On the homology of the posteriormost gill arch in polypterids (Cladistia, Actinopterygii)

Polypterids are unusual among ray-finned fishes in possessing only four rather than five gill arches. We review the two current hypotheses regarding the homology of the last gill arch in polypterids: that it represents (1) the fifth or (2) the fourth arch of other actinopterygians. Arguments for the alternative hypotheses drawn from different anatomical systems are compiled and evaluated. We conclude that in polypterids the last arch represents the fourth arch of other Actinopterygii and the fifth arch is absent. (C) 2003 The Linnean Society of London, Zoological Journal of the Linnean Society.