The pliosaurs first appeared in the Early Jurassic, alongside their ancestors, the plesiosauroids. The pliosaurs became the tigers of the Mesozoic seas, chasing and overpowering the large sea-creatures such as sharks, large squid, ichthyosaurs and even their relatives, the plesiosauroids. To do this, they evolved a large head with very strong teeth and jaws, powered by huge jaw muscles. Some pliosaurs had heads almost 10 ft/3 m long. Their bodies were streamlined for speed by progressively shortening the neck. Some had as few as 13 vertebrae, compared to at least 28 in the shortest-necked plesiosaur. They also became larger, in order to tackle even larger prey.
name: Macroplata time: Early Jurassic locality: Europe (England) size: 15 ft/4.5 m long
This early pliosaur had a slender crocodilelike skull which was only a little larger proportionally than that of early plesiosaurs. And it still had quite a long neck, with 29 slightly shortened vertebrae, which was fully twice the length of its head.
As in the plesiosaurs, the pliosaurs progressively improved the limbs into powerful paddles, with a great increase in the number of bones in the digits. But the hindlimb, not the forelimb, became the larger, implying a difference in the use of the limbs in the 2 groups.
name: Peloneustes time: Late Jurassic locality: Europe (England and USSR) size: 10 ft/3 m long
Although smaller than Macroplata, this Late Jurassic pliosaur shows an advance in the trend to increase the size of the head and to shorten the neck. It had only about 20 neck vertebrae, so that the large head was almost equal to it in length.
With this more steamlined shape, Peloneustes was able to swim rapidly after its fast-moving prey, such as squid, cuttlefish and ammonites. The long head of Peloneustes gave it the reach that its short neck did not allow. Its teeth were adapted for its specialized diet: they were fewer and less sharp than those of the fish-eating plesiosaurs, and better for catching soft-bodied squid and crushing the hard shells of ammonites.
The stomach contents have been preserved in some of the pliosaurs found. The bulk of the undigested mass consisted of hooks from the suckers that cover the feeding arms of cephalopods.
name: Liopleurodon time: Late Jurassic locality: Europe (England, France,
Germany and USSR) size: 39 ft/12 m long This large pliosaur was typical of the later members of the family. It was whalelike in appearance, with a heavy head, short, thick neck and a streamlined body. From the structure of the limb girdles, it is evident that this pliosaur was extremely maneuverable in the water, and could swim at all depths.
The front flippers were used in an up-and-down motion, like those of the plesiosaurs. The strong downward stroke would have pushed water to the rear, so propelling the animal forward. On the recovery stroke, the flippers would have been lifted up automatically by the passage of water over their hydro-dynamic shape.
The hind flippers would have thrust back against the water in a powerful kicking motion, and then been turned, to offer the least resistance to the water on the recovery stroke.
This combination of movements would have made for efficient, fast long-distance swimming. This, in turn, enabled the pliosaur to sustain the chase after its fast-moving cephalopod prey.
name: Kronosaurus time: Early Cretaceous locality: Australia (Queensland) size: 42 ft/12.8 m long
The Australian Kronosaurus is the largest-known pliosaur. Its skull was flat-topped and massively long, measuring 9 ft/2.7 m — almost a quarter of the total body length, and therefore substantially larger and more powerful than that of the greatest carnivorous dinosaur, Tyrannosaurus (see pp. 118-121).
Throughout the Triassic and Jurassic periods, the modern continent of Australia had been dry land, but in the early part of the Cretaceous, the seas flooded in to submerge many areas. The environment was warm and the seas shallow, and these conditions would have supported large populations of fish and their cephalopod predators. Kronosaurus and other short-necked pliosaurs were highly maneuverable swimmers, and they would have found rich feeding grounds in these shallow seas.
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