Plesiosaurs, the Beautiful Bottom-Feeders

In 1821, British geologists Henry De la Beche and William Conybeare presented a bizarre, previously-unknown fossil creature to their colleagues in the Geological Society of London. They called their monster Plesiosaurus. A paddle-legged marine reptile akin to the recently-discovered, shark-shaped animals known as ichthyosaurs, the new animal was cast as “a link between the Ichthyosaurus and Crocodile” in the “connected chain of organized beings.”

Don’t let the language lead you on. De la Beche and Conybeare were not advocating an evolutionary view. Quite the opposite.  Speculation about the transmutation of species was “absurd and extravagant,” the naturalists said. Instead, the Plesiosaurus testified to a natural plan in which “every place capable of supporting animal life should be so filled, and that every possible mode of sustenance should be taken advantage of.” The stout-bodied and paddle-limbed monster was part of the static balance of nature, and the thought that such links indicated true transformation of one being into another was “most ridiculous.” Plesiosaurus simply took up an intermediate place between semi-aquatic crocodiles and the fish-like ichthyosaurs so that there would be no gap in the natural order.

What the naturalists did not realize was that Plesiosaurus was not an intermediate form between crocodiles and ichthyosaurs, after all, but a unique and starkly different sort of marine reptile. Missing bones were what caused them to miscast their mosnter. The specimen De la Beche and Conybeare initially studied was missing the skull and had only twelve neck vertebrae. There was no reason to think that the neck was very much longer, and the skulls of ichthyosaurs and crocodiles hinted that Plesiosaurus, too, would have had an elongated noggin. Three years later, though, Conybeare reported on “an almost perfect skeleton of the Plesiosaurus” which has been discovered by paleontologist and highly-skilled fossil hunter Mary Anning, and he gave the marine reptile a makeover. The better specimen had a small, short skull at the end of a ridiculously long neck consisting of thirty five vertebrae. No other animal had a neck quite like it.

Just what Plesiosaurus was doing with such a long neck was a mystery. Clearly the creature carried such a bizarre form to fill some essential role in the economy of nature, but there was no modern analog to draw clues from. While the animal’s body shape suggested that it swam like a shell-less turtle, Conybeare proposed, might the long neck of Plesiosaurus have obligated the animal to swim at the surface, “arching its long neck like the swan, and occasionally darting it down at the fish which happened to float within its reach?” Alternatively, Conybeare imagined that Plesiosaurus was the underwater equivalent of a snake in the grass, relying on “the suddenness and agility of the attack which [the length of flexibility of its neck] enabled it to make on every animal fitted for its prey, which came within its extensive sweep.”

Conybeare’s twin visions of Plesiosaurus sculling the Mesozoic seas with their heads held high and ambushing unwary fish just beneath the waves remained the canonical visions of these creatures for over a century and a half. In his painting of Cretaceous marine reptiles created in the mid-1950’s, for example, the artist Rudolph Zallinger presented long-necked plesiosaurs in both modes, and in his 1981 book The Dinosaurs William Stout painted a gallery of snake-necked plesiosaurs raising their necks out of the sea to snap at a heard of passing sauropod dinosaurs. These were the plesiosaurs as I first encountered them in my childhood, but paleontologists have known for quite some time that these marine reptiles were not actually capable of such poses.

Contrary to popular images of the marine reptiles, the necks of Plesiosaurus and its kin were not as flexible as cooked spaghetti noodles. Conybeare himself recognized that the necks of these animals were limited in their range of motion by the flattened articular surfaces between the bones, and later paleontologists like marine-reptile expert Samuel Wendell Williston confirmed that the necks of plesiosaurs could not be raised high and were constricted in their ability to sweep from side-to-side. Mike Everhart explained the reasons why in his book Oceans of Kansas: “The close fit, the relatively flat surface of the articulation between the vertebrae, and the lack of space between [projections of bone from the body of the vertebrae called] neural spines and transverse processes on adjoining vertebrae probably meant that there was little movement between the individual vertebrae” in long-necked plesiosaurs.

The inflexibility of the plesiosaur neck makes these creatures all the more puzzling. If they were incapable of raising their heads above the water to ambush prey from above and lacked the flexibility to suddenly dart their heads out of the shadows to gulp down fish, then how were they feeding? Clearly the elongated necks of these animals evolved for some adaptive reason and were useful enough that a diverse array of plesiosaurs were widespread in both space and time. Everhart proposed that the long neck of a hunting plesiosaur would allow the predator to get its head close to a school of fish while its body remained relatively concealed. This idea is a plausible connection between the anatomy of long-necked plesiosaurs and gut contents which indicate that at least some of these animals fed on fish and cephalopods, but plesiosaurs were not limited to snacking on free-swimming prey.

Contrary to the classic image of plesiosaurs of sneaky ambush predators stalking fish through the azure waters of the Jurassic and Cretaceous oceans, some plesiosaurs plucked fast food off the seabottom. The evidence comes from two sets of gut contents described in 2005 by paleontologists Colin McHenry, Alex Cook, and Stephen Wroe in a one-page Science report. Both sets of gut contents – ensconced within the skeletal stomach cavities of two long-necked plesiosaurs found in Australia – were chock-full of fragments from invertebrates which sat and scuttled on the seafloor. (One set was dominated by fragments of bivalve shells, while the other held several crustacean parts.) These plesiosaurs were not fish and squid specialists like their Cretaceous relatives elsewhere, but, at least near the time of their death, subsisted on clams and crustaceans.

From the time of their initial description, long-necked plesiosaurs were thought to have a specialized way of life. Their conical teeth and long necks seemed to make them adept ambush predators which took on schools of small fish while their big-headed pliosaur cousins and mosasaurs tackled larger prey. Yet the brief note by McHenry, Cook, and Wrote indicate that it isn’t as simple as that. “[E]ven structures as apparently specialized as the elasmosaurid neck,” they wrote, “do not necessarily indicate narrow ecology.” Some plesiosaurs were adept fish hunters, others picked up clams from the seafloor, and the discovery of ichthyosaur embryo remnants in plesiosaur gut contents indicates that these creatures were not above scavenging when they had the chance. The skeletal anatomy of plesiosaurs can give us some idea of what their were capable of, but, to really get at their feeding behavior, it’s best to go with their guts.

Top image: A reconstruction of the long-necked plesiosaur Thalassomedon at the American Museum of Natural History. Image from Flickr user Ryan Somma.

References:

Conybeare, W. 1824. On the Discovery of an Almost Perfect Skeleton of the Plesiosaurus. in Weishampel, D., and White, N. eds. The Dinosaur Papers 1676-1906. Washington: Smithsonian Books. pp. 60-67

De la Beche, H.; Conybeare, W. 1821. Notice of the Discovery of a New Fossil Animal, Forming a Link between the Ichthyosaurus and Crocodile, Together with General Remarks on the Osteology of Ichthyosaurus. in Weishampel, D., and White, N. eds. The Dinosaur Papers 1676-1906. Washington: Smithsonian Books. pp. 35-50.

Everhart, M. 2005. Oceans of Kansas. Bloomington: Indiana University Press. pp. 133-134

McHenry CR, Cook AG, & Wroe S (2005). Bottom-feeding plesiosaurs. Science (New York, N.Y.), 310 (5745) PMID: 16210529