The early members of this group of long-necked marine reptiles were the basal stock from which all other plesiosaurs developed. The group first appeared in Early Jurassic times, and flourished throughout that period. One family, the elasmosaurs (below), continued to the very end of the Cretaceous period, and were the last of the group to survive.
The general trend among the various families of plesiosaur was toward the development of longer necks and limbs. This reached an extreme in later members of the group, some of which had necks as long as their bodies and tails combined, and huge, paddlelike flippers that would have swept them inexorably through the water. The forelimbs of these plesiosaurs were always somewhat larger than the hindlimbs.
The plesiosauroids fed on modest-sized fishes and squid. Their long necks enabled them to raise their head high above the surface of the sea and scan the waves in search of traces of their prey.
name: Plesiosaurus time: Early Jurassic locality: Europe (England and
Germany) size: 7 ft 6 in/2.3 m long Plesiosaurs seem to have changed little during their 135 million years of evolution. The earliest member of the group, Plesiosaurus, had already developed all the main structural features that characterize these marine reptiles.
There were several species of the genus Plesiosaurus. P. macrocephalus, for example, illustrated on p. 74, had a lar
ger head than most species, but in other respects its structure sets the pattern for all its relatives.
A plesiosaur was built for maneuverability, rather than speed. Its fish-hunting habits would have required quite precise movements. For example, a forward stroke by the flippers on one side of the body, coupled with a backward stroke by the flippers on the other side, would have turned the animal's short body almost on the spot. Its long neck could then dart out swiftly to catch fast-swimming prey.
name: Cryptocleidus time: Late Jurassic locality: Europe (England) size: 13 ft/4 m long
Cryptocleidus and other members of its family retained the same moderately long neck proportions as Plesiosaurus. But they evolved a large number of very sharply pointed curved teeth which in-termeshed when the jaws were closed. This arrangement formed a fine trap for holding very small fishes or shrimps.
Like other Late Jurassic plesiosaurs, Cryptocleidus had perfected the transformation of the limbs into flippers by greatly increasing the number of bones in each of the 5 digits to produce a long, flexible paddle.
name: Muraenosaurus time: Late Jurassic locality: Europe (England and
France) size: 20 ft/6 m long
The most successful family of plesiosaurs were the elasmosaurs, of which Muraenosaurus is an early member. They evolved in the middle of the Jurassic period and survived to the end of the Cretaceous. Elasmosaurs had the longest necks of all plesiosaurs.
The neck of Muraenosaurus was as long as its body and tail combined and
was supported by 44 vertebrae. The head, perched at the end of this cranelike neck, was tiny — only about one-sixteenth of the total body length.
The typically short, stiff plesiosaur body had become quite stout and inflexible in Muraenosaurus. This rigidity would have helped to make the flippers more effective propulsion organs.
muraenosaurus name: Elasmosaurus time: Late Cretaceous locality: Asia (Japan) and North
America (Kansas) size: 46 ft/14 m long "Snakes threaded through the bodies of turtles" — this description of the long-necked plesiosaurs was coined by Dean Conybeare, a nineteenth-century English paleontologist who did much of the initial work on these marine reptiles.
Conybeare's description is more than vindicated in Elasmosaurus, which was the longest member of the elasmosaur family, and, in fact, the longest-known plesiosaur. More than half of its total length was neck — 26 ft/8 m out of a total of 46 ft/14 m.
The length of Elasmosaurus' neck was due to a great number of vertebrae, 71 in all — many more than in the earliest plesiosaurs, which had about 28 neck vertebrae.
This long structure would have enabled Elasmosaurus to curl its neck around sideways, making almost 2 full circles on either side of its body. It would have been only half as flexible in the vertical plane. However, had Elasmosaurus swung its neck around underwater while swimming, it would have met with great resistance from the water.
Some paleontologists suggest, therefore, that the habit of such long-necked reptiles was to paddle along on the surface, their necks held clear of the water. When fish or other prey were spotted from this vantage point, the long neck was plunged into the sea, and the prey snapped up. The modern anhinga, or snake bird, has a long neck, and hunts in much the same way.
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