Water Reptiles of the Past and Present

by Samuel Wendell Williston

As in other parts of the skeleton, there has been a reduction in the number of parts of the reptile skull from that of the more primitive forms, and a better adaptation of those which remain for the special uses they subserve. This reduction in number has been caused in part by the actual loss of bones, in part by the fusion of contiguous ones. The most primitive reptiles had no less than seventy-two separate bones in the skull;[1] the human skull has but twenty-eight inclusive of the ear bones. There is but little variation, either in the number or in the relations of bones, in the mammalian skull. If one knows the human skull thoroughly he can easily understand the structure of the skull of any mammal. The same cannot be said of the skulls of reptiles; one would be greatly puzzled in the comparison of the skulls of turtles and crocodiles, if he knew nothing about other forms. And it is safe to formulate another general law in evolution here: Characters which have been longest inherited are least liable to change. The earliest reptiles had at least four pairs of bones which have disappeared in all later reptiles; and they had some bones in pairs which have fused in later reptiles, either with their mates or with contiguous bones. The crocodile has at least two pairs of bones which have disappeared in turtles. On the other hand, the turtle has at least one pair of free bones which have been fused with adjacent bones in the crocodiles, and one pair that is fused which is free in the latter. The lizard has one pair of bones that has been wholly wanting in other reptiles for millions of years, while on the other hand it has lost some bones that are present in all other modern reptiles. The four parts of the occipital bone of mammals, basioccipital, exoccipitals, and supraoccipital, are almost invariably free and there is a single occipital condyle, except in the Theriodontia.

Fig. 6.—Labidosaurus, a cotylosaur. Skull from above: pm, premaxilla; n, nasal; m, maxilla; l, lacrimal; p, prefrontal; fr, frontal; pf, postfrontal; po, postorbital; j, jugal; pa, parietal; sq, squamosal; ds, dermosupraoccipital; pf, parietal foramen.

In this reduction or fusion of parts, or in addition thereto, there has been a general lightening-up of the whole skull-structure in reptiles from the rather massive and protected form of the older to the lighter, less protected, and more fragile type of the later ones, since speed, greater agility, better sense organs, and doubtless greater brain power have rendered unnecessary or useless the older kinds, just as modern methods and modern arms have rendered useless the coat of mail of the Middle Ages.

Fig. 7.—Edaphosaurus, a theromorph reptile from the Permian of Texas. Skull with single temporal vacuity.

The old reptiles had a continuous covering or roof for the skull, pierced only by the openings for the nostrils in front—the nares—the orbits for the eyes near the middle, and a smaller median opening back of them for the so-called “pineal eye.” The temporal region, that is, the region back of the orbits on each side, was completely roofed over by bone for the support and protection of the jaw muscles. In later reptiles this region has been lightened, either by holes that pierce it or by the emargination of its free borders, as in the turtles. The openings have occurred in different ways, and with the loss of different bones in various lines of descent. In one large group of reptiles, comprising the pterodactyls, dinosaurs, phytosaurs, crocodiles, and rhynchocephalians, there are two openings on each side, called the supratemporal and lateral temporal vacuities. In another still larger group there is a single vacuity on each side, all members of which it has been thought were markedly related to each other. Some of these, the lizards, snakes, and mosasaurs, the ichthyosaurs, and probably the proganosaurs, have the single opening high up on the side, corresponding apparently to the supra temporal vacuity of the double-arched forms, as those with two openings are called. Many others, however, like the whole order Therapsida and the Theromorpha, have the single opening lower down and bounded differently; their relationships are doubtful, since it is very much of a question how the single opening has arisen. There have been many theories to account for the origin of the temporal vacuities, but all are yet speculations. Notwithstanding these doubts, which more recent discoveries have intensified, there can be none that the structure of this region of the skull offers important and reliable characters for the classification of the reptiles into the larger groups, but, unfortunately, we are very uncertain yet as to what this classification should be. We are confident that all those reptiles having two temporal vacuities on each side are related to each other; we are yet very much in doubt as to the classification of all other reptiles, or at least all others having only a single temporal vacuity on each side.

Fig. 8.—Sphenodon (tuatera). Skull from side and above: pm, premaxilla; n, nasal; prf, prefrontal; f, frontal; pf, postfrontal; p, parietal; po, postorbital; sq, squamosal; m, maxilla; j, jugal; qj, quadratojugal; q, quadrate; c, coronoid; sa, surangular; art, articular; pa, prearticular; d, dentary; an, angular.

Better evidences of relationships, or the absence of relationships, are offered by the presence of certain bones in the skulls in some orders that are lost in others, since it may be accepted as an axiom that new bones have not appeared in the skulls of reptiles, birds, or mammals; and that no bone which has once disappeared has ever been functionally regained by the descendants of those that lost it. The presence, then, of an extra bone in the temporal region of the lizards or the ichthyosaurs is proof that they have had a long and independent descent from reptiles which possessed it.