Systematics and evolution of cladoxylopsids: a consequential yet underappreciated tracheophyte group with a deep fossil history

Abstract The cladoxylopsids are the first of the major euphyllophyte groups recognized in Middle Devonian and younger floras to acquire individuality from among a plexus of simple Early Devonian euphyllophytes. The first plants to evolve arboreous growth habits were members of this group, which has also been hypothesized to lie at the origin of ferns and equisetophytes. However, despite such multifaceted importance, and despite having been first recognized more than one-and-a-half centuries ago and subsequently documented from Early Devonian to Mississippian localities worldwide, our understanding of the cladoxylopsid clade is still quite incomplete. Nevertheless, the last 30 yr have seen consequential discoveries that provided key insights on their development, architecture, and evolution. Our review of the cladoxylopsids, aimed at providing an entry point to the study of this particularly challenging group, addresses their taxonomic history and the current status and diversity of the different subgroups (iridopterids, and pseudosporochnalean and cladoxylalean cladoxylopsids); their geographic distribution across their stratigraphic range; and evolutionary trends in their stelar architecture and ultimate appendage morphology, position in the body plan and taxis, and their ontogeny and growth habits as formalized in architectural models. Reviewing all of these aspects led us to identify the main gaps in our understanding of cladoxylopsids and to formulate questions about the developmental and functional significance of their unique vascular system in relation to their morphologies, the evolutionary patterns of their reproductive organs, and the still unresolved phylogenetic relationships of the group with other major lineages of pteridophytes and lignophytes within the euphyllophytes.