Plesiosaur Anatomy

The Plesiosauria were more fully adapted to the ocean than the Nothosauria. It is difficult to imagine that they willfully crawled out of the water except when they were still young and small. Imagine how clumsy they would have been on land, dragging themselves along on their paddles. It would have made them extremely vulner--able to dinosaurs and other terrestrial predators. Even the sturdy belly ribs of a 30- to 50-foot (9 to 15 m) plesiosaur or pliosaur might not have been strong enough to support its enormous weight out of the water.

One plesiosaur from the Late Cretaceous was found with the remains of a pterosaur (flying reptile) in its stomach area along with a fish and a cephalopod still in its shell, swallowed whole. Another example found in 1998 provided even more evidence that plesio-saurs were adapted to eat hard-shelled creatures. The specimen from Japan had many remains of ammonite jaws left in its stomach area. The nautiluslike shells that once housed these creatures had evidently been dissolved by the digestive system of the plesiosaur.

Characteristics of the plesiosaur body included:

Medium to long length. Plesiosaurs ranged in length from about 12 to 46 feet (4 to 14 m). Most of this length, however, was taken up by the long neck and short tail. The body was proportionately small compared with the length of the neck.

Long necks, short tails. Unlike nothosaurs, in plesiosaurs, the neck was always much longer than the tail. The neck had from 29 to 72 vertebrae.

Broad but compact body. The plesiosaur body was stiff with an ellipsoid shape. The spine was not flexible, which provided further strength and leverage for the mighty paddles to do their work.

Strong ribs and belly ribs. The stiffness of the plesiosaur body was largely because of its numerous and heavily built ribs. These animals had strong, closely spaced ribs that extended down their sides as well as belly ribs that reinforced their underside. The belly ribs were so long that they almost met the ends of the side ribs, forming a stiff, bony cage for the body. The belly ribs were important for strengthening and protecting the body when the animal was in the water.

Small skull. The skull was broad and short, with a wide, flat snout with a pointed tip. It was triangular in shape when viewed from above.

Teeth and jaws. Plesiosaur teeth were long, sharp, and conical. They were embedded in bony sockets that lined the sides and front of the upper and lower jaws. These teeth were widely enough spaced so that they would interlock when the jaws clamped down on prey. The teeth sometimes pointed outwards from the jaw. Some plesiosaurs, such as Dolichorhynchops ("long snout face") and Libonectes ("southwest wind swimmer"), had long, caninelike fangs in the front part of the jaws. They could lance soft, fleshy prey or penetrate the hard shell of mollusks.

Long, wing--shaped paddles. The paddles of a plesiosaur were highly elongated because of the addition of bones to its fingers and toes. All four paddles were about the same length. They were wing-shaped and allowed the animal to move them up and down to "fly" underwater like a penguin. The ichthyosaurs were most certainly fast swimmers as well.

The long necks of the nothosaurs and plesiosauroids likely slowed them down. It would have been difficult for many long-necked plesiosauroids to pursue their prey in a high-speed chase. Just how the long neck affected plesiosauroid behavior is a matter of speculation. These animals could have hunted using an ambush style, waiting for fishes to pass and lashing out with their necks to snare a few with their wicked teeth. This makes sense for an animal with a long neck, because it could conceal its body behind a rock or other obstacle so that it was hidden from the view of approaching fish. Another view of plesiosaur hunting was proposed in 2006 by paleontologist Leslie Noe of the Sedgwick Museum in Cambridge, England. By modeling the neck of a long-necked plesiosaur, Noe determined that the interlocking vertebrae of the neck and back would have made it difficult for the animal to lift its head, rotate its neck, or move the neck from side to side. The most natural movement would have been a downward bend just below the body of the animal. Noe pictured the plesiosaur as a bottom feeder, picking up mollusks from the ocean floor as it hovered overhead. This hypothesis has been supported by the discovery of mollusk remains as presumed gut contents in the ribcages of some plesiosaur specimens.

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