While hornworts clearly represent a monophyletic group, taxonomic boundaries and interrelationships among hornwort species and genera remain confused. In an effort to re-examine the phylogeny of hornworts we obtained more than 20 sequences for the rbcL (chloroplast DNA) gene from representatives of each of the five generally accepted genera of hornworts. The following hypotheses and observations surface from the analyses of these data: 1) The taxa fall into well-supported clades but the hypothesized interrelationship of these taxa is novel; 2) Anthoceros is basal to a clade containing two lineages: Notothylas plus Phaeoceros and Dendroceros plus Megaceros; 3) Taxonomic limits and the monophyly of taxa are challenged by the placement of Phaeoceros hirticalyx and P. coriaceus in the Megaceros clade and Anthoceros lamuniferus within A. punctatus; 4) Both Phaeoceros laevis s.lat. and Anthoceros punctatus exhibit genetic divergence among samples that suggests the possible presence of cryptic species; 5) Genetic divergence among putative genera is substantial, validating the suggestion that there are deep divergence points among extant taxa; 6) Rate heterogeneity suggests that differential rates of RNA editing in this gene will not pose a problem for subsequent analyses. An examination of morphological features in relation to this phylogeny provides novel interpretations of character evolution, especially modifications in spore wall structure, chloroplast microanatomy and stomata.

Key words: biogeography, cryptic speciation, Hornworts, phylogeny, rbcL, stomates