The Rhizomorphic Lycopsids: A Case-Study in Paleobotanical Classification

Abstract

The rhizomorphic lycopsids are a putatively monophyletic group delimited by the rhizomor- phic syndrome: centralized determinate growth and secondary thickening in their rooting systems. A cladistic analysis of the most completely understood species provides the basis for two alternative, new classifications of the group, one following strict phylogenetic classification criteria, the other more traditionally Linnean. Both are based on the same preferred most-parsimonious cladogram and place the rhizomorphic lycopsids in one order, the Isoetales. Also, both classifications delimit the same suprageneric groups: Hizemodendron + Lepidodendron + Lepidophloios, Diaphorodendron + Synchysidendron, Sigillaria, and Chaloneria + Isoietes are well supported as monophyletic clades, but the basal plexus of bisporangiate-coned ulodendrids (Paralycopodites + Oxroadia + Paurodendron) is paraphyletic. Both classifications differ from all previous studies in 1) assigning all rhizomorphic lycopsids to a single order, and 2) recognizing the trees Diaphorodendron plus Synchysidendron as a distinct family, the Diaphorodendraceae. A generally applicable paleobotanical taxonomic philosophy is outlined that broadly reflects a phylogenetic framework based on living species (if available) and recon- structed whole-plant fossil species. These core species provide a framework into which are interpolated satellite taxa: fossil isolated organs and partial plants.