Abstract
A large, light green, mat-forming Fossombronia was discovered during explorations in the Río Anzu Reserve in eastern Ecuador in 2008. The plants were found only on scattered limestone boulders along the Río Anzu as it flowed through a narrow limestone gorge. A morphological study of specimens collected in 2008, using a combination of optical and scannning electron microscopy, was conducted. A unique suite of characters of this riverine Fossombronia, when compared with those of currently recognized species, supports its recognition as a new species, which is named and described herein as Fossombronia jostii Crand.-Stotl. & Gradst. The restriction of this species to a limestone, riverine habitat is shared by F. texana and F. wrightii, with which it shares some, but not all morphological characters. The impact of a major flooding event on the taxon is discussed, and a key is provided to the six species of Fossombronia currently known from Ecuador.