THE RHIZOMORPHIC LYCOPSIDS - A CASE-STUDY IN PALEOBOTANICAL CLASSIFICATION

The rhizomorphic lycopsids are a putatively monophyletic group delimit ed by the rhizomorphic syndrome: centralized determinate growth and se condary thickening in their rooting systems. A cladistic analysis of t he most completely understood species provides the basis for two alter native, new classifications of the group, one following strict phyloge netic classification criteria, the other more traditionally Linnean. B oth are based on the same preferred most-parsimonious cladogram and pl ace the rhizomorphic lycopsids in one order, the Isoetales. Also, both classifications delimit the same suprageneric groups: Hizemodendron Lepidodendron + Lepidophloios, Diaphorodendron + Synchysidendron, Sig illaria, and Chaloneria + Isoetes are well supported as monophyletic c lades, but the basal plexus of bisporangiate-coned ulodendrids (Paraly copodites + Oxroadia + Paurodendron) is paraphyletic. Both classificat ions differ from all previous studies in 1) assigning all rhizomorphic lycopsids to a single order, and 2) recognizing the trees Diaphoroden dron plus Synchysidendron as a distinct family the Diaphorodendraceae. A generally applicable paleobotanical taxonomic philosophy is outline d that broadly reflects a phylogenetic framework based on living speci es (if available) and reconstructed whole-plant fossil species. These core species provide a framework into which are interpolated satellite taxa: fossil isolated organs and partial plants.