The fossil record of fish ontogenies: insights into developmental patterns and processes.

One of the properties of fossils is to provide unique ontogenies that have the potential to inform us of developmental patterns and processes in the past. Although fossilized ontogenies are fairly rare, size series of relatively complete specimens for more than 90 fish species have been documented in the literature. These fossilized ontogenies are known for most major phylogenetic groups of fishes and have a broad stratigraphic range extending from the Silurian to the Quaternary with a good representation during the Devonian. Classically, size series have been studied in terms of size and shape differences, where subsequently allometric changes were used as indicators of heterochronic changes in Paleozoic placoderms and sarcopterygians. Quantitative analyses of fossilized ontogenies of dipnoans have been interpreted in terms of morphological integration and fluctuating asymmetry. Recently, reconstructed sequences of ossification have been used to identify recurrent patterns of similar development in actinopterygians and sarcopterygians in order to infer phenotypic developmental modularity and saltatory pattern of development. Phylogenetic and temporal landmarks are put forward for some of the major developmental patterns in the evolution of fishes.