Samuel Wendell Williston

“ In no reptile did the atlas attain the specialization of the mammals, even approximately, but it most nearly approached it in the Theriodonts. In very few do the two bones of the arch fuse with the intercentrum into a complete arch ring, or does the pleurocentrum unite with the axis as a real odontoid. In few is there any degree of rotation about it, not more than between the axis and the following vertebra. This lack of torsion, in most reptiles at least, was compensated for by the ball-and-socket joint between the single condyle and the atlas, lost in mammals. ”

“ Doubtless this same more mammal-like or turtle-like mode of progression was characteristic of the Dromasauria, Anomodontia, and Theriodontia, and likewise resulted in the reduction of the phalanges and shortening of the toes. One can imagine the difficulties of locomotion if our toes were six inches long! The Cotylosauria have short and broad feet, and many of the later ones, like Pariasaurus and Telerpeton have the astragalus and calcaneum fused. ”

“ In the order here provisionally called the Protorosauria the skull is completely known in none, but best in Araeoscelis, the oldest certainly-known reptile with a single typical upper temporal vacuity. The roof bones are all paired in all, so far as known. In Araeoscelis, Pleurosaurus, and probably Protorosaurus, there is a parietal foramen, but none in Saphaeosaurus. The lacrimal is small or vestigial in all. The postfrontal is present in Araeoscelis, and only in this genus are there indications of the presence of the dermosupraoccipitals. Probably all have teeth on the palatal bones. ”

“ With much reason it has long been urged that the anterior projection of the ischium represents the real pubis. [3] In early life it is largely cartilaginous, but becomes fully ossified in the adult. The so-called pubis is probably homologous with the prepubis of the pterodactyls. It has no pubic foramen.
The ilium of the Pterosauria, like that of all bipedal reptiles is produced anteriorly by the sides of the vertebrae, very much so in some forms. ”

“ In no reptiles is the pelvis more aberrant than in the Crocodilia (Fig. 121) . So characteristic is its structure that it at once distinguishes the order from all others. The ilium is a strong bone firmly united with the two pairs of stout sacral ribs, of which the posterior is the larger. Below, the ilium articulates with the ischium only, to form the acetabulum. In front of the acetabulum it is produced forward to join ligamentously with an anterior process of the ischium, enclosing between them a foramen of considerable size for the passage of the obturator nerve. ”

“ Because of the inflexible carapace the structure and posture of the limbs are much modified. The forearm is so twisted upon the humerus that the foot is brought more or less directly forward in walking. As in the dinosaurs there is a greater or less reduction of the postaxial fingers and a strengthening of those on the other side of the feet. Only rarely, in certain aquatic types, have the outer fingers of the hand more than three phalanges, probably because of an incipient hyperphalangy.
Regarding the general classification of the Chelonia there is still dispute. ”

“ The palatines also often meet for a short distance below and in front of the internal nares, forming a rudimentary secondary palate.
The temporal region primitively was wholly roofed over, and yet is, in some marine turtles, by the large postorbital, quadratojugal, and squamosal. Usually it is more or less exposed by the emargination of the roof from behind or below, or from both sides; and the squamosals and quadratojugals may even become vestigial in the process as in the terapenes. The quadrate is always large, its ear-cavity sometimes wholly surrounded by bone. The stapes is slender. ”

“ It is the general belief that the loss of mesopodial bones has been due to their fusion with adjacent ones; it is doubtless true for the most part, but not always. That there has been an actual loss of the first centrale and fifth distale, the first to be lost in both carpus and tarsus, seems certain, as shown in specimens of various early reptiles, where unoccupied spaces for cartilaginous elements have been preserved. Moreover, their actual loss in living reptiles has long since been affirmed. ”

“ The skull of the Testudinata or Chelonia is never elongate, though relatively slender in some of the more predaceous kinds. It always lacks the septomaxillae, postfrontals, supratemporals, dermosupraoccipitals, tabulars, ectopterygoids, ethmoids, and the parietal foramen. The nasals are usually absent, and the lacrimals are present only in some ancient forms. The prefrontals are large, meeting in the middle line. The prevomers are single and sometimes obsolete; they usually articulate above with the prefrontals. The parasphenoid has been lately recognized as a distinct bone in certain forms. ”

“ As a rule the hind limbs of terrestrial reptiles, as of terrestrial mammals, are more specialized than the anterior ones; that is, there are fewer bones, and the ones remaining are more developed than those of the front feet. Among aquatic and volant reptiles, on the other hand, where locomotion is chiefly effected by the fore limbs, these are more specialized. ”

“ That they and the sternum to which they are supposed to have given origin are really the ends of true ribs is improbable, since no other tetrapods are known in which the dorsal ribs meet on the under side of the body, or even approach each other. It would seem more reasonable that the abdominal ribs of all reptiles are of parenchymatous or cartilaginous origin, and that the anterior ones fused to form the sternum. ”

“ The single large temporal opening is bounded below by the postorbitals and squamosal, above by the parietals, which are more or less elevated in the middle in a crest. There is a pineal foramen. The squamosals, in some at least, join each other back of the parietals on the upper surface of the skull. There are no interparietals, tabulars, or supratemporals.
On the occipital surface the supraoccipital is excavated more or less by the foramen magnum, which may extend to the roof, dividing the bone. The paroccipitals are always fused with the exoccipitals. ”

“ The pelvis of amphibious or aquatic reptiles is also modified not a little. It lost all connection with the spinal column in the Mosasauria and later Ichthyosauria, but is firmly connected, as usual, in other water animals. The slender ilium of the mosasaurs, like that of the ichthyosaurs, lay loosely in the flesh with its upper end in apposition or ligamentously connected with the end of a transverse process or rib of a single vertebra. The narrow ischia and pubes meet in a symphysis, and there is a pubic foramen. ”

“ Rarely among the amphibians are there more than one one; certain temnospondyls and modern urodeles [5] are known to have two. It is quite certain, however, that reptiles began their career with but a single rib-bearing sacral vertebra, inasmuch as Seymouria of the Cotylosauria is known to have no more (Fig. 1) . A second vertebra (Fig. 84) , however, was soon added from the basal caudal by the enlargement of the ribs to come in contact with the ilium on each side. And this
Fig. 84. Sacrum and tail vertebrae of Varanosaurus (Theromorpha) . ”

“ The extreme aquatic adaptations of the plesiosaurs have caused certain modifications in the structure and relations of the bones of the skull that are unique among reptiles.
The general shape of the skull seems to bear a definite relation to the length of the neck, always shorter in the long-necked forms, slender, sometimes very slender in the short-necked types. The premaxillae are always greatly elongated, extending back at least as far as the front part of the orbit, in the long-faced kinds even to articulate with the parietals, above or between the frontals. ”

“ The skull of reptiles, as of other vertebrates, has undergone many changes in adaptation to food, offensive and defensive habits. It has lost not a few bones in various forms, and others have united or formed new associations; to such an extent, indeed, that there are several in later reptiles about whose homologies there has been and yet is dispute. ”

“ The skull of the ichthyosaurs, while retaining not a few primitive characters, has been highly and peculiarly modified in many ways. The greatly elongated premaxilla, unlike those of other aquatic reptiles, is broadly separated above by the very large nasal, and bears numerous teeth; the maxillae are short. All bones are paired. The frontals are small. The very large orbits have the usual bounding bones, prefrontal, postfrontal, postorbital, jugal, and lacrimal, but their relations are somewhat changed. ”

“ But the very great differences presented by the later, Triassic, forms, especially those included under the Cynodontia, differences as great as those between any other two orders of reptiles, render a division or divisions imperative, even though it may result, as is so often the case in other groups of animal and vegetable life, in the structural differences between members of the same group being greater than those limiting the groups themselves. ”

“ Digits probably webbed in life.
The Nothosauria were all aquatic in habit, but not exclusively so like the plesiosaurs, the feet still retaining terrestrial characters, with but minor aquatic adaptations. The parasternals, like those of the Plesiosauria, are very stout, apparently also an aquatic adaptation. The body was never slender, though less broad than that of the plesiosaurs, and it is not probable that they were rapid swimmers. ”

“ In addition to the fused calcaneum and astragalus, the centrale and fifth tarsale have disappeared and the first three tarsalia are fused in the adult.
The tarsus of the Pterosauria (Fig. 155 d) , like the carpus, is highly specialized. In the early forms the astragalus is suturally united with the tibia, the calcaneum fused with the astragalus. In the later forms the astragalus is indistinguishably united with the end of the tibia, the calcaneum fused or lost as in birds, forming a large, pulley-like articulation. ”

“ The first digit to be lost is the fifth, and only in a few dinosaurs is the first digit wholly lost. In the more upright-walking kinds, those in which the feet of the two sides are brought more nearly parallel in walking, the greater strength of the foot passes more to the preaxial side, and both the fourth and fifth digits may be obsolete or lost, and very rarely the third also. This weakening of the postaxial digits is especially noticeable in the dinosaurs (Figs. 141, 156) and turtles (Fig. 154) , in which the posture in locomotion is more like that of mammals. ”

“ These articular surfaces appear to be present in all known genera. In the Crocodilia, occurring as far back as Jurassic times, it is a single bone in the adult, roof-shaped, arising from paired cartilages. In Iguanodon (Fig. 80 l) , of the predentate dinosaurs, as also in several genera of the Sauropoda, and the Triassic Plateosaurus of the Theropoda, it is paired, as in the modern Sphenodon (Fig. 80 d) , also articulating with the atlas. ”

“ The fifth carpale may be absent, fused with the fourth, or separate and distinct. Indeed, in some old animals the third, fourth, and fifth carpalia and the pisiform may all be coössified.
The changes of the wrist and hand in adaptation to aquatic life are more profound than those of terrestrial reptiles. The earliest observed effect of water habits is delayed ossification, not only of the mesopodial bones, but of the bones of the skeleton in general, a large amount of cartilage remaining in the joints. ”

“ Epipodials
Radius and Ulna. The two bones of the forearm or antibrachium are always complete in reptiles and movable upon each other, freely in most terrestrial reptiles, flexibly in the aquatic types, that is, without rotation of the radius; they may be more or less fixed in the chelonians (Fig. 145 a) , though not crossed. The forearm in this order has a peculiar twist on the humerus by which the dorsal surface of the forearm, wrist, and hand is turned forward at right angles to the humerus without pronation or rotation of the radius (Fig. 145 a) . ”

“ In many reptiles, however, perhaps in part because of the closer articulation of the vertebrae, the head has migrated backward to articulate with a facet or process on the anterior end of the centrum, the parapophysis, and there it has remained in the cervical vertebrae of most reptiles, and in the dorsal vertebrae of the Squamata and their allies. ”

“ Two genera only, the living Sphenodon and the Jurassic Homœosaurus, can be located with certainty in this suborder. Sphenodon has long enjoyed the reputation of being the most primitive of living reptiles, as evidenced by the persistent dorsal intercentra, deeply amphicoelous vertebrae, and the single-headed ribs of primitive type. So far as known Homœosaurus agrees closely, except that it has no uncinate process on the ribs, a character in which Sphenodon is almost unique among reptiles. ”

“ The preaxial condyle, usually the smaller, gives articulation to the tibia; the postaxial condyle, to the fibula, and in part to the tibia behind. The shaft of the femur is sometimes markedly curved (Figs. 155, 157) , sigmoidally in the more slender kinds. It is always longer and more slender than the humerus, its distal width seldom if ever equal to more than half the length of the bone. ”

“ Axial ribs are more often present, but are lost in not a few reptiles, articularly the Pterosauria (Fig. 80 e) and Chelonia (Fig. 80 m) . In early crocodiles the axial rib articulated with diapophysis and parapophysis; in later crocodiles the diapophysial articulation is lost, though a vestige often remains, and the single headed rib has migrated forward on the odontoid.
The dorsal ribs of the Eunotosauria and all Chelonia [2] have expanded to meet or fuse with each other, forming more or less of a carapace (Fig. 91) . ”

“ The distal expansion is always great in the Cotylosauria and Anomodontia, as also in some Theromorpha (Figs. 129, 131) . Doubtless in these animals, or some of them at least, the peculiar humerus is to be correlated with the screw-like motion in the glenoid fossa, the horizontal position of the humerus in locomotion, and the more turtle-like mode of progression. The digits in such animals are never long, and the ungual phalanges are short and stout. ”

“ On the outer side, where the three bones meet, there is a cup-like depression, sometimes a hole, the acetabulum, for the articulation of the thigh bone. In only two groups of reptiles, the Crocodilia and Plesiosauria, is the pubis excluded from union with the ilium. In the snakes and snake-like lizards there are at most only vestiges of the pelvic bones. ”