Water Reptiles of the Past and Present

by Samuel Wendell Williston

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.