Water Reptiles of the Past and Present

by Samuel Wendell Williston

Fig. 108.—Geosaurus; skull from side and from above.
(After Fraas.)

The skull of Geosaurus is rather small in comparison with the length of the body, smaller proportionally than in any living crocodile, but not much smaller than that of the teleosaurs. The snout is long and slender, much like that of the teleosaurs and gavials, but the bones of the whole upper surface are quite smooth, not roughened and pitted like those of modern forms. The skull of Dakosaurus, another genus of thalattosuchians, is much less elongate than that of Geosaurus, but has the other characteristics of Geosaurus. The eyes are provided with a stout ring of sclerotic bones, with a pupillary opening of less than one inch. We have seen that all other strictly aquatic reptiles have similar eye bones, but no other crocodiles have them. The internal openings of the nostrils are large and long, but they are not situated far back, as in the modern crocodiles, not even so far back as in the early teleosaurs. They had no need of the peculiar breathing apparatus of the amphibious crocodiles, since all their prey must have been water-breathing creatures. Their eyes were directed laterally, not more or less upward, as in their nearest relatives. Nearly all other crocodiles have an opening through the hind end of the lower jaw, but the thalattosuchians did not. The teeth were about as numerous as in the modern gavials, but they projected freely only a short distance above the gums in life, and they were very slender and sharply pointed, excellently well adapted for catching smooth and slippery fishes. Their vertebrae, like those of all other reptiles of their time, were biconcave. Those of the neck resembled those of the teleosaurs, save that there were only seven, fewer than is the case with any other members of the order. In becoming adapted to their peculiar mode of life these crocodiles lost two vertebrae from the neck. All modern crocodiles have two ribs attached to the first vertebra; the thalattosaurs had but one, another evidence of primitive characters. While the number of vertebrae in the neck was reduced, in the back it was increased to eighteen; all other crocodiles have but fifteen or sixteen. The trunk was long, another adaptation to water life. There were two firmly united vertebrae in the sacrum, as in the modern forms. The reason for the persistence of this terrestrial character we shall see later.

Fig. 109.—Tail, scapula (sc), and coracoid (c) of Geosaurus.
(After Fraas.)

The tail was very long and strong, nearly as long as all the remainder of the body, and relatively much longer than in other crocodiles. It is of interest to observe that the head, neck, body, and tail had almost the same relative proportions as those of the great sea-lizards, the mosasaurs. The terminal bones of the tail are very peculiar, and very different from the corresponding bones of other crocodiles. While the spines of the tail bones along the anterior part are only moderately stout and long, and are directed obliquely backward, near the terminal part they become suddenly much broader and are directed upward, and, a little farther along, obliquely forward. The chevron bones on the under side also here become broader and longer. The end of the tail curves markedly downward to end in a slender point. It will be remembered that a similar downward curvature of the end of the tail observed in nearly all specimens of ichthyosaurs induced in Owen the belief that the animals had a fleshy terminal fin, a belief which later discoveries of the carbonized remains of the flesh confirmed. The peculiar structures observed in various specimens of these sea-crocodiles, even though no impressions or remains of the fleshy parts have been discovered, is quite conclusive evidence that these animals also had a broad, fleshy, terminal fin. No other explanation of the structure is possible.

Fig. 110.—Geosaurus. Elongate hind leg, and paddle-like front leg.
(After Fraas.)

The ribs are not at all stout and are not much curved. They are directed posteriorly in the known specimens preserved in the matrix in such a way as to indicate a slender thorax and abdomen, not the broad body of the modern crocodiles. The abdominal ribs, that is, those protecting the region on the under side of the body between the breast bone and the pelvis, are strongly developed in Geosaurus. The sternum, always present in other crocodiles, is wanting in Geosaurus. The shoulder-blades and coracoids, often changed in shape in water reptiles, are not unlike those of the amphibious crocodiles, but are smaller and flatter.

The fore limbs, to use Professor Fraas’s words, “are among the most interesting of all the peculiarities of Geosaurus,” and show very clearly that these animals were excellent swimmers. The humerus is remarkably short and broad; the two bones of the forearm, the radius and ulna, are broad, rounded, or angular plates, not long bones, as in other crocodiles; the wrist bones also are broad and plate-like. The three bones of the thumb, that is of the digit which received most strongly the impact of the water in swimming, are also broad and flat. All of these bones are marvelously aquatic in type, as will be evident from a comparison of them with the corresponding bones of the ichthyosaurs and mosasaurs. The bones of the other fingers, however, were not much changed from the ordinary crocodilian form, as a further comparison of them with the fingers of a land crocodile will show. Furthermore the whole limb or paddle was very small in comparison with the hind leg, and it was attached very near to the head. The relative proportions of the front and hind limbs in the geosaurs, gavials, and alligators are instructive as showing the progressive decrease in size of the front legs from the subaquatic, through the semiaquatic, to the almost purely aquatic type. The hind legs, strangely enough, were not very different in size and structure from those of the gavials or teleosaurs. The thigh bone is long and slender, though the bones of the leg and ankle are somewhat shortened and flattened, as are also those of the first toe. There were no claws on the hind feet, a distinctly aquatic adaptation, and the toes were certainly webbed. The pelvis, while not especially stout, is of good size, and was firmly attached to the sacrum.

Perhaps all these characters may best be summed up in the words of Professor Fraas, as translated: