EVOLUTION OF GNATHOSTOME LATERAL-LINE ONTOGENIES

An outgroup analysis of multiple ontogenies provides the most robust a pproach to understanding phylogeny. Such an analysis of the lateral li ne system among extinct and extant gnathostomes reveals that lateral l ine placodes constitute the basic onto genetic unit responsible for th e development of this system. Six pairs of lateral line placodes appea r to have existed in the earliest gnathostomes, and eight stages (stag es A-I-I) can be recognized in their differentiation. Terminal truncat ion (heterochronic changes) in the primitive sequence of placodal deve lopment has occurred in one or more placodes in each gnathostome radia tion, with the most extensive truncations occurring in arthrodire plac oderms, lepidosirenid lungfishes and extant amphibians. The most exten sive nonterminal changes in the primitive sequence of placodal develop ment involve the failure of electroreceptors to form within the latera l zones of the elongatiang sensory ridges of the placodes. This nonter minal change appears to have occurred independently in ancestral neopt erygian bony fishes, in many amphibians and, possibly, in the extinct acanthodians. At least two teleost radiations, osteoglossomorphs and o stariophysines, have re-evolved electroreceptors which may represent a dditional nonterminal changes in placodal patterning or, possibly, a c hange in the embryonic som:ce of these receptors.