Argythamnia (Euphorbiaceae) from biogeographic provinces of the Chaco and Pampa in South America

Argythamnia ( Euphorbiaceae ) comprises approximately 70 species and is widely distributed in the New World, mainly restricted to seasonal environments. It is represented by seven species in the biogeographic provinces of the Chaco and Pampa, which are characterized by vegetation mosaics of grasslands and forests. Some species are poorly known, with only 3–5 collections restricted to specific areas, whereas others have many records and a wide distribution throughout these two provinces. Morphological similarities among many species make identification difficult. After several expeditions to observe and collect samples in the field, bibliographic revision, and examination of herbarium specimens, we present a taxonomic treatment of Argythamnia in the Chaco and Pampa provinces, where we recognize five species and provide descriptions, an identification key, and a distribution map. In addition, we propose three synonyms, four neotypifications, four lectotypifications, and report the first records of A. salina in Brazil, and the first record of A. montevidensis in the Chaco from the state of Mato Grosso do Sul, Brazil, a species known to date in this country only from the Pampa of Rio Grande do Sul state.