Early vertebrate evolution: pharyngeal structure and the origin of gnathostomes

Contrary to one widely held view, pharyngeal morphology does not separate jawless vertebrates (lampreys) from gnathostome fish. Despite differences in branchial arch location, gills of lampreys and gnathostomes are fundamentally alike. Phylogenetic considerations might take a fruitful new direction by asking how the branchial arches came to differ in the two groups.
Thanks are extended to Quentin Bone, Philippe Janvier, R. P. S. Jefferies, Malcolm Jollie, Ken Kardong, Colin Patterson, Barbara Stahl, Keith Thomson and Richard Wassersug for discussion of the ideas in this paper, and to Barbara Comstock and Sue Rose for typing the manuscript.