Redefinition of Stachytarpheta sprucei (Verbenaceae) reveals a remarkable geographic disjunction in tropical forests of South America

Stachytarpheta is the second largest genus of Verbenaceae, mainly distributed in xeric habitats of South America. The morphological revision of South American specimens clarified the taxonomic identity of three previously accepted species from informal group Gesnerioides: S. sprucei (from the Amazon Forest domain), S. alata , and S. tomentosa (from the Atlantic Forest domain). A new taxonomic arrangement is proposed here, with the synonymization of S. alata and S. tomentosa under S. sprucei . Consequently, the newly circumscribed S. sprucei evidences a remarkable geographic disjunction, with populations separated by the Caatinga, Cerrado, and Chaco domains (the South American dry diagonal). It inhabits inselbergs, tepuis, and savannas in the Amazon Forest domain, and top of inselbergs surrounded by forest in the Atlantic Forest domain. A detailed species description, taxonomic comments, a geographic distribution map, photos of living specimens, and an identification key to the species from the Gesnerioides group are included.