Degeneracy: Its Causes, Signs and Results

by Eugene S. Talbot

The Degenerate Teeth and Jaws

Next to the ears, the jaws and teeth (as was to be expected from the variability of these organs in allied animals) are most affected by degeneracy. This is particularly true of the vertebrates, especially mammals, as might have been anticipated from their phylogeny. At the head of the vertebrates is man; at the foot is the lancelet (amphioxus), which is perhaps most akin to those semi-vertebrates the ascidians, who, in their larval phase, are higher than when adult, and whose life-history excellently illustrates that potent phase of evolution, degeneracy.

The lancelet has a spinal cord enclosed in a half-gristly canal (the notochord). It is practically destitute of a brain. The cerebral vesicle which represents this is a plain cavity without true subdivision into ventricles. There is no cranium. The eye (central in position) is a mere pigment spot with which it is able to distinguish light from darkness. The nose (behind this) is a small pit, lined with cilia, for purposes of smell. Into this the cerebral vesicle of the larval lancelet opens. The mouth is well guarded against the intrusion of noxious substances which have to pass through a vestibule richly provided with sensitive cells, resembling the taste buds of the human mouth. There is no heart. In this, as in the case of the eye, the lancelet is lower than the ascidians, the insects, crustaceans, and many molluscs. It approximates those worms which, despite a very elaborate vascular system, are destitute of a heart, the function of which is performed by contractile blood-vessels. From an embryologic and morphologic standpoint the proximate ancestor of the vertebrates may have been a free swimming animal, intermediate between an ascidian tadpole and the lancelet, and the primordial ancestor, a worm-like animal organised on a level with the star-fish. The vertebrates embryologically develop from this stage to the lampreys; thence to the cartilaginous fish (shark); to the amphibia (frog, toad, axolotl); to the reptiles; and thence to the oviparous mammals (duck-bill and echidna or spiny ant-eater); to the lemurs, and through forms like the Pithecanthropus erectus to man. Mammal teeth pass, in evolution, from the simple types found in that oviparous edentate, the spiny ant-eater of Australia, to those of the indeciduous ancestors of the sloths and armadilloes, and their descendants, inclusive of the dolphins and whales, whose teeth, both in the fetal Greenland and adult sperm whale, preserve this old type. (The whales have degenerated from the hoofed mammals to suit their environment.) While, as in the edentates, these teeth may be few, they may also, as in the insectivorous marsupials, approximate those of the reptilia in number (sixty or seventy on a side) and characteristic location.

The evolution of this primitive tooth to the bicuspid and molar type has been explained by two theories: that of concrescence and that of differentiation.[216]

A number of conical teeth, in line as they lie in the jaws of the sperm whale, represent the primitive dentition.[217] In time a number of these teeth, according to the concresent theory, cluster together so as to form the four cusps of a human molar, each one of the whale tooth points forming one of the cusps of the mammalian tooth. Vertically succeeding teeth might also be grouped. What evidence is there in favour of this theory? and what is there against it? All primitive reptiles from which the mammals have descended, and many of the existing mammals, have a large number of isolated teeth of a conical form. Further, by shortening of the jaws, the embryonic germ from which each of the numerous tooth-caps is budded off in course of development could have been brought together in such a manner that any cusps originally stretched out in a line would form groups of a variable number of cusps, according to the more or less complex pattern of the crown. Against the acceptance of this theory stands the fact that cusps quite similar in all respects to each of the cusps which form the angles of the human molar are even now being added to the teeth in certain animals, such as the elephant, whose molar teeth cusps are being thus complicated. In the mesozoic period certain animals with tricuspid teeth occur. According to the theory of concrescence these teeth ought not to show any increase of cusps in later geologic periods, but down through the ages to the present time successors of those animals continue to present a very much larger number of cusps. How is this increase of cusps to be accounted for? Has there been a reserve store of conical teeth to increase the number? Most obviously to every student of the fossil history of cusps there is no reserve store, but new cusps are constantly rising upon the original crown itself by cusp addition.

In the Triassic occur the first mammalia with conical, round, reptilian teeth. There are also some aberrant types which possess complex or multitubercular teeth.

These teeth begin to show the first trace of cusp addition.

In Fig. 1, [Plate A], the teeth of the dromatherium of the coal beds of North Carolina occur on the sides of the main cone, cusps or rudimentary cuspules. On either side of the main cone are two cuspules. In the same deposit occurred another animal represented by a single tooth (Fig. 3), in which these cusps are slightly larger. These cusps have obviously been added to the side of the teeth and are now growing. In teeth of the Jurassic period, found in large numbers both in America and in England, but still of very minute size, are observed the same three cusps. These cusps have now taken two different positions; in one case they have the arrangement presented in [Plate B]. The middle cusp is relatively lower, and the lateral cusps are relatively higher; in fact these cones are almost equal in size. These teeth are termed triconodont, as having three nearly equal cones. But associated with this is the spalacotherium, the teeth of which are represented in [Plate A], Fig. 4. This tooth illustrates the transformation of a tooth (triconodont) with three cusps in line into a tooth with three cusps forming a triangle. Here the primitive cusp is the apex of a triangle of which the two lateral cusps are the base. This tooth, in this single genus, is the key of comparison of the teeth of all mammalia. By this can be determined that part of a human molar which corresponds with a conical reptilian tooth. This stage is the triangle stage; the next stage is the development of a heel or spur upon this triangle (see in the amphitherium, Fig. 5). The opossum still distinctly preserves the ancient triangle. Look at it in profile, inside or in top view, and see that the anterior part of the tooth is unmodified. This triangle is traceable through a number of intermediate types. In Miacia (Fig. 6), a primitive carnivore, is a high triangle and a heel; looked at from above (Fig. 6a), the heel is seen to have spread out broader so that it is as broad as the triangle. The three molars of this animal illustrate a most important principle, namely, that the anterior, triangular portion of the crown has been simply levelled down to the posterior portion.