Water Reptiles of the Past and Present

by Samuel Wendell Williston

The general aspect of the ichthyosaurs was very dolphin-like. The body was everywhere naked and probably dark in color. The head was produced in front into a long, slender snout, and was closely joined to the body posteriorly without indications of a neck. The body itself was cylindrical, expanded in front by the large thorax and abdomen, but rapidly diminishing into the long, slender, and strong tail. Close behind the head were the front paddles, which in some species were broad and shovel-like, in others elongated and pointed. The hind paddles were smaller than the front ones, sometimes greatly reduced in size, their function replaced by that of the very broad tail.

From the foregoing descriptions and the restoration shown in [Fig. 51], we see how very fish-like, or rather dolphin-like, these animals were in the external form—so fish-like that the name Ichthyosaurus is not misleading, though Koenig gave it in the mistaken belief that they were really allied to the fishes. When to these external features certain other fish-like details of the skeleton are added, we do not wonder that the early observers were so long in doubt about them. A more careful examination of the skeleton will, however, disclose so many truly reptilian characters that their external appearance and habits lose all significance.

The vertebrae are deeply biconcave and fish-like, it is true, but a consideration of the reasons therefor will convince us that any other kind of vertebrae would be more remarkable. At the time when the ichthyosaurs must have originated, at the time when the first known ichthyosaurs appeared in geological history indeed, all reptiles had biconcave vertebrae, and for the most part at least deeply biconcave ones. The vertebrae remained fish-like throughout all their history, perpetuating their type until most other reptiles had developed a firmer one, because such vertebrae were best adapted for the quick, pliant movements of the spinal column so necessary for the well-being of the animals in the water. In the modern dolphins, animals in shape, size, and habits most wonderfully allied to what these old reptiles must have been, the small, flat-ended vertebrae are widely separated by disks of flexible cartilage.

Not only were their vertebrae fish-like in form, but there are other characters in the spinal column of a primitive or generalized nature. As in all aquatic animals, the articulating processes between the vertebrae are either weak or wanting in the posterior part of the column. And they were not only small, but were situated, in many, high up, very remarkably resembling the peculiar arrangement of the articulations in the dolphins.

There is no sacrum, that is, there were no united vertebrae posteriorly for the attachment and support of the pelvis, as no such support was needed. In only one other group of aquatic reptiles was the sacrum lost, though it has wholly disappeared in the cetaceans and sirenians among mammals. The chevron bones of the tail, usually bony arches on the under side of the tail for the protection of the blood-vessels, in crawling reptiles, were very imperfectly developed in the later forms, though normal in shape in the early ones. The ribs are numerous, long, and slender, very much resembling those of the fish-eating dolphins. They usually had, however, two attachments to the body of the vertebra and none to the arch, differing in this respect from all other animals.

Of the shoulder bones, the scapula or shoulder-blade, as usual among water animals, is short and broad. In the place of a sternum the coracoids joined each other broadly in the middle, just as they did in the oldest known land reptiles. And there were clavicles and an interclavicle. Below the abdomen behind were numerous slender bones called ventral ribs. The pelvis is very weak, and was suspended below the spinal column in the fleshy walls of the abdomen. The hind legs were so small that little support was necessary for them, and, because they were not used either for the support of the body or for propulsion, they did not require a firm union with the skeleton. Doubtless had the ichthyosaurs continued to the present time, they would have lost entirely the hind legs, as have the cetaceans.

Fig. 55.—Pectoral girdle of Baptanodon (Ophthalmosaurus), an American Upper Jurassic ichthyosaur. (After Gilmore.)

It is in the limbs that most extraordinary differences from all other animals are seen. So great are these differences that it has been a puzzle to naturalists to understand how they could have arisen. In no other animals above the fishes, that is, in no other reptiles, in no amphibians, birds, or mammals, are there ever more than five fingers or five toes, the number with which air-breathing animals began. Fingers and toes may be lost and often are lost in all groups of life, until a single one in each limb may remain, as in the domestic horse. An increase of fingers and toes, however, seems to be an impossibility in evolution, and doubtless of real fingers and toes it is an equal impossibility. All naturalists are now agreed that a specialized character can never revert to a generalized condition, or rather to a generalized structure, that an organ once functionally lost can never be regained by descendants. A character once lost is lost forever; horses of the future can never have more than one finger or one toe in each limb.