Models for the origin of the anthropoid postcranium

In traditional interpretations of primate evolution prosimians are portrayed as the stem group of the Anthropoidea, and thus prosimian postcranial anatomy is considered the foundation from which the presumably more derived anthropoid postcranium evolved. This view has been questioned by other researchers who have pointed out several postcranial features of anthropoids, especially platyrrhines, which are in fact phenetically more similar to those of archaic primates and other eutherians than to prosimians. This implies that anthropoids have retained primitive primate character states for these traits, while all prosimians have derived states. If this interpretation is correct, the anthropoid postcranium is a better model for the ancestral euprimate than is any prosimian postcranium, and no currently known fossil or living prosimian can be ancestral to anthropoids. Although these two hypotheses have very different implications for the course of evolution of the primate postcranium, for the relationships of prosimians and anthropoids, and for the timing of important cladistic events in primate evolution, it proves difficult to find a decisive test for them. One line of evidence which helps to resolve this issue is an assessment of the probability that anthropoid-eutherian resemblances are homologousvs the probability that strepsirhine-tarsiiform resemblances are homologous. Since resemblances between prosimian groups are very marked and extend to many areas of the body, they are very likely to be homologous. On the contrary, most of the features in which anthropoids resemble eutherians have convergently evolved elsewhere in mammals including primates, and are therefore less likely to be homologous. On this basis it is concluded that the traditional view is correct: the special resemblances between anthropoids and other eutherians are interpreted as non homologous (i.e., reversals).