SO far as we can gather, these grotesque sea-monsters roamed the shallower seas, feeding upon fish and especially upon cuttle fish, whose remains we actually find in the fossilised stomach-region. As the neck was comparatively immov-
able, we can only imagine that the prey was obtained through speed and the quick snapping movement of the jaws. From a few fossilised stomach contents that we have, we know that the plesiosaurs swallowed pebbles, consequently known as gastroliths or stomach-stones, whose function was to grind up the hard ingested matter and thus aid digestion. It would seem that like most other reptiles the plesiosaurs laid eggs, and that, though well enough adapted for life in the water, they had to return to land to lay their eggs, the inverse process to that we saw in the amphibians.
The fossilised remains are especially common in some geological horizons, and some of the specimens are truly enormous, with skulls six feet long. Thus we can clearly visualise these ancient monsters ploughing their way through the seas near land, playing havoc among the lesser kinds of creatures and probably occasionally attacking their own kind.
Living in the seas at the same time, but better adapted for marine life, were the ‘fish-lizards,’ or Ichthyosaurs. These equally well-known and numerous animals rivalled their contemporaries in size, but were of strikingly different appearance and habit. They were similar to porpoises or large fish. They had a long skull with narrow jaws containing sharp and ribbed conical teeth. The narrow skull had prominent eyes and a small third eye. The neck, in contrast to that of the plesiosaurs, was short, while the body was quite porpoise-like. It had a single, triangular, fleshy dorsal fin, and a tail with a large triangular tail fin, which was supported by the downwardly bent tail vertebras. It is interesting to note that in fish with a similarly shaped tail the supporting vertebras are turned sharply upwards.
As a modification for life in the sea, the nostrils were placed far back on the head and just in front of the eyes; the latter, too, were protected by a ring of hard plates which were also of assistance in focussing. Whereas the sections of the plesio-saurian backbone were moderately long and tended towards inflexibility, the ichthyosaurs had very short centra and consequently considerable flexibility of body.
Swimming was still accomplished by paddles, but here the front limbs greatly exceeded the hind in size, and the paddles consisted of a mosaic of modified bones in a cartilage setting. The skin was smooth and unarmoured. The ichthyosaurs were therefore much more highly adapted for fife in the seas, and must have been powerful swimmers and ferocious enemies.