Water Reptiles of the Past and Present

by Samuel Wendell Williston

SKULL AND TEETH

The skull of reptiles is much more primitive or generalized in structure than is that of mammals, to such an extent, indeed, that there is yet much doubt as to the precise homologies of some of the bones composing it; and, inasmuch as the names were originally given, for the most part, to the bones of the human skull, there is still some confusion among students as to the proper names in all cases, a confusion that doubtless will not be wholly dissipated until we know much more about the early or more primitive reptiles than we do at present.

Fig. 4

Fig. 5

Fig. 4.—Seymouria, a primitive cotylosaurian. Skull, from above: pm, premaxilla; n, nasal; l, lacrimal; p, prefrontal; f, frontal; pf, postfrontal; it, intertemporal; st, supratemporal; sq, squamosal; ds, dermosupraoccipital; t, tabulare; j, jugal; po, postorbital; m, maxilla; s, surangular; ang, angular; pa, parietal.

Fig. 5.—Seymouria, skull from the side. Explanations as in fig. 4.

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.

The mandible of the earliest reptiles was composed of not less than seven separate and distinct bones, as shown in the [accompanying figures]. The mandible of no modern reptile has more than six, and some have fewer. The mandible of mammals is composed of a single bone, the dentary; those reptiles, the Theriodontia, which doubtless were ancestral to the mammals in Triassic times, have all the bones, except the dentary, much reduced, or even vestigial. The prearticular bone, as shown, so far as known, has been absent in all reptiles since Triassic times, except the ichthyosaurs, plesiosaurs, Sphenodon, and turtles, all reptiles of ancient origin. The coronoid bone primitively extended the whole length of the teeth on the inner side; in all reptiles, except the plesiosaurs, since Triassic times it is either reduced to a small bone back of the teeth or is absent. So also the splenial has been greatly reduced in size in all later reptiles and may be wanting as in Sphenodon and modern turtles. The articular of reptiles, it is now generally believed, is represented in mammals by one of the ear bones, the quadrate by another.

Fig. 9.—Mandible of Trimerorhachis, a stegocephalian amphibian, ancestrally related to the reptiles: A from within; B from without. The coronoid is composed of three bones, the true coronoid (cor), the intercoronoid (icor), and the precoronoid (pc). The splenial is composed of two, the true splenial (sp) and the postsplenial (psp). The prearticular (pa) is broad, the dentary (d) is small; and the angular (an) is only slightly visible on the inner side.

Fig. 10.—Mandible of Labidosaurus, a cotylosaur reptile: A from within; B from without. The coronoid (cor) is a single bone, but extends far forward. The splenial (sp) is also a single bone, replacing the two of the amphibians. The prearticular (pa) is narrower, and the angular (ang) appears broadly on the inner side. The dentary (d) is much larger and the surangular (sa) is distinct. The articular (art) is small.

Fig. 11.—Mandible of Alligator, a modern, highly specialized reptile, from within. The coronoid (cor) is small and is situated far back; the splenial (sp) does not extend to the symphysis; the prearticular (pa) has disappeared, or has fused with the angular (an) or articular (art). The dentary (d) has become the chief bone of the mandible.

The teeth of reptiles are of much less importance, as a rule, in the determination of relationships than are the teeth of mammals. Rarely are their shapes of specific, and often not of generic, importance, though their number and relative sizes may be. The teeth of mammals, as a rule, are forty-four or less in number, and they are always inserted in distinct sockets in the jaw bones. Among reptiles they are indefinite in number, and may be attached to any of the bones of the palate and sometimes also to the coronoid of the mandibles. Furthermore, except in those reptiles related to the immediate ancestors of the mammals, they are alike or nearly alike in the jaws, that is, homodont, not distinguishable into incisors, canines, and molars. They may be inserted in separate sockets (thecodont), in grooves, or simply be co-ossified to the surface of the bone (acrodont). And they are usually reproduced indefinitely by new teeth growing at the side of the base or below them. More usually they are pointed and curved; sometimes they are flattened, with sharp cutting edges in front and behind in the more strictly carnivorous reptiles; in those of herbivorous habits they are more dilated and roughened on the crown, not pointed; in not a few they are low, broad, and flat and are used only for crushing the hard shells of invertebrates. With the very few exceptions among certain dinosaurs, they never have more than one root for attachment. The evolutional tendency for reptiles, as for the mammals, is to loose teeth, especially those of the palate. Among living reptiles it is only the most primitive types, such as the lizards, snakes, and the tuatera, which have teeth on the palatal bones, and in none are there teeth on the vomers, as was the rule in the ancient reptiles. The lizards may have them on pterygoids and palatines, and the tuatera has them on the palatines only. There may be as many as eighty on each jaw, above and below, and hundreds of smaller ones on the palate, or they may be reduced in number to five or six, or even to a single one; some reptiles, like the turtles and later pterodactyls, have none. The teeth of reptiles are composed of the same kinds of tissues as are the teeth of mammals, that is, of dentine and enamel, but the enamel is always thin, perhaps because the teeth are so easily replaced that a thicker protective covering is not needed. The arrangement of the dentine in primitive reptiles is complicated, that is, plicated or folded in labyrinthine figures, like that of many stegocephalian amphibians, the Labyrinthodontia, especially. This labyrinthine structure of the dentine persisted longest in the ichthyosaurs.