An outgroup analysis of multiple ontogenies provides the most robust approach to understanding phylogeny. Such an analysis of the lateral line system among extinct and extant gnathostomes reveals that lateral line placodes constitute the basic ontogenetic unit responsible for the development of this system. Six pairs of lateral line placodes appear to have existed in the earliest gnathostomes, and eight stages (stages A-H) can be recognized in their differentiation. Terminal truncation (heterochronic changes) in the primitive sequence of placodal development has occurred in one or more placodes in each gnathostome radiation, with the most extensive truncations occurring in arthrodire placoderms, lepidosirenid lungfishes and extant amphibians. The most extensive nonterminal changes in the primitive sequence of placodal development involve the failure of electroreceptors to form within the lateral zones of the elongatiang sensory ridges of the placodes. This nonterminal change appears to have occurred independently in ancestral neopterygian bony fishes, in many amphibians and, possibly, in the extinct acanthodians. At least two teleost radiations, osteoglossomorphs and ostariophysines, have re-evolved electroreceptors which may represent additional nonterminal changes in placodal patterning or, possibly, a change in the embryonic source of these receptors.