28 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



upon the other, but the two live in very constant association, each conferring 

 some benefit upon the other. Such associations are not uncommon in 

 fossils. An interesting paper by J. M. Clarke cites several instances of 

 both parasitism and commensalism among extinct forms. 1 



Other relations than the two above should not be neglected. Prominent 

 among the effects of interrelation are the adaptations of the carnivorous 

 and herbivorous fauna to each other. Where two such groups have lived 

 together for a sufficient time for the relations of one to the other to become 

 well established, extreme and perfect adaptations of structure are to be 

 expected the means of defense by armor, thickened shells, mimicry, con- 

 cealment, etc., will have reached a notable degree of development, while 

 the carnivorous forms will exhibit equally extreme adaptations to over- 

 coming the defense. If the fauna is a new one, or if two faunae have been 

 recently brought together by wide migration or the transgression of a sea 

 into an inland basin or another sea, the relations will be more simple. The 

 measure of perfection in the balance between offense and defense will beyond 

 doubt give some clue to the length of time the whole fauna has been estab- 

 lished. 



It is less probable that forms will be found which have failed to become 

 adapted more or less accurately to the physical surroundings, as they have 

 in most cases experienced no sudden change, but in the case of suddenly 

 increasing salinity of sea-water or the relatively sudden transgression of a 

 sea over a lowland such conditions might be discovered. Dacque tells us 

 that changes in facies, organic or inorganic, are gradual; changes between 

 beds are sudden. 



No study of an extinct fauna would be complete were we to neglect to 

 balance all conclusions drawn from structure against the presence of the 

 features indicated. For instance, the food of a vertebrate may commonly 

 be inferred from the character of the teeth carnivorous (molluscivorous, 

 durophagous, conchifragous, etc.), herbivorous, or omnivorous. Rodent 

 teeth, browsing teeth, grazing teeth, etc., all imply definite feeding habits. 

 Such observations should be checked by a search for the possible food-supply 

 if it is believed that the remains occur in the original habitat. Frequently 

 it is possible to determine by a comparison of the armor and the weapons 

 of offense which animal has been selected as a prey by definite raptorial 

 forms. It has been suggested that the growing length of the teeth of the 

 saber-toothed tiger was correlated with the increasing thickness of the cara- 

 pace of the glyptodonts and that in the Permian vertebrates of North 

 America the development of strong tusks in Dimetrodon may have been 

 associated with the development of armor in many of the amphibians and 

 smaller reptiles. 



1 Clarke, J. M., The Beginnings of Dependent Life. N. Y. State Museum Bull. 121, p. 146, 

 1908. 



