The Plesiosaurs

Most grade school children can identify a plesiosaur if they see one, even if they incorrectly call it a dinosaur (no dinosaurs, to our knowledge, became aquatic). Plesiosaurs were marine reptiles that thrived during the Mesozoic Era; though living in tandem with terrestrial dinosaurs, they were not dinosaurs themselves. Plesiosaurs first appeared in the Triassic Period around 200 million years ago; they diversified and thrived in the Jurassic and Cretaceous before going extinct at the Cretaceous-Tertiary Extinction Event (known in parlance as the K-T Event) some sixty-six million years ago. Over a hundred species have been discovered, giving paleontologists a fantastic window through which to study them and the world they inhabited.

Attenborosaurus flaunting its tail fluke

Plesiosaurs generally had flat bodies and short tails, and their limbs had evolved into four long flippers. The flippers were powered by strong muscles attached to wide bony plates formed by the shoulder girdle and pelvis. All four limbs were used to propel the animal through the water by up-and-down movements; the flippers gave plesiosaurs a ‘flying movement’ through the water, and their tails likely served for directional control (in contrast with ichthyosaurs and the Cretaceous mosasaurs, who used their tails for propulsion). One species of plesiosaur, Attenborosaurus, had a vertical tail fin that would be difficult to fossilize; a possibility of a tail fluke, at least in some species, has been confirmed by recent studies on the caudal neural spines of multiple plesiosaurs. Plesiosaurs were reptiles; as such they breathed air with lungs rather than gills. Oddly enough for reptiles, they gave birth to live young and were likely warm-blooded. Recent computer simulations indicate that plesiosaurs could swim up to 1.8 km/hr if they were cold-blooded and 5.4 km/hr if warm-blooded; thus they were far slower than modern whales, twenty percent slower than the most advanced ichthyosaurs, but five percent faster than the Cretaceous mosasaurs (this speed benefit, however micro, would be helpful, as we have fossilized clues that mosasaurs liked to dine on plesiosaurs). Plesiosaurs come in two main morphological types: the ‘plesiomorph’ build consisted of long necks and small heads; these ‘plesiosaurs’ were relatively slow and fed on small sea animals. The ‘pliosauromorph’ build consisted of short necks and long heads; these ‘pliosaurs’ were the top marine predators, fast hunters of large prey. The Plesiosauria group reflects these morphological types in the group’s subdivision into the long-necked Plesiosauroidea and the short-necked Pliosauridea. 

Plesiosaurs belonged to the Sauropterygia, a group that consists of marine reptiles with terrestrial origins. At some point shortly after the Permian-Triassic extinction, some land-loving reptiles began returning to the sea. An early thread of sauropterygians broke into two branches during the Late Triassic: the Pistosauria and the Nothosauridae (it’s worth noting that many scientists believe that the Nothosaur lineage gave rise to the plesiosaurs rather than the plesiosaurs and nothosaurs arising from a common ancestor). Those creatures belonging to Pistosauria became more adapted to marine life than their nothosaur cousins; the pistosaurs sported stiffened vertebral columns and hands and feet that turned into full-fledged flippers (unlike the nothosaurs, whose hands half-assed the aquatic lifestyle by evolving webbing between the fingers and toes). At some point the pistosaurs became warm-blooded and viviparous (giving birth to live young). The earliest pistosaurians were coastal animals, but later developments enabled some to split off into the wider ocean. This was made possible by reinforced shoulder girdles, flatter pelvises, stiffer joints, shorter tails, and more pointed plesiosaurs. These ‘deep ocean’ pistosaurs are known as the plesiosaurs. Though nothosaurs likely stuck to the coasts, plesiosaurs lived a little more dangerously by venturing deeper into the oceans. Plesiosaurs started off small (one of the earliest plesiosaurs, Thalassiodracon, was only six feet long) but bloomed big by the end of the Cretaceous (Mauisaurus reached fifty-five feet in length). 

the titanic Mauisaurus

Several species of plesiosaur show up in early Jurassic fossil beds, hinting that they diversified as early as the Late Triassic. Plesiosaurs in the early Jurassic were at most sixteen feet long, but by the cusp of the Middle Jurassic they were more numerous and some species developed longer necks (some species reached up to thirty-three feet in length). In the Middle Jurassic, behemoth pliosaurs evolved: they had large heads and short necks, and included such forbidding species as Liopleurodon (which reached up to forty feet and clocked in around twenty-five tons) and Simolestes. Their skulls could reach up to ten feet long (the early Cretaceous Kronosaurus would have a head twelve feet long!), and their bodies could span up to sixty feet in length. Most would’ve weighed around ten tons. These pliosaurs had large, conical teeth and were the apex predators of the day.

Pliosaurus - a.k.a. Predator X - hunting in a Mesozoic ocean

In the Early Cretaceous, small plesiosaurs with stunted necks radiated; but later on in the Early Cretaceous, the elasmosaurs appeared. These infamous plesiosaurs are famous for being among the longest of their kind, reaching up to fifty feet in length due to their long necks that contained as many as seventy-six vertebrae, more than any other known vertebrate. It’s no surprise that half of Elasmosaurus’ length was in its neck and head alone! The pliosaurs, such as the monstrous Pliosaurus funkei (known as ‘Predator X’), shared the ocean with them. The aforementioned Pliosaurus was fifty feet long and weighed around forty-five tons; most notably, its jaws could produce a bite force of 33,000 psi, perhaps the largest bite of any animal in earth’s history. At the beginning of the Late Cretaceous, the ichthyosaurs became extinct; it’s theorized that a new type of plesiosaur, the Polycotylidae, evolved to fill their vacant niches. These plesiosaurs had short necks and peculiarly elongated heads with narrow snouts (reminiscent of the late great ichthyosaurs). Elasmosaurs continued to abound, but all plesiosaurs – of both morphological types – went extinct after the K-T event.

a pliosaur snags himself a plesiosaur buffet

The diet of the big-headed, short-necked pliosaurs is pretty straightforward: they were apex predators, at the top of their food chains, perfectly designed to ambush and pursue prey of all sizes. Their teeth could pierce any soft-bodied prey, especially fish, and their skull structure and jaws were suited for grabbing and shearing their prey. They had great eyesight, and they could reach greater speeds than their plesiosaur counterparts. The diet – and hunting methods – of the long-necked, small-headed plesiosaurs is a different matter entirely. Though it’s generally accepted that they dined on shellfish, bony fish, and hard-bodied cephalopods – their jaws and teeth could pierce tough shells, and some specimens have been found with cephalopod shells in their stomach areas – there’s a lot of contention about how they used their necks. When a creature evolves such a long neck – whether it’s a modern giraffe or a Jurassic sauropod – the question is, “Why?” What purpose did it serve in the plesiosaur’s life? Some have speculated that they used their necks to intercept fast-moving fish, but computer models show that the neck couldn’t move very fast through water due to skin friction. Others have speculated that plesiosaurs rested on the seafloor and used their head to ‘sweep around’ for prey, or that they would swim to the surface and plunge their necks downwards in search of a meal. These last two theories assume that the neck was flexible, but we now know plesiosaur necks were actually quite rigid with limited vertical movement; and so we return to the age-old question, “What’s the point?” As to this, there’s limited agreement. Some scientists wonder if the long neck enabled the plesiosaur to surprise schools of fish before the sight or pressure-wave of the body could alert them; or perhaps they were bottom feeders, using their stiff necks to plough the seafloor and eat benthos (marine organisms living in the ‘benthic zone’ close to the seafloor); or perhaps they were plankton feeders, filtering plankton like modern whales? One species of plesiosaur, Aristonectes, had hundreds of teeth which it could use to sieve small crustaceans from the water. All this is conjecture; what is known is that plesiosaurs weren’t adapted to catching large, fast-moving prey. An interesting twist is that some plesiosaur remains have been found with gastrolith stones in their stomach; the size of the stones indicate they were swallowed on purpose, and it’s believed that the gastroliths may have helped to break down cephalopods in a muscular gizzard. 

a plesiosaur on the prowl for shellfish

One of the strangest things about the plesiosaurs is that they gave birth to live young. Up until the tail end of the 1900s it was believed that plesiosaurs crawled up onto the beach to lay their young. This was a good assumption: they were reptiles, after all, and laying eggs is kind of a reptile thing. Some paleontologists, however, questioned the assumption: first of all, plesiosaur limbs didn’t retain functional elbow or knee joints, which would be needed for the creature to raise itself to lay eggs; secondly, it’s hard to imagine titanic plesiosaurs being able to survive crawling onto dry land to deposit eggs. Scientists knew that ichthyosaurs (also marine reptiles) bore live young, evidenced by fossilized embryos; it wasn’t until 1987, however, that the fossil of a pregnant Polycotylus was unearthed showing that it gave birth to a single large juvenile. This hinted that plesiosaurs, like modern whales, gave birth to live young and operated by a k-strategy for survival, in which they bore less progeny but made up for it by practicing paternal care. 

a Polycotylus giving birth