TEETH 



187 



The elasmobranchs, indeed, do have a certain succession of teeth, 

 Init only one at a time as single teeth are lost. Lower vertebrates, 

 reptiles conspicuously, have a somewhat indefinite number of sets, and 

 are said therefore to be polyphyodont. Only the mammals have two 

 definite sets, and are therefore diphyodont. But monotremes, sirenians, 

 and toothed whales retain their milk teeth throughout life, have no second 

 set, and are said to be monophyodont. 



In general, then, the course of evolution has been from a large and 

 indefinite number of simple teeth all alike, not fixed firmly in place, 

 and borne by any part of the mouth, to a reduced and definite number, set 

 firmly in alveoli, confined to the jaws only, and differentiated into three 



A C 



Fig. 142. — Jaws of some primitive Jurassic mammals. The resemblance of these 

 jaws and teeth to those of the theriodont reptiles of the same period suggests a similar 

 genetic origin. (Redrawn from Romer, after Simpson.) 



sorts. Along with this, has gone a shortening of the jaws and a change 

 of food habits. But whether the change in diet caused the change in 

 teeth, or the change in teeth made possible the change in foods, is still 

 an unsolved problem. 



EVOLUTION OF COMPOUND TEETH 



Compound teeth resembling the molars of mammals first appear in 

 certain late Permian and early Triassic reptiles, the theromorphs. Since 

 amphibia and the earlier reptiles had simple conical teeth, the conclusion 

 has been drawn that compound teeth are derived from conical teeth, and 

 morphologists have advanced two theories as to how this evolution came 

 about. 



The Differentiation Theory of Cope and Osborn assumes that the 

 teeth of vertebrates were originally of the simple conical type found in 

 most Reptiles. Such were the teeth of the premammalian Stegocephala 

 and of primitive Theromorph reptiles. 



The first multitubercular type of molars of modern mammals appears 

 in such a Triassic mammal as Dromatherium, the teeth of which had a 

 large median cone or protocone in line with two smaller cones, a paracone 

 in front and a metacone behind. Corresponding parts in the teeth of 



