Article: Growth gradients among fossil monotremes and marsupials
N. G. Stephenson
Although a few groups appear to be characterized by relative stability of size, growth gradients, whether isometric or allometric, have played an important role in the evolution of Australian monotremes and marsupials.Intra-speciflc size reduction, resulting from isometric change, has been a widespread phenomenon from Pleistocene to Recent. Its implications were not recognized by early taxonomists, whose undue emphasis on minor size differences as a criterion of speciation, and even failure to compare fossil and recent forms, have led to nomenclatorial confusion in some families. Further study of the available fossil and recent material in such groups suggests cases of conspecificity, and the consequent priority of either a palaeospecific or neospecific name.In certain marsupials such as the Diprotodontids and the wombats, there is evidence of a trans-specific increase in size, involving allometric as well as isometric changes, and leading to gigantism and subsequent extinction. Evidence of the progressive stages by which such gigantism was achieved in the Diprododontinae is given, and a new genus and species of this subfamily of the Diprotodontidae is described.