HETEROCHRONIC VARIATION IN THE DEVELOPMENTAL TIMING OF CRANIAL OSSIFICATIONS IN POECILIID FISHES (CYPRINODONTIFORMES)

The quantitative description of ossification sequences and other developmental events with respect to body size provides a basis for assessing the ontogenetic patterns underlying differences in morphological structure. To the extent that such sequences are evolutionarily conservative, they may also provide a basis for phylogenetic inference. Ossification profiles were examined in five species of poeciliid fishes of the genera Poecilia, Xiphophorus, and Poeciliopsis, selected to represent three lineages of varying evolutionary distinctiveness. Although ossification sequences are highly correlated among species, numerous small timing differences and reversals are evident, differences that are accentuated more among than within genera. The timing profiles by themselves, when used to construct hypothetical phylogenetic trees, contain sufficient historical information to recover evolutionary relationships consistent with conventional systematic criteria based primarily on adult osteology and external morphology. Character-state changes mapped onto the resulting trees can be interpreted directly as heterochronic accelerations and retardations of ossification.