Ontogeny of the ectotympanic-petrosal plate relationship in strepsirhine prosimians.

The two major groups of living strepsirhine prosimians (the lorises and lemurs) display two different forms of the ectotympanic-petrosal plate relationship (EPPR). Ontogenetically, the factors producing this difference primarily relate to (1) the degree of elaboration of the sutural tissues at the ectotympanic-petrosal plate interface and (2) the method of pneumatization of the petrosal bone. These two processes are, in all likelihood, the same ones which determined the EPPRs fossil strepsirhines as well. The important theoretical point which emerges is that the transition from the intrabullar to the extrabullar condition of the ectotympanic (or vice versa) was probably effected partly or wholly in young rather than adults stages of ancestral strepsirhines.