Equisetum is a small (15 extant species), easily recognized, and highly distinctive genus of vascular plants with a cosmopolitan distribution. Only Australia, New Zealand, and Antarctica lack native representatives. Most species are found between 40 and 60 degrees north latitude and they are generally confined to seasonally wet ground. Two subgenera have been recognized based on stomatal position and growth form: subg. Equisetum (8 species; superficial stomates; stems branched) and subg. Hippochaete (7 species; sunken stomates; stems unbranched). Fossils assigned to Equisetum or Equisitites date back to the Triassic Period or later, with possible records extending to the Carboniferous. Prior attempts at understanding Equisetum systematics, phylogeny, and character evolution have been hampered by the high degree of variability in the genus, as well as by rampant hybridization among members within each subgenus. Carefully avoiding hybrids, we present the first explicit phylogenetic study of Equisetum, including all 15 species, based on a combined analysis of two chloroplast markers, trnL-F and rbcL, and demonstrate robust support for two monophyletic groups that correspond to the two subgenera recognized by earlier workers. The species relationships resolved within each of these groups, however, are mostly in disagreement with previous views. In addition, the South American species E. bogotense is not a member of either of these two groups, but rather it is isolated as sister to the other 14 species. With a robust phylogeny in hand, we explore questions related to morphological character evolution and biogeography in this ancient genus. Contrary to earlier interpretations, unisexual gametophytes and a diminutive stature appear to be ancestral conditions. Several taxonomic characters such as dimorphism, previously considered to be good indicators of species relationships, are shown to be homoplastic. Using fossil evidence and geological vicariance of continents to calibrate divergence times, we also investigate possible biogeographic hypotheses.

Key words: biogeography, character evolution, Equisetum, fossils, horsetails, phylogeny, rbcL, trnL-F