Based on phylogenetic and morphological evidence, Tridimeris
chiapensis Escobar-Castellanos & Ortiz-Rodr., sp. n. (Annonaceae), a new species from the karst forest of southern Mexico, is described and illustrated. The new species differs from Tridimeris
hahniana, the only described species in the genus, in that the latter has flowers with sepals densely tomentose outside, one (rarely two) carpel(s) per flower and fruits densely covered with golden-brown hairs, while Tridimeris
chiapensis has flowers with glabrous sepals outside, two to five carpels per flower and glabrous fruits. Furthermore, a shallow triangular white patch at the base of the inner petals is found in Tridimeris
chiapensis, a morphological character shared with the sister genus Sapranthus but absent in Tridimeris
hahniana. Geographically, both species occur allopatrically. With just one known locality and seven individuals of Tridimeris
chiapensis recorded in one sampling hectare, and based on application of the criteria established by the IUCN, we conclude tentatively that the species is critically endangered.
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