The figures show clearly some of the remarkable aquatic adaptations of the animal, especially the short neck, the very long and narrow body, and the extraordinarily long and flattened tail. The head is elongate triangular in shape, resembling very much that of the mosasaurs; and the external nostrils are likewise situated remotely from the end of the snout, as in the mosasaurs. The extremity of the snout has a beak-like projection. The teeth are much longer, more pointed, and more recurved than is the case with most land reptiles, indicating their use for the capture and retention of slippery, quick-moving prey.
The single-headed ribs are short, proving that the body was slender and doubtless cylindrical, more like that of a snake. The tail was not only enormously elongated, but it was also compressed into a flat and effective propelling organ in the water. This flattening of the tail is apparent from the skeleton, with its elongated chevrons below and spines above, and it is also proved by the fortunate preservation of the extremity of the tail of one specimen, showing not only the impressions of the scales in the matrix, but also the outlines that the soft parts had in life. To quote from Lortet, in translation: “The tail was covered wholly with small scales, regularly hexagonal in shape, shining and nacreous, larger on the under side than above. The upper border of the tail was surmounted by a broad crest, extending to its extremity, and composed of large, oval scales.” The body doubtless was wholly covered with scales, though it is not probable that the caudal crest continued along the back.
Fig. 64.—Life restoration of Pleurosaurus.
The limbs begin to show an aquatic adaptation, though not very pronounced. They are much shorter and smaller than are those of land-crawling reptiles; and the bones of the second series, that is, the radius and ulna, tibia and fibula, are relatively short, the beginning of adaptation to water habits. It is very probable that the feet were webbed, though the fifth digit, as usual, is shorter than the fourth. Doubtless on land the creature moved about in a serpentine way, for it could not have progressed very rapidly by the aid of its legs alone. The hind legs are longer than the front legs, and they were connected firmly with the body by means of a sacrum. The number of vertebrae in the neck is only five. The number of dorsal vertebrae is forty-three, a larger number than is known in any other air-breathing vertebrate with legs.
Upon the whole, these lizard-like, almost snake-like pleurosaurians present some very curious adaptations to water life. In water they were doubtless speedy, swimming in serpentine undulations, with the small legs for the most part folded against the body and only of occasional use. Doubtless, too, had the pleurosaurs lived longer in geological history, they would have become quite snake-or eel-like, just as have some modern salamanders.
In all probability the pleurosaurs lived habitually in fresh-water, perhaps visiting the shores for refuge, or for the hatching of their young. That they were not on the way toward a terrestrial snake-like body is evident from the flattened tail, and especially the crest of scales above; the tail was like that of the sea-snakes of the present time. Pleurosaurus, then, affords the solitary instance among reptiles of aquatic adaptation by the diminution of both front and hind extremities and the acquisition of a snake-like body and snake-like habits.
CHAPTER XI
SQUAMATA
The order Squamata, so called because of the dermal covering of overlapping horny scales, comprises the great majority of living reptiles. Although the scaly covering is characteristic of nearly all the members of the order, the most essential differences distinguishing them from other reptiles are, as usual, found in the skeleton, and especially in the skull. The quadrate bone, that to which the lower jaw is articulated on each side, is not wedged in immovably between other bones of the skull, as in all other reptiles, but is, instead, freely articulated with the cranium in such a way that its lower end moves both backward and forward, as well as inward and outward. This freedom of movement has in the past been thought to be due to the loss of a lower temporal arch, a bony bar connecting the lower end of the quadrate with the hind end of the upper jaw, which is very characteristic, for instance, of the Rhynchocephalia. Indeed, because of the many primitive characters which the lizards possess, it has generally been supposed that the order was an early branch of the rhynchocephalian stem. But we are now quite sure that the lizards are as primitive as the Rhynchocephalia, and that their origin, as an independent branch of the reptilian stem, goes quite as far if not farther back—quite sure that the ancestors of the lizards never had a lower temporal arcade and two temporal vacuities, but that the looseness of the quadrate bone has been due to the gradual loss of a bone which covered the whole side of the skull until only the upper part of it was left. In other words, the ancestral skull of the Squamata must have been like that of Araeoscelis, more fully described under the Protorosauria, a group than which there is perhaps none more closely allied to the Squamata.
The bones of the roof of the mouth of the Squamata—that is, of the palate—are narrow and long, and are not closely articulated, as in most other reptiles; they often bear teeth, a primitive character. The teeth of all living lizards and snakes are not inserted in sockets, as are those of the crocodiles, but are co-ossified to the margins or sides of the jaws or the bones of the palate. But this is probably not a primitive character; doubtless the teeth of the early lizards were inserted in sockets like those of most other reptiles. The shoulder bones are absent in many and vestigial in some others. When present and fully developed, they comprise the shoulder-blades or scapulae, a single coracoid on each side, the clavicles, and an interclavicle. The vertebrae, except in some lizards, are procoelous, that is, with the body concave in front and convex behind, a peculiar structure that was developed only in crawling animals. In addition to the usual articulations for the union of the vertebrae there are also, in some of the lizards and mosasaurs and all of the snakes, additional ones called the zygosphene and zygantrum, which will be best understood by reference to [Fig. 12, p. 28]. But little less characteristic than the loose articulation of the lower jaws, so unique in this order of reptiles, is the manner of attachment of the ribs. They are always single-headed, articulating only with the body or lower part of the vertebra. The single-headed ribs of the plesiosaurs articulate with a projection on each side of the arch of the vertebra; those of the turtles to the space between the adjacent vertebrae; nearly all other reptiles have double-headed ribs, articulating in various ways. This character, it is seen, though apparently a simple one, immediately distinguishes a lizard or a snake from all other animals, except the thalattosaurs and protorosaurs.