analysis and electric stimulation of spe- 

 cific jaw muscles have established that the 

 fulcrum of the jaw-skull lever is the jaw 

 joint on the side of the mouth opposite the 

 camassial pair in use. The maximum bite 

 force is transmitted across the head and fo- 

 cused on the midpoint of the opposite jaw, 

 where the camassials are situated. 



Since camassials are so distinctive and 

 since teeth in general are the most endur- 

 ing fossils, their presence in the fossil 

 record enables us to estimate the antiquity 

 of the order Carnivora. The earliest- 

 known camassials date from at least sixty- 

 five million years ago, the time when di- 

 nosaurs became extinct and mammals 

 began to gain ascendancy. There are hints 

 of camassial development in even earlier 

 mammalian predators, perhaps seventy 

 million years old. 



The development of camassials was 

 such a pivotal adaptation that it governed 

 all subsequent carnivore evolution. The 

 system has been fine-mned within limits 

 for various species, and in some cases the 

 original shearing function of the camas- 

 sials has been lost. But the teeth them- 

 selves have never been lost in the course 

 of evolution of any of the camivore lines. 



The teeth of dogs and cats exemplify 

 the differences in skeletal stracture that 

 separate the two major divisions within 

 the order Camivora — the suborders Cani- 

 formia and Feliformia. Among hving car- 

 nivores, those whose camassials are least 

 modified from the original type include 

 the members of the dog, wolf, and fox 

 family — the Canidae. In canids the lower 

 camassial retains a broad shelf (talonid) at 

 its back end that occludes with the upper 

 first molar (the tooth that lies immediately 

 behind the upper camassial). The lower 

 camassial thus has a dual function, shear- 

 ing at the front and cmshing behind. Far- 

 ther back in the jaw, the second molars 

 above and below have several cusps and 

 continue this cmshing function. 



Like many other mammals, therefore, 

 dogs and their close relafives use molars 

 for chewing, mixing their food with saliva 

 so that digestion begins in the mouth. As a 

 result, dogs can process a variety of foods, 



including meat, bone, sinew, inverte- 

 brates, and plants. This has great survival 

 value because the wider the range of food, 

 the greater the animal's ability to shift 

 from resource to resource as local condi- 

 tions dictate. 



Cats, on the other hand, lack the talonid 

 "heel" on the lower camassial and have 

 also lost all the other lower molars. Their 

 only upper molar is a tiny bladelike tooth 

 lying directly behind the large upper car- 

 nassial. In some cats, the upper camassial 

 has an extra cusp in front that may be en- 

 larged, extending the camassial blade for- 

 ward. Thus, cats have become specialized 

 for a purely meat-eating life style: they are 

 "hypercamivores." Their teeth slice meat 

 and deliver the chunks whole to the stom- 

 ach, with little digestion in the mouth. 



The dog family, Canidae, originated 

 early in the evolution of the caniforms. 

 Another early group to appear was the 

 family of giant bear-dogs, the Amphicy- 

 onidae. Members of this lineage — now ex- 

 tinct — retained three upper molars, as did 

 the most primitive canids. But in the evo- 

 lutionary line leading to all other caniform 

 families — which include bears, sea lions, 

 weasels, and raccoons — the last upper 

 molar was lost at least thirty-five million 

 years ago. Eventually, raccoons, weasels, 

 and their relatives also lost the last lower 

 molar. These modifications do not signal 

 greater specialization for eating flesh be- 

 cause the remaining molars, including the 

 lower camassial, retain their crushing 

 function. 



Early in the evolution of the bear lin- 

 eage, the camassials themselves began to 

 take on a greater cmshing function. The 

 upper camassial became smaller, and its 

 inner cusp became prominent, approxi- 

 mating an upper molar in form. In the 

 lower camassial the shearing cusps be- 

 came blunted and lowered, while the 

 cmshing talonid "heel" enlarged to make 

 up half or more of the tooth crown. The 

 upper and lower molars behind the camas- 

 sials also grew larger as their low, broad 

 cusps formed effective cmshing surfaces. 

 These transformations, accomplished by 

 twenty-five million years ago, were stmc- 



In a scene set in the Great Plains six million years ago, 



distant relatives of modem dogs {lower right) feed 



on a camel carcass. These extinct carnivores, known as 



Osteoboms, sheared meat and crushed bone 



with their camassials — the teeth that are the 



hallmark of the order Camivora. 



Detail of mural by Jay H. Matternes; National Museum of Natural History, Smittisonian Institution 



tural responses to omnivorous and herbiv- 

 orous diets, resembling those of living 

 bears. 



This adaptation away from camivory is 

 called hypocamivory. It is not confined to 

 the bear lineage but also appears in 

 roughly similar form among raccoons, 

 badgers, and even some fossil canids. It is 

 most spectacularly developed in the denti- 

 tion of the giant panda, an herbivorous 

 member of the bear lineage that feeds ex- 

 clusively on bamboo in the mountains of 

 southeastem Asia. 



Among the pinnipeds — sea lions, wal- 

 mses, and seals — ail the teeth behind the 

 canines have the same simple shape. How 

 the camassials evolved to this form can be 

 traced through a succession of fossils. An 

 extinct, twenty-four-million-year-old spe- 

 cies of the genus Enaliarctos, whose 

 skeleton indicates it was fairly well 

 adapted for open-ocean swimming, still 

 retained camassials and molars. Succes- 

 sive species of this genus possessed pro- 

 gressively modified camassials that even- 

 tually came to resemble the premolars in 

 front of them. This transformation may 

 have followed a shift from larger prey, 

 eaten in shallow water or on land, to small 

 fish, swallowed whole in the manner of 

 hving pinnipeds. 



The feliform group also has evolved 

 striking modifications of the camassials 

 and succeeding molars. The hypercamivo- 

 rous cats occupy an extreme branch of the 

 feliform evolutionary tree. Other feliform 

 families, including those containing the 

 civets, mongooses, and hyenas, eat a 

 wider range of food. Still, all feliforms 

 have reduced the molar teeth involved in 

 mastication. Living hyenas, which cmsh 

 bone with their robust premolars, have 

 gone as far as cats in the loss of all lower 

 molars behind the camassial, and their 

 only upper molar is a small blade. 



Only a few feliform species have be- 

 come adapted to an otterlike life in 

 streams. None have evolved that are com- 

 parable to seals or walmses. But two spe- 

 cies, the aardwolf of Africa (an ant-eating 

 hyena) and the falanouc of Madagascar (a 

 civet), have become specialists in feeding 

 on insects, especially ants and termites, an 

 abundant tropical protein resource. As in 

 other ant-eating mammals, whose prey 

 need little mastication, their teeth are very 

 small and simple. The aardwoLf's camas- 

 sials no longer have a sheanng function. 



Within the order Camivora, certain dis- 

 tinct groups have come to resemble one 

 another in details of their teeth. For ex- 

 ample, the hunting dog and the Asian 



75 



