numerous sharp teeth. During the Early Jurassic, many plesiosaur species were generally smaller, typically growing to a maximum of about five meters (16 feet) long. These animals occupied a niche similar to modern medium-sized marine predators.
The Largest Known Plesiosaur Giants
The maximum size within the Plesiosauria order was achieved by the Pliosaurs, the short-necked, robust-bodied members, which were the apex predators of their time. These giants developed massive, elongated skulls that could measure over two meters (6.5 feet) in length. Fragmentary remains of a specimen from the Kimmeridge Clay Formation in England suggest a length of up to 14.4 meters (47 feet).
Other notable giants include the genus Pliosaurus, with some specimens like the one formerly nicknamed “Predator X” (Pliosaurus funkei) estimated to have been around 12 meters (39 feet) long. Similarly, the powerful Kronosaurus from the Early Cretaceous of Australia and Colombia reached lengths of about 9 to 11 meters (29 to 36 feet). Based on skeletal scaling, the largest of these pliosaurs are estimated to have weighed over 20 tonnes (44,000 pounds), a mass comparable to a large sperm whale.
The long-necked lineage also produced enormous species, such as Elasmosaurus, which could reach lengths of 13 to 14 meters (43 to 46 feet). However, a significant portion of this length was neck, resulting in a proportionally smaller body mass compared to the bulky Pliosaur giants. The largest plesiosaurs were among the largest marine predators to have existed.