Water Reptiles of the Past and Present

by Samuel Wendell Williston

PHYTOSAURIA

The Phytosauria, so far as known, were all reptiles of considerable size, greatly resembling the crocodiles, and especially the gavials in form and habit, but differing very greatly in having the external nostrils situated far back near the eyes; in having no false palate so characteristic of the Crocodilia; in having a more primitive shoulder-girdle, consisting of a short coracoid, interclavicle, and clavicles; and in having the ordinary type of pelvis, that is, with the pubis entering into the acetabular articulation for the femur. They were all, like the crocodiles, covered more or less by a bony armor; there are two openings on each side of the temporal region; there is no pineal opening; the vertebrae are gently biconcave, precisely like those of the early or mesosuchian crocodiles; there is always an opening of considerable size, called the preorbital foramen, in front of the eyes, as in some crocodiles, many dinosaurs, and most pterodactyls; there is also an opening through the back part of the mandibles as in crocodiles; and the double-headed ribs are attached exclusively to the transverse process of the arch, precisely as in the crocodiles, dinosaurs, and pterodactyls. From all these it is evident that the phytosaurs are related most nearly to the crocodiles and dinosaurs, and are probably an early branch of the stem from which they, the pterodactyls and the birds, arose, a branch that persisted only a short time, geologically speaking, and went entirely out of existence at the close of Triassic times, leaving no descendants behind. Nevertheless, in this comparatively brief life-span they developed not a few distinctive forms and became widely distributed over the earth. Their remains are known from the Upper Trias of Germany, England, and Scotland, India, South Africa, and from Massachusetts, North and South Carolina, and many places in the Rocky Mountains. No true phytosaurs are yet known from South America, but in all probability they will be discovered there when the Triassic deposits of that continent have been better explored for fossils. In the Rocky Mountains, especially, their remains are widely scattered, they have been found in many localities in Wyoming, Colorado, Oklahoma, Utah, and New Mexico. Though for the most part their known remains from these localities are yet fragmentary, not less than four distinct genera have been described from these regions: “Belodon,” Angistorhinus, Paleorhinus, and Episcoposaurus. From the Carolinas and Massachusetts a single genus, though described under numerous names, has been made known, originally called by Emmons Rutiodon (Rhytidodon). And from Europe and India at least as many more different genera are known. All these genera are, however, so closely allied that they are placed in the single family Belodontidae.

Fig. 92.—Restoration of Mystriosuchus,
an Upper Triassic phytosaur.

Fig. 93.—Skull of Mystriosuchus, a phytosaur: pm, premaxilla; m, maxilla; na, nasal; f, frontal; p, prefrontal; l, lacrimal; pf, postfrontal; po, postorbital; pa, parietal; sg, squamosal; qj, quadratojugal; pl, palatine; t, transverse; in, internal nares; en, external nares; pt, pterygoid; bs, basisphenoid; eo, exoccipital. (After McGregor.)

Fig. 94.—Dorsal vertebrae
of phytosaur: az, anterior zygapophysis;
pz, posterior zygapophysis; d,
c, articulations of rib.

Fig. 95.—Scapula and
coracoid of Rutiodon carolinensis,
an American phytosaur.
(After McGregor.)

Fig. 96.—Belodon; restoration of head,
from above.

Fig. 97.—Mystriosuchus: restoration of head,
from above.

In Belodon ([Fig. 96]), the earliest known and most typical genus, perhaps, the moderately elongated face has a high crest reaching nearly to its front end, and this type is known both from Europe and from New Mexico. Others have the face long and slender, even longer and more slender than in the ancient teleosaur crocodiles or the modern gavials. In some forms the teeth are cylindrical and slender throughout, and there may be as many as fifty on each jaw, or two hundred in all; while in others only the anterior teeth are cylindrical and the posterior teeth are flattened and serrate along their cutting edges. In the body not very great differences have been observed. Some are more slender than others, and there are minor differences in the shapes and sizes and numbers of the bony scutes along the back and on the throat.

But they are all alike in their essential characters, a very long beak with numerous teeth; the foremost ones on the expanded, more or less spoon-shaped front extremity, are more or less, sometimes greatly, elongated. The jaws may be likened to a long and slender pair of tongs with nipping teeth at the front end. The strong, long, and flattened tail is sufficient evidence that the phytosaurs were excellent swimmers, but, aside from that and the posterior location of the external nostrils, directly over the internal, few other aquatic adaptations are observed in the skeleton. There are no sclerotic bony plates about the eyes, or at least none have so far been discovered, although among the numerous known specimens they would confidently be expected were they really present in the skeleton; and the presence of bony armor negatives markedly aquatic habits.

Doubtless on the whole the habits of the phytosaurs were not very unlike those of the modern gavials, which they so strongly resemble in form, size, and general characters. But they differ very greatly from the gavials in the extreme posterior position of the nostrils, and in the greatly elongated teeth of the front end of the beak, teeth which must have had some especial and peculiar use. Nor is the position of the nares to be accounted for satisfactorily by reference to aquatic habits. It has been suggested that the creatures used the very long and slender beak in prodding and probing in the sand and mud for soft-bodied invertebrates, worms and the like, for which the teeth would be especially fitted; and that the posterior position of the nostrils may be in part, perhaps wholly, accounted for by this habit, which permitted the reptiles to breathe without extricating the beak from the mud or shallow waters. That the animals were wholly and intensely carnivorous in habit is attested by their teeth; although they are called “plant saurians,” they never had anything to do with plants in the way of food. Unfortunately so far no specimens have ever been found showing the remains of stomach contents, nor have any been found showing impressions of the form of the body or of any of its parts. Until such specimens are found, as they doubtless will be eventually, one can be less sure of the precise details in their life reconstructions. However, the skeleton is now known nearly completely, and this suffices to give a very approximately correct idea of what the animals were like when alive.

CHAPTER XV
CROCODILIA

The order of reptiles to which the name Crocodilia is technically applied comprises less than twenty-five living species, popularly known as crocodiles, alligators, caimans, and gavials. They are often of great size, ugly and repulsive in appearance, cruel and vicious in habit, wholly carnivorous, and denizens, almost exclusively, of fresh-water lakes or rivers in tropical and subtropical regions; a few only venture into the sea near the shores. They are all excellent and powerful swimmers, but are by no means exclusively aquatic in habit, many of them spending a large part of the time on the shores; and they invariably seek the land for the deposition and hatching of their eggs. In size they are the largest of living reptiles, some of the existing species reaching a length of twenty-five feet, while some extinct species were probably fully twice that length.

The geological history of the crocodiles is a very ancient one, reaching back at least as far as the early Jurassic and probably to the Triassic. The culmination of the order, at least so far as size, variety, and numbers are concerned, was doubtless before the close of the Mesozoic. The early crocodiles, however, were of a more generalized structure in some respects, though specialized in others, because of which naturalists in the past have usually divided the order into three or four chief subdivisions, or suborders, two of which, the Mesosuchia and the Thalattosuchia, became extinct before or during Cretaceous time. The third suborder, the Parasuchia of many textbooks, has now been unanimously separated by paleontologists from the Crocodilia as an independent order. The fourth suborder of the textbooks, the Eusuchia, a word meaning true crocodiles, appeared in geological history, so far as we yet know, shortly before the extinction of the Mesosuchia, and is best known from the forms now living, all of which belong to it. Although the modern crocodiles can hardly be called, as a group, purely aquatic reptiles, we shall find it of interest, because of their intimate relation to the older and more strictly marine forms, to speak of them somewhat in detail.

MODERN CROCODILES,
EUSUCHIA

The crocodiles of the present—and we use the word in the technical sense of Crocodilia—because of their general resemblance to the lizards, or true “saurians,” were classed with them by the older naturalists, whence comes the popular name alligator, a corruption of the Spanish el lagarto, or “the lizard,” given to some of the South American forms by early explorers. But this resemblance is a superficial one only, as was early recognized by comparative anatomists. The crocodiles, indeed, are only remotely related to the lizards.

Fig. 98.—Senegal crocodile.
(By permission of the New York Zoölogical Society.)

The head or cranium is flattened and broad, the facial part or snout sometimes greatly elongated and slender, and the external nostrils are always situated at the front end. The bones of the upper surface of the cranium and face have many pit-like depressions. The neck is short and stout, and but little movable. The body is somewhat depressed and flattened, not cylindrical and slender, as in the more typical water reptiles. The tail is much elongated and compressed, forming a powerful means of propulsion in swimming, its breadth being increased by a vertical row of horny plates above. The limbs are of the ordinary elongated type—ambulatory rather than swimming legs—and are not of much use for propelling the body in the water; the front legs indeed are usually held close to the body while the animal is swimming. The toes, however, are partially connected by webs, to a slight extent only in the alligators and crocodiles, but much more so in the long-snouted gavials. The feet have five toes in front and four behind; and the loss of the fifth toe can only be ascribed to terrestrial habits. The body is covered more or less with horny scutes or scales, beneath which are several rows of thickened, pitted, bony plates on the dorsal side, and sometimes also on the under side, forming a more or less extensive bony armor. The eyes have movable lids, as in most lizards, and the ear-opening is small.

But the external appearance of these reptiles is not sufficient to distinguish them widely from other groups, and we must resort to the internal structure, especially that of the skeleton, for the more essential differential characters. The most crucial of these, the one which more than any other determines their relationships, consists in the position and shape of the bone with which the lower jaw articulates, the quadrate bone, so characteristic of reptiles. As in the plesiosaurian and ichthyosaurian skulls, it is firmly united with the adjoining bones, not articulating freely with them, as in the lizards and snakes. But this fixed relation of the bones is very unlike that of the plesiosaurs, ichthyosaurs, and turtles, in that it is held in place by two bony bars or arches, the upper extending forward to unite with the bones at the back part of the orbit, the lower, with the hind extremity of the upper jaw. The lower jaws are rigidly united in front, sometimes for a long distance; they have, almost always, a hole or opening through the hinder part, known in but few other reptiles. The bones of the palate are all firmly united, forming a nearly complete roof, very unlike the condition in the mosasaurs and lizards. The palate also is very peculiar in the development of a plate of bone below the nasal chambers, forming a complete bony canal on each side through which the respiratory air passes far back to the internal opening of the nostrils above the pharynx, and not, as in other reptiles—save the immediate ancestors of the mammals—entering the mouth at the front end. This peculiar arrangement of the air-passages, so like that of ourselves, has much to do with the water habits of the crocodiles, as we shall see.

The teeth are conical in shape, and are always restricted to the edges of the jaws, above and below. They are inserted deeply and firmly in sockets, and are replaced frequently by new ones growing beneath them, pushing the older ones out as their usefulness becomes impaired by injury or by use. In some species there are as many as thirty teeth in each side of the jaws, above and below, although the broad-headed kinds have a smaller number.

Fig. 99.

Fig. 100.

Fig. 99.—Skull of Alligator mississippiensis, from below.

Fig. 100.—The same, from above: bo, basioccipital; bs, basisphenoid; f, frontal; j, jugal; l, lacrimal; m, maxilla; n, nasal; p, parietal; pa, palatine; pm, premaxilla; pf, prefrontal; pr, postfrontal; pt, pterygoid; q, quadrate; qj, quadratojugal; tr, transverse.

The neck is short, as has been stated, but it always includes in living forms nine vertebrae, a number probably slightly in excess of that of their terrestrial forbears. By the peculiar mode of attachment of the short “hatchet-shaped” ribs, much lateral movement of the neck is prohibited, nor is any very great vertical movement possible. The vertebrae of the whole column, save the atlas, the second sacral, and the first caudal—which is a very remarkable anomaly—are concave in front and convex behind, agreeing in this respect with those of all other living reptiles, save the turtles, the tuatera, and some lizards. The ribs of the neck have their two heads attached, one to the body of the vertebra, the other to the arch, but those of the dorsal region, though double-headed, have both become attached to the transverse projection of the arch, a seemingly trivial character, but one which immediately distinguishes all crocodiles from all other water reptiles, and from all terrestrial reptiles, indeed, save the Parasuchia, Pterosauria, and Dinosauria. The pelvis is firmly attached to the spinal column by two sacral vertebrae.

Fig. 101.—Vertebrae of gavial from the side (cervical), and from in front (dorsal): az, anterior zygapophysis; pz, posterior zygapophysis; d, diapophysis; r, cervical rib; c, articulation for head; t, for tubercle of dorsal rib.

The collar-bones, or clavicles, are wanting in crocodiles; there is a slender interclavicle; and the shoulder-blade and coracoid are well developed ([Fig. 102]). The bones of the pelvis are loosely united with each other as they are in most reptiles ([Fig. 104]). The pubes, the anterior bones below, unlike those of all other reptiles, do not help to form the acetabulum or socket for the articulation of the thigh bone, nor do they articulate with each other. This single character sharply distinguishes a crocodile from all other reptiles, living or extinct, and is almost the only character that separates the order from the dinosaurs, aside from the peculiar structure of the nasal passages in the skull. On the under side of the body, connected with the front end of the pubes, there are seven or eight pairs of abdominal ribs, corresponding to the plastron of the turtles and similar to those of the ichthyosaurs and plesiosaurs. The mosasaurs have no such ribs.

Fig. 102.

Fig. 103.

Fig. 102.—Scapula (sc) and coracoid (cor) of gavial.

Fig. 103.—Front foot of crocodile: u, ulna; r, radius; re, radiale; ue, ulnare; p, pisiform.

Furthermore, the crocodiles differ from all other living reptiles in having a four-chambered heart, like that of birds and mammals, that is, a heart with two auricles and two ventricles. This more perfect structure of the circulatory organs does not, however, insure at all times a complete separation of the pure or arterial blood from the impure or venous blood, since the blood may be more or less intermixed outside of the heart by a connection between the venous and the arterial systems. Whether these imperfectly developed organs, so suggestive of a higher and more perfect mode of respiration, are the vestiges of what were once among some reptiles functional structures, or whether they are rudiments of a higher organization, developing independently in these creatures, cannot be positively determined, but it seems very probable that, far back in geological times, some reptiles, especially the pterodactyls and dinosaurs, had their respiratory and circulatory systems more like those of the birds and mammals of today. Unfortunately, however, if such was the case, we may never be able to prove it, although proof would not be impossible; stranger things than fossil hearts have been found by paleontologists!

The stomach, moreover, in the crocodiles is fashioned somewhat after that of the birds, with an imperfect division into crop and gizzard. Some crocodiles of today have the habit of swallowing hard pebbles, as do many birds. There is an old myth that the crocodile of the Nile swallows a pebble on each of its birthdays, thus giving reliable information as to its age by the number found in its gizzard at its death! And this habit has been suggested for some of the most ancient crocodiles, the teleosaurs, by the recurring presence of siliceous pebbles found with the remains of their skeletons. And we have seen this pebble-swallowing habit was also characteristic of the plesiosaurs, with whose remains “stomach-stones,” or gastroliths, as they have been called, are often found.

Fig. 104.—Pelvis of crocodile: il, ilium; is, ischium; pu, pubis.

All of these various characters of the skeleton and fleshy parts are pretty conclusive evidence that the crocodiles, ugly creatures that they are, today enjoy the highest rank among cold-blooded animals. They are perhaps in some respects of not so high a type of reptiles as were some of the extinct reptiles, but that they have survived so long, so many millions of years, is pretty good evidence of endurance, to say the least.

Living crocodiles belong to three distinct groups or families: the true crocodiles and alligators; the long-snouted crocodiles or Borneo gavials; and the true gavials of India. Members of the first of these families are really only subaquatic, or amphibious in habit; they move about on land with entire freedom, and often seek their food there. Certain marked aquatic characters they do possess, in the skull and tail, as we shall see. They are indigenous to southern China, India, Africa, Madagascar, the southern part of the United States, Central America, and the northern part of South America. The members of this family are distinguished by the more or less broad and flat head, the possession of comparatively few teeth of large size, and by having the toes less completely webbed. The crocodiles proper differ from the caimans and alligators especially in the arrangement of the teeth. During later geological times, that is, during early Tertiary times, the geographical range of the Crocodilidae was much more extended than it is at present, the remains of many often very large species, being found in the lake deposits of the northwestern parts of the United States, Great Britain, Germany, France, etc. Yet earlier, in the late Cretaceous rocks of the United States, in Texas, and Wyoming especially, there have been found rather scanty remains of a gigantic crocodile which must have been nearly fifty feet in length when living.

The second family, the Tomistomidae, or long-snouted crocodiles, comprises but two living species, both restricted at the present time to Borneo. These crocodiles have a moderately slender snout, because of which they are sometimes called gavials, though it is not nearly so slender as that of the true Gangetic gavial. This family is probably older than either of the other living ones, and is the only one known with certainty to have lived during much of the Cretaceous times, several species of considerable size having been found in New Jersey and Europe. Their feet are better webbed than are those of the true crocodiles and alligators, the front feet partly, the hind feet wholly so. In general structure they seem to be the most primitive of the living Crocodilia, and may have been the ancestors of all modern forms.

Fig. 105.—Gavial.
(By permission of the New York Zoological Society)

The third family, the Gavialidae, also comprises but two living species, both restricted in habitat to the rivers of India. Of these the famed gavial of the Ganges is the better known and the more highly specialized. The skull of this species has an exceedingly long and slender snout; the teeth are numerous, small, and slender; and the feet are more fully webbed than are those of other members of the order. In habits the gavials are more distinctly aquatic than are the crocodiles and alligators. They feed almost exclusively upon small fishes, for the seizure and retention of which their small and sharply pointed teeth are admirably adapted. The hind feet are relatively long, a character that will be better understood when comparison is made with those of the Thalattosuchia. Although attaining a large size, fully twenty-five feet in length, they are comparatively harmless animals, never attacking human beings or other large animals, as do some of the crocodiles proper. The gavials have lived a long time in the Indian regions, the Gangetic gavial itself having been found in deposits of Pleiocene age, perhaps the oldest known of all living species of air-breathing vertebrates. Some of the extinct gavials of the same region attained a length of nearly or quite fifty feet, possibly the longest, if not the largest, of all swimming reptiles of ancient or modern times. Extinct gavials have been reported from South America, but are not yet fully known.

While the fish-eating gavials swallow their prey whole, the crocodiles, caimans, and alligators prey upon all living animals that come within their reach, whether large or small, and they will often leave the water to seize their intended victims, such as pigs, sheep, birds, or even human beings. Their teeth, as has been already stated, are much larger, longer, and more irregular in size than those of the gavials. Their victims are often drawn under the water and drowned, the peculiar posterior position of the internal nostrils permitting the animals to breathe with the mouth and to firmly hold their prey under water, while the extremity of the snout and the external nostrils are above the surface.

As the firm, unyielding bony palate, the fixed position and articulation of the lower jaws, and their rigid attachment to each other in front do not permit creatures of large size to enter the gullet whole, the crocodiles and alligators must tear their food to pieces, which they do by quick, strong jerks from side to side, aided by the powerful tail; or they may twist off a limb or some other part of their victims by a rapid rotation of the whole body, two assisting in this operation, rotating in opposite directions.

Living crocodiles lay from twenty to sixty eggs, according to the species; these eggs are sometimes the size of a goose egg, and are covered with a hard shell. They are laid either in a deep excavation in the sand and covered over by the parent; or under leaves and straw. The female remains on guard until the eggs are hatched, of which she is apprized, it is said, by a peculiar noise uttered by the partly imprisoned young. She thereupon reopens the nest, and guides her liberated infants to the water, where she leaves them to their fate. Whether this remarkable habit is one that has been acquired in recent times or not is uncertain, but because it has been observed in a number of unrelated forms, it is probable that the instinct is of long inheritance, and may account for certain peculiarities of structure in some of the ancient members of the order. Doubtless the habit arose because of the unprotected places in which the eggs are necessarily laid on the shores and beaches, and because the eggs are comparatively so few in number. The sea-turtles likewise lay their eggs in hollows scooped out of the sand of the beaches, but the parents give no further care to their eggs, nor to their newly hatched offspring, a neglect which is compensated for by the much larger number of eggs they lay, because of which the chances are much greater that a few will survive the more numerous vicissitudes to which the eggs and young turtles are exposed.

ANCIENT CROCODILES,
MESOSUCHIA

The name Mesosuchia, meaning “middle crocodiles,” by which the ancient members of the Crocodilia have generally been known, was given by Huxley in the belief that they were intermediate between the “true” or modern crocodiles and an ancient group which he united with the order under the name “Parasuchia.” A fuller and better knowledge of the members of this last group has proved very conclusively that they are really less allied to the crocodiles than are some other orders of reptiles, the dinosaurs for instance, and should be properly classed by themselves as a distinct order. And, more recently, it has also become quite apparent that the old crocodiles should not be separated so widely from the modern ones as Huxley proposed; that the differences distinguishing them from the recent members of the order are really not of more than family importance. We thus have left but two chief divisions of the Crocodilia, the Eusuchia and Thalattosuchia; and the latter group even, by some authors, perhaps rightly, are included under the true crocodiles as a family only.

Fig. 106.—Teleosaurus; skull, from above.

These older crocodiles, the Mesosuchia of Huxley, comprise a considerable number of extinct forms which lived as far back as the early part of the Jurassic, and continued nearly to, if not actually into, Cenozoic time, that is, to the Eocene. They differ from all living forms, chiefly in having, not concavo-convex but biconcave backbones, that is, the more primitive vertebrae with which all reptiles began. Nor was the internal opening of the nasal passages so far back in the mouth as in the later forms. In other respects they did not differ very greatly from some of those now living. All the earliest kinds that we know of—the teleosaurs—had a long, slender snout, resembling very much that of the modern gavials. And they were, for the most part, incased in a more complete bony armor, on both the dorsal and the ventral sides; and the front legs were smaller than those of the gavials even. The resemblance of the living teleosaurs to the modern gavials must have been very great, although the heavier bony armor indicates a less exclusively aquatic life. They probably lived more in the shallow waters of the seas near the shores.

Near the close of the Jurassic appeared for the first time, so far as we now know, broad-headed mesosuchian crocodiles, forms having less numerous and stronger teeth, and resembling closely modern alligators. It has been believed that these broad-headed kinds were of later origin than the more slender-nosed teleosaurs, but a moment’s consideration will make it evident how improbable such an evolution must be. The crocodiles must have descended from strictly terrestrial reptiles, and no terrestrial reptiles have a slender nose. That they should have acquired a slender face in adaptation to water habits and then returned to the more primitive land type with a broad face and less strictly aquatic habits is contrary to all our experience in paleontology. From this it is altogether probable that broad-faced crocodiles of later times must have been the descendants of broad-faced kinds that were in existence during all the Jurassic times, but of which we as yet have no knowledge. These broad-faced Jurassic crocodiles were, for the most part, small creatures, much smaller than the teleosaurs even, and smaller than any species of crocodiles now living. Their remains are known only from fresh-water or shore deposits, and are, for the most part, associated with those of land and fresh-water animals. About the time of their first known appearance in geological history, the small mammals and birds had also become more or less abundant, and it was suggested by Owen that these land animals had something to do with the development of the ancient amphibious crocodiles. Perhaps this was the case with respect to their greater abundance and development, and with certain peculiarities of their structure, but that the gavial-like teleosaurs should have come back to the land and reverted to a more primitive form seems quite improbable.

During Cretaceous times, especially in America, numerous forms of these old mesosuchian crocodiles were doubtless in existence, both slender-nosed and broad-nosed, and some of them must have been of considerable size, though none known was as large as some of the late crocodiles. This type, with biconcave vertebrae, continued to live on, in both North and South America, to the latter part of Cretaceous times, and it is even possible that some continued on into the Tertiary. But long before the close of the Cretaceous, the modern kind appeared, those with concavo-convex vertebrae, and more posterior internal nostrils. The earliest are known from New Jersey (Thoracosaurus, Holops), so like the Borneo gavials of today that they are properly classified in the same family, the Tomistomidae or Gavialidae. If all the later, procoelian type, that is, those with concavo-convex vertebrae, originated from a single form when the amphicoelian or mesosuchian type became extinct, Huxley’s classification into the Mesosuchia and Eusuchia would perhaps be proper, but we have much reason to suppose that the change in the kinds of vertebra and in the position of the nostrils was only incidental, and may have occurred in more than one line of descent, that is, it may have occurred in the broad-headed kinds of the Jurassic to the broad-headed crocodiles of today, as also in the gavial-like forms of the Cretaceous to the gavials of the present. And this is the reason why naturalists no longer recognize the classification of Huxley, which, partly perhaps because of the prestige of his name, has so long been accepted in our chief works on natural history.

MARINE CROCODILES,
THALATTOSUCHIA

While the ancient crocodiles of which we have spoken resembled the modern ones so closely in form of body and probably in habits, there were certain others of the old Jurassic seas which departed so widely both in structure and in habits, from their associates that they are by some authors given a place wholly by themselves as a distinct group. This has been called by Professor Fraas the Thalattosuchia, a word meaning “sea-crocodiles.” They were a very early side-branch from the great genealogical tree of the Crocodilia, a branch which departed so widely from their associates in adapting themselves to a peculiar and aberrant mode of existence that they cannot be considered as typical crocodiles, although so closely related to them in other respects that therecannot be the least doubt regarding their ancestry. None of the crocodiles which we have considered, whether ancient or modern, can truthfully be called purely aquatic. They never ceased to use their limbs for locomotion on land, more or less of the time, or for the support of the body; and many of them have subsisted, in part at least, on land animals. How easy it may have been for some of them to become almost wholly emancipated from land habits we may easily conjecture. The gavials, as we have seen, are more at home in the water than upon land; their food is chiefly found in the water; but, so long as their habits restrict them chiefly to fresh-water, or to the vicinity of the shores, their limbs continue to be used as much for crawling as for swimming. Were the gavials to be driven out to sea by the stress of fresh-water conditions or attracted thereto by a greater abundance of more easily obtainable or better food, so far from land that they no longer would have much use for their legs for the support or propulsion of their bodies, in the course of time they would doubtless change to a more purely aquatic type. And in that change there would be material modifications of their structure: their limbs would become better adapted to movements in the water; the skin would become smoother, and the bony and horny scales would be lost, since they would be not only useless in the water, but actually detrimental to the well-being of the animals; and the tail would develop into a more powerful organ of propulsion, as a means of increasing their speed in obtaining food or in escaping their enemies, such as the sharks. In fact, we can only imagine that in the transformation precisely those modifications would occur which we actually find in these old sea-crocodiles, the Thalattosuchia. And they are of especial interest to us here because nowhere do we find a better example of Nature in the act of transforming a terrestrial or subterrestrial animal into a truly aquatic one.

Fig. 107.—Geosaurus, an Upper Jurassic thalattosuchian crocodile, drawn from restoration and figures by Fraas.

The group comprises only a few forms, so far as known. All were of modest size among extinct reptiles, from ten to twenty feet in length, and all are from the Upper Jurassic deposits of Europe. They did not exist very long, probably because they found it impossible to discard old habits and old structures entirely and become absolutely emancipated from the land; their breeding habits possibly were too deeply impressed into their structure readily to change, as did those of other sea-reptiles. Some of their remarkable aquatic adaptations have long been known, but only within a dozen years has our knowledge of them become at all complete. Three or four genera have been described, but only a few forms are well known, of which Geosaurus may be taken as most typical. To this we shall confine our descriptions.

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:

We recognize in Geosaurus an unusually slenderly built crocodile, in appearance very different from all true crocodiles. The smooth, rounded skull, with its greatly elongated and slender snout, and the deep-lying, small eyes, reminds one most of the ichthyosaurs. The skull merges into the slender, elongated trunk without a visible neck, and the body was provided neither above nor below with horny or bony armor, but was, probably, as in the whales, covered with a smooth, soft skin. The anterior extremities, attached far forward, are developed as paddles, and served rather as organs of equilibration than as a means of propulsion, which was the function of the elongated hind legs and the extraordinarily strong and powerful tail, which supported at its end a large fin. The entire impression given of the animal is that of an excellent swimmer, with all the peculiar aquatic adaptations. In the skeleton, however, all the characters of the original crocodiles are preserved. Most remarkable are the laterally placed eyes, protected by the stout sclerotic bones, and the overhanging bones of the orbits. So, too, the large temporal openings of the skull, doubtless due to the absence of the bony plates in the integument, give to the animal a strangely abnormal appearance for a crocodile.

We have observed that all the truly aquatic air-breathing animals, save the plesiosaurs, have either lost the hind legs or else have them greatly reduced in size, and the disproportionately large size of these members in Geosaurus seems inexplicable. But an explanation is not, I think, hard to find. In the adaptation to water life the first to become modified for the control of the body are the front legs. The hind legs never have any really important use when the tail is a powerful propeller. The hind legs of the geosaurs are still essentially legs and not paddles, and they were doubtless used either occasionally for propulsion on land, or perhaps for pushing the body about on the bottom of shallow waters. And the presence of a well-developed ventral armor of bony ribs possibly also indicates more or less of the terrestrial crawling habit. As soon as the hind legs cease to be used for crawling they take on only a feeble use for the equilibration of the body, and speedily become small, until finally they disappear. That the hind legs of these creatures were of some use in the water is certain, because of the modifications in their structure, and especially because of the loss of the claws; but that they were of important use as propellers is hardly probable. We may therefore infer that the thalattosuchians, while distinctively sea-reptiles, had not entirely lost their land habits. Moreover, it is highly probable that their egg-laying habits, which would hardly change with a greater adaptation to water life, compelled the animals recurrently to visit the shores. To have finally lost their hind legs they must have become viviparous in habit, since it seems to be impossible for any true air-breathers to be hatched in water. Perhaps this insurmountable habit was the final cause of their extinction in competition with the truly viviparous aquatic flesh-eaters. The thalattosuchians had but a brief existence in geological history, during the latter part of the Jurassic period only, so far as certainly known, nor did they become widely dispersed over the earth; they are known from Europe, possibly from Brazil.