What Is a Plesiosaur? Four Flippers and a Long Unsettled Argument

The distinguishing feature is four flippers of roughly equal size, which is unique among swimming vertebrates, since every other group propels itself primarily with a tail or with one pair of limbs. The flippers are built of a shortened upper limb bone and greatly elongated digits enclosed in a stiff paddle, producing a rigid hydrofoil rather than a flexible fin. The body is broad and stiffened by a dense arrangement of belly ribs, which limits body flexing and means the animal could not swim by undulating. The tail is short and is not a propulsive organ. Two broad forms exist, with long-necked plesiosaurs carrying up to seventy-six neck vertebrae, more than any other animal, and short-necked pliosaurs with enormous heads and powerful jaws, which are the group's large predators. Both are air-breathing reptiles, and they gave birth to live young rather than laying eggs, which is demonstrated directly by a fossil containing a single large foetus.

How four flippers propel an animal is not obvious and the question has attracted a long sequence of proposals:

• •Rowing, with the flippers pushed backwards like oars, which is the original and simplest proposal and is inefficient for a stiff paddle
• •Underwater flight, with the flippers moving in a figure-of-eight as in a turtle or a sea lion, which generates lift and is more efficient
• •A combination, with different species using different modes
• •Coordinated use of front and rear pairs, where the rear flippers work in the wake of the front pair, which computational and robotic modelling suggests can either help or hinder depending on timing
• •Modelling and physical experiments published in recent years generally find that the front flippers do most of the propulsion and the rear pair contributes mainly to stability and manoeuvring
• •The disagreement persists because the shoulder and hip joints permit a range of motion that the fossils constrain only loosely

The extreme neck of some species is as puzzling as it is famous. It is stiffer than it looks, since the vertebrae articulate in a way that limits sideways and vertical flexing, so the swan-like curves in older reconstructions are not supported and the neck was held closer to straight. That rules out the traditional image of a neck striking like a snake. The favoured explanations involve approaching prey without the body being noticed, since a small head at the end of a long neck disturbs the water far less than a large body would, which suits hunting fish or bottom-dwelling invertebrates in low visibility. Gastroliths are found in many specimens and have been interpreted as ballast rather than as grinding stones, which would help with buoyancy control for an animal that needed to stay down. The neck is also a hydrodynamic cost, and the fact that it evolved repeatedly and persisted for over a hundred million years indicates the benefit was real.

The group occupies an unusual position in public awareness because of the persistent claim that one survives in a Scottish loch. That claim has been examined and the objections are conclusive rather than merely sceptical: the loch formed after the last glaciation and is far too young, it could not support a breeding population of large air-breathing predators, an air-breathing animal would surface constantly and be seen and photographed, and the food supply is inadequate. Environmental DNA surveys of the loch published in 2019 found no reptile DNA at all. The association nonetheless persists and has arguably helped public interest in the real animals while attaching them to something false. More productively, plesiosaur fossils from the English coast were among the finds that established Mary Anning's reputation in the early nineteenth century and contributed directly to the recognition that extinction had occurred, which was a genuinely radical conclusion at the time and one of the foundations of the discipline.

Four flippers of equal size make this the only vertebrate group propelling itself that way, with a stiff body and a short non-propulsive tail. Modelling suggests the front pair does most of the work and the rear pair steers. The famous long neck was stiffer than illustrations show, and it probably allowed a small head to approach prey without the body being detected. They gave birth to live young.