Water Reptiles of the Past and Present

by Samuel Wendell Williston

Fig. 56.—Front paddle
of Ophthalmosaurus
(after Andrews):
h, humerus; r, radius; u,
ulna; p, pisiform; re, radiale;
int, intermedium; ue, ulnare.

Fig. 57.—Front
paddle of Merriamia,
a Triassic ichthyosaur.
(After Merriam.)
Explanations as in
Fig. 56.

And there was an increase in the ichthyosaurs, in some not only of the number of digits in each limb, but in all of the number of bones in each digit, a character found also in the unrelated mosasaurs and plesiosaurs. This increase in finger and toe bones, or hyperphalangy as it is called, is one of the most peculiar of all the adaptations to water life, changing the feet and hands from the ordinary walking type to the fish-like swimming type. The bones beyond the humerus and femur in the ichthyosaurs were so increased in number and so changed in form and relations that they bear little resemblance to the corresponding bones of other reptiles. They are merely polygonal platelets of bone, articulating on all sides and fitting closely together, permitting flexibility, but not much else.

It is now believed that the increase, not only of additional digits, sometimes to as many as ten in each hand and foot, but of the finger and toe bones as well, was the result of a sort of vegetative reproduction. The margins and ends of the flippers were doubtless hardened by cartilage or fibrous material, and because of the action of the limbs this cartilagenous material broke up into nodules each of which took on ossification finally. Among the whales, where hyperphalangy also occurs, though to a less extent, it has been thought that the increase in number has been due simply to the ossification of the parts of each bone normally present, that is, to the epiphyses, which became separated from the shaft of each bone. But this explanation will hardly suffice for the fingers and toes of the plesiosaurs and ichthyosaurs, for there are altogether too many such ossifications; and besides, the bones in these animals, as in most reptiles, did not have epiphyses, or terminal separate ossifications of the bones of the skeleton.

It will be observed from the figures that the arm and thigh bones of Ichthyosaurus are very much shortened—a striking adaptation to water life, so conspicuously seen in the modern whales and dolphins as well as in the mosasaurs, thalattosaurs, etc. So characteristic indeed is this shortening that, were every other bone of the skeleton of an ichthyosaur unknown save the humerus or femur, it would be quite certain from these alone that the animal was thoroughly aquatic in habit.

About sixty years ago a rather aberrant form of ichthyosaur, now known as Mixosaurus, was discovered in rocks of Triassic age, that is, of much greater age than any ichthyosaurs previously found, in which not only the forearm but also the lower leg bones were longer, resembling more the corresponding bones of land animals. It was from the examination of specimens in 1887 of these mixosaurs that the late Professor Baur became convinced that the ichthyosaurs were the descendants of land reptiles, and not directly of the fishes as they were universally thought to have been at that time. As Professor Baur very pertinently said, if the ichthyosaurs were descended from the fishes directly, the earliest forms should be more nearly like the fishes than the later ones, whereas just the opposite was the real fact. The arguments which he gave in support of his contention were so convincing that they found immediate acceptance among all naturalists. Fortunately within the past fifteen years many other remains of early ichthyosaurs from the Triassic rocks of California have been brought to light by Professor Merriam, remains which throw a flood of light upon the early, though not the earliest, history of these strange reptiles. He has recognized among the forms he has discovered, not only new species, but several new genera, and perhaps new families of ichthyosaurs. His studies have demonstrated so well the stages of evolution between the early ichthyosaurs and the later ones in their progressive adaptation to water life that it will be of interest to summarize them here.

In the early ichthyosaurs locomotion was largely by the aid of the limbs; in the later ones almost exclusively by the aid of the caudal fin. In the former the paddles were larger and the bones longer, more like those of land animals; in the latter they were relatively smaller and shorter, and more fin-like. In the digits of the early forms the finger and toe bones were more elongated and fewer in number. The hind limbs were nearly as large as the front ones in the Triassic, often very much smaller in the later ichthyosaurs; and the increased number of digits occurs only in the later forms.

In the Triassic ichthyosaurs, all classed in the family Mixosauridae, the pelvis was larger and more firmly connected with the body than in the later forms.

The skull of the early forms was relatively shorter, as compared with the trunk, the jaws shorter as compared with the head, the eyes were relatively small, the teeth in some less numerous, and set in distinct sockets like those of land reptiles; the vertebrae were relatively longer and less fish-like, and their articulations more like those of land reptiles.