The Tepuianthaceae consists of the genus Tepuianthus, containing 7 taxa that are narrow endemics within the Guayana Highlands region of Venezuela and adjacent Colombia and Brazil. Hypotheses of the systematic affinities of this family have suggested relationships with the Celastrales (Cronquist) and Rutales (Takhtajan, Thorne), though the Tepuianthaceae is unplaced in the APG system. Parsimony analysis of sequence data from 18S rDNA, atpB, and rbcL places Tepuianthus within the Malvales, sister to the Thymelaeaceae, sensu lato (including Gonystylaceae). The secondary phloem of Tepuianthus is characteristic of Malvales in that it is tangentially stratified into fibrous and nonfibrous layers, with radially dilated phloem rays. Potential morphological synapomorphies with Thymelaeaceae include the absence of stipules, unilacunar nodes, a single ovule per locule with apical-axile placentation, ovules with a ventral raphe, and ovules with the micropyle formed exclusively from the inner integument. We propose to taxonomically accommodate Tepuianthus in a new Thymelaeaceae subfamily. The unusual pollen of Tepuianthus may represent an intermediate state between a tricolporate type--ubiquitous in early-branching lineages of major clades within Malvales--and the highly derived crotonoid exines found in Thymelaeaceae. The presence of a well-formed, 5-merous corolla in addition to a series of extrastaminal, glandular scales in Tepuianthus leads to the reinterpretation of similar scales in Gonystylus as not representing a vestigial corolla as previously thought. The phylogenetic position of Tepuianthus is biogeographically significant in that, like the malvalean dipterocarps Pakaraimaea and Pseudomonotes, it represents an early-branching, presumably relictual, Guayana Shield-centered lineage of an otherwise mostly paleotropical clade.

Key words: Malvales, phylogeny, Tepiuanthus, Tepuianthaceae, Thymelaeaceae