Ontogenetic differences between males and females result in sexual dimorphism, but this process is poorly understood in the majority of mammalian taxa. Sexual dimorphism is particularly extreme in the otariids (Carnivora: Otariidae), and to examine the origin, structure, and temporal patterns of otariid morphological diversity, we focus here on 3 otariid species: Arctocephalus australis (southern fur seal), Callorhinus ursinus (northern fur seal), and Otaria byronia (southern sea lion). Our aims are to compare the ontogeny of skull shape across species, and to evaluate the ontogeny of sexual dimorphism, testing the hypothesis that dimorphism arises by extrapolation of a shared ontogeny to the larger sizes that are characteristic of males. We found that dimorphism increased over ontogeny but was not due solely to allometric extrapolation, because different rates of development were found in some species. Specifically, the relationships between changes in shape and size increase were different between sexes in A. australis and O. byronia, but equal in C. ursinus. It is possible to implicate heterochrony in the origins of the modifications undergone during the ontogeny of males and females of A. australis and O. byronia, considering the differences in the rates of development between the sexes of both species, but it is certain that allometric repatterning also is involved in these.