TEETH 133 



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 alveoU, confined to the jaws only, and differentiated into three 

 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 

 amphibians 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 

 the lower jaw are called protoconid, paraconid and metaconid. Teeth of 

 this sort are known as triconodont. Besides the three cones, triconodont 

 teeth have a basal rim, the cingulum, which forms part of the crown. 

 Marsupial-Uke mammals of the Tertiary had teeth of this triconodont 

 sort. See Fig. 124. 



The secondary tubercles of such teeth show a tendency to enlarge 

 to the size of the protocone. A further advance occurs when the three 

 cones assume a triangular relation to one another, the secondary cones 

 of the upper jaw migrating inwards, those of the lower jaw outwards. 

 Teeth of this tritubercular sort occur in Amphitherium of the Jurassic 

 period. 



Later, in mammals, appeared a posterior projection or talon, and a 

 fourth tubercle, the hypocone and hypoconid. With these additions, 

 the molar teeth assumed more and more the modern form with six cusps. 

 It took many million years to accomplish these changes, which were 



