The trophic role assumed by an animal taxon in a community is best understood when the structure of the community is viewed in historical perspective (Whittaker 1977). In many environments it is not physical and behavioral limitations which guide a group's evolutionary potential, but the opportunity to exploit available adaptive zones, which, because of the nature of the fauna, were open (Hecht 1975, p. 248). [The term adaptive zone is used as defined by Van Valen (1971, p. 421) … “the niche of any taxon, especially a supraspecific one.” Van Valen distinguished adaptive zones from the “ways of life” of taxa. Adaptive zones are part of the environment and exist independently of taxa that exploit them.] It is often assumed or implied that a systematic group can, in its adaptive radiation, fill most of the available niches within a particular adaptive zone (Hecht 1975, p. 247). This type of “taxocene” analysis (Whittaker 1972, p. 218; Levandowsky and White 1977, p. 133), which promotes these views, has obvious shortcomings. It is now evident that mammals, for example, were unable to equally exploit and partition all available adaptive zones on every continent. The failure of carnivorous mammals to do this to a complete degree in Australia (see Hecht 1975, p. 247) and in South America (see Marshall 1977, p. 709) has some historical basis in the development of the stratification of the vertebrate fauna.