Untangling the taxonomic knot of Croton anomalus (Euphorbiaceae), a Neotropical dry forest shrub

Croton anomalus was described by Henri Pittier in 1930 from a collection made in Estado Lara, Venezuela, and the use of the name has so far been restricted to several states in this country. A reevaluation of the species has led to its recircumscription and recognition in several other countries, including Bolivia, Brazil, Colombia, French Guiana, Guyana, Mexico, and Suriname. Previously, it was confused with species referred to here as the “Croton anomalus group”, namely C. acapulcensis, C. blanchetianus, C. chiapensis, C. jacobinensis (=C. sonderianus), and C. stahelianus. We integrated morphological, phylogenetic, and ecological evidence to understand species limits and relationships within the Croton anomalus group. We first studied ca. 650 herbarium specimens covering the geographic range of the group, and we inferred species phylogenetic relationships using DNA sequences from the nuclear and plastid regions (ITS and trnL-F). We also used ecological niche modeling to infer potential suitable areas for the occurrence of the studied species and to determine the variables that most contribute to their distribution model. Both morphological and phylogenetic data provide evidence for the synonymization of C. acapulcensis, C. chiapensis, and C. stahelianus under C. anomalus. On the other hand, our results support the recognition of C. blanchetianus and C. jacobinensis as two independent lineages, both distinct from C. anomalus. An emended description of C. anomalus is provided, as well as the designation of lectotypes, illustrations, updates of distribution data, and morphological comparisons with closely related species. Regarding niche modeling, annual precipitation and the precipitation of the warmest quarter were the most important variables explaining species distributions. Croton anomalus showed suitable areas in most seasonally dry tropical forests in the Neotropics, while C. blanchetianus and C. jacobinensis had their most suitable areas restricted to the Caatinga Dry Forest (Brazil), and Caatinga + northern South America, respectively. Our study shows the importance of taxonomic revisions using integrative approaches to disentangling species boundaries and to elucidate their biogeography and conservation status.