Paedomorphosis and the evolution of the Dipnoi

Abstract

Paedomorphosis has played an important evolutionary role in several vertebrate lineages, yet there are few studies in which fossils have been used to document this evolutionary pattern directly. The unusually good fossil record of lungfishes offers an excellent opportunity to do this. An approach to recognizing paedomorphic character complexes in fossil lineages is formalized, and the interdependence of ontogenetic and phylogenetic studies is reemphasized. Concordant evolutionary changes in seven characters representing a variety of organ systems are discussed. For example, in Devonian lungfishes, the tail is heterocercal, endochondrally ossified bones are present, and the fin rays are similar to those of other Osteichthyes. Recent lungfishes have protocercal tails, suppression of endochondral ossification, and greatly reduced fin rays. The data suggest that paedomorphosis played a pervasive role in the evolution of lungfishes and give further evidence for the evolutionary importance of heterochrony.