Water Reptiles of the Past and Present

by Samuel Wendell Williston

On the other hand, certain unrelated reptiles of the past, the dolichosaurs, nothosaurs, and plesiosaurs, with a short non-propelling tail, developed a long neck—sometimes an excessively long one in the plesiosaurs. The turtles, some of which have attained a high adaptation to water life, have invariably a short tail and a freely movable, relatively long neck, a neck which Dr. Hay tells us has increased in length from the beginning of their race by the simple elongation of the vertebrae, as in the giraffe, and never by the addition of vertebrae. We may then account it a rule that swimming animals with a long neck have a short tail, and those with a short tail have a long flexible neck. Even in the plesiosaurs there is some variation of the length of the tail in correlation with the neck. Short-tailed animals must necessarily propel themselves through the water by the aid of their legs, especially the hind legs. If one watches an actively swimming alligator he will observe that the front legs are folded or collapsed by the side of the body, while the hind legs, much bent, are used only slightly in propulsion. The animal swims by a marked sinuous or serpentine movement, like that of a snake upon land, extending throughout the tail and part of the body, at least. An animal propelling itself by its limbs could not move sinuously, and use its legs actively at the same time, and it is probable that the long neck has been evolved compensatorily.

With this shortening of the neck and sinuosity of movement there is developed in every case a long trunk as well as a long tail. The trunk becomes more slender and cylindrical, more like that of a snake, with an actual increase of the bones composing it, reaching the great number of forty-three vertebrae in that most sinuous of all water reptiles with legs, Pleurosaurus of the Protorosauria. And the tail, primitively having perhaps sixty or seventy vertebrae, may have as many as one hundred and fifty in the more typical aquatic forms. This elongation of trunk and tail must be of great advantage to the swimming reptile, just as the racing scull is a more perfect type of speedy craft than a flat-bottomed scow. Dr. Woodward has said that the fate of all fishes, if they continue their evolution long enough, is to become eel-like.

Not only was the tail greatly elongated in swimming reptiles, but it was also more or less flattened. In the beginning of water adaptation the flattening was throughout the tail, as in the living alligators and crocodiles. As the adaptation to water life became more perfect, the flattening became more and more restricted to the extremity; that is, the flattening begins like that of a salamander and in the end becomes like that of a fish, a terminal fin. And some of the actual stages in the evolution of the fish-like fin have been observed by Dr. Merriam in the earlier and more primitive ichthyosaurs of California. In those animals swimming chiefly in a horizontal direction the tail fin has become like that of fishes, that is, vertical; but in those animals which use the tail chiefly for ascending and descending rapidly in the water the fin is developed in a horizontal position, examples of which are seen in the flukes of whales and sirenians.

All animals living upon the land require firm articulations between the different bones of the skeleton, and especially between the vertebrae, for the support and control of the body. Among aquatic animals there is a strong tendency toward looseness of joints, with increasing flexibility. Fishes have the articular processes between the arches of the vertebrae feebly or not at all developed, and the centra or bodies of the vertebrae have thick pads of cartilage between them. Firm union between the vertebrae would restrict freedom of movement, and firmness is not required when the body is surrounded on all sides by water of nearly the same specific gravity as the body itself. And it is doubtless for the same reasons that the articulations of all strictly aquatic reptiles have for the most part become looser and less firm, especially those between the different vertebrae.

The same looseness of articulation is also found in the ribs of aquatic animals. In most animals, and in all those which walk erect, like the mammals, each rib is firmly attached to the backbone by two distinct joints, the head and tubercle, with an interval between them. This double attachment prevents much in-and-out movement of the ribs and gives a firm support for the attachment of the muscles of respiration, as well as for those supporting the viscera. This firmness is unnecessary in animals living always in the water, and the ribs therefore in all aquatic animals tend to become single-headed and loose. The lower or capitular articulation has been lost in part, or almost wholly, in many cetaceans. It has been said that a whale cast up on land will die of suffocation, not for the lack of air, for it is an air-breathing animal like ourselves, but because it can no longer use its respiratory muscles attached to the loosely articulated ribs; it suffocates because the ribs collapse.

As would be expected, the greatest modifications of structure in the adaptation of air-breathers to water life are found in the limbs. No other parts of the body have such different functions in water and on land as the limbs and fins. The limbs of a dog, or a cat, or a man are feeble organs for swimming in comparison with the fins of a fish, and if the land animal must compete with fishes to prey upon them for food it must acquire like swimming powers. As a matter of fact, the limbs of all typically aquatic air-breathing animals have lost nearly all external resemblance to the legs of walking and running animals, and have become more or less fin-like in function—fin-like in shape and function, but never fin-like in actual structure. No creature can go back and begin over again, any more than a man can again become a child with all its possibilities for improvement and development. If an animal cannot modify the organs it already possesses so as to adapt them to new and changed uses by the aid of evolutionary forces it must fail in the struggle. It can never acquire new material, never get new fingers and toes, new organs or parts of organs; all its possibilities lie in the improved and new uses it can make of the material which it received from its ancestors.

The beginning of aquatic adaptation of the limbs lies in the membranous webs between the toes of frogs, salamanders, ducks, seal, otters, etc., where the feet are used largely or entirely for propulsion through the water, in the absence of a propelling tail. And this membrane, in the majority of cases, is the extent of aquatic adaptation in air-breathing animals. In those animals, however, such as most of the reptiles described in the following pages, where the tail has developed as the propelling organ, the limbs lose to a greater or less extent their propelling function and become merely organs of equilibration and control. Of the two pairs of fins of fishes it is evident that the anterior ones have the more important equilibrational function; the hind ones have a much less important use as guiding organs; as a matter of fact, in not a few fishes the hind or pelvic fins have actually migrated forward to supplement the function of the pectoral fins. It is for these reasons that those animals best adapted of all for life in the water—the whales and sirenians—have lost the hind legs completely. In other tail-propelled air-breathers the hind legs have become progressively smaller and less powerful than the front ones. In all short-tailed water animals, however, where the legs, and especially the hind legs, have the important function of propulsion to subserve, they still retain the large size and firm connections with the body, examples of which will be seen in the seals, sea-otters, marine turtles, and plesiosaurs.

Because the legs are no longer needed for the support or propulsion of the body in long-tailed air-breathers, their connection with the body becomes less and less firm, long before their entire disappearance. In animals using the legs for crawling or walking the bones of an arm and thigh are elongated, and the joints are always well formed, permitting varied, extensive, and firm movements. Just the reverse is the tendency in all those animals that propel themselves by the aid of the tail in the water, since here what is needed is broad, short limbs, not long and slender ones.

Most reptiles have five digits on each hand or foot; the bones of the wrist and ankle are well formed, as in mammals, and the digits are elongate, with a very definite arrangement of the bones composing them, as already described, never exceeding five in any one finger or toe.

In the paddles of water reptiles, as the limbs are usually called, the bones of the first segment, that is, the humerus and femur, are always greatly shortened in those having a propelling tail, and even in some with a short tail, such as the seals, and in a lesser degree in the sea-otters. On the other hand, in those animals which use the legs chiefly for direct propulsion these bones are elongated, as exemplified by the plesiosaurs and marine turtles. In all save the seals and their kind, and the otters, whose legs are used rather as sculls than as oars, the bones of the next segment, the radius and ulna of the front pair, the tibia and fibula of the hind pair, are always shortened, and one can tell the stage of aquatic adaptation, as exemplified, for instance, in the plesiosaurs and ichthyosaurs by the degree of shortening of these bones. Indeed, the first suggestion in any crawling animal of water habits is shown in the relative lengths of the epipodial bones, as these bones are called. Furthermore, cursorial or terrestrial habits are suggested by the relative size of the smaller bone of the leg, that on the little-toe side, the fibula. In birds, pterodactyls, and most running animals, it disappears in part or wholly. In swimming animals it tends to grow larger than the tibia, as will be conspicuously seen in the paddle of the mosasaurs.