The two South American dry diagonals

South American drylands roughly form two diagonals both termed in the biogeographical literature as the "South American dry diagonal" (SADD). However, they correspond to two different geographical areas. One comprises the Caatinga, Cerrado and Chaco domains, thus encompassing the areas between northeastern Brazil and northwestern Argentina. The other stretches from Patagonia in southern Argentina to the Pacific deserts of northern Chile and Peru, thus also including the Monte, Prepuna and dry Puna domains. I termed them the eastern and western SADDs, respectively (i.e., eSADD and wSADD). In this mini review I attempt to summarize the major climatic features of the two South American dry diagonals, their possible origins, biogeographical patterns within and around them and to explore possible interconnections. The eSADD is generally more humid than the wSADD and has more pronounced rainfall seasonality, with precipitation concentrated in summer, while the wSADD tends to be less seasonal due to year-round aridity, with little precipitation largely occurring in winter. The origin of both diagonals appears to go back to the middle Miocene, associated with global cooling. Biogeographical studies show that these diagonals are important in structuring South American distribution patterns south of the Equator, both acting as barriers for humid-adapted lineages and corridors for arid-adapted taxa. Remarkably, the two diagonals appear to have few plant and animal taxa in common, which may explain why biogeographers speaking about one diagonal seem to ignore the existence of the other.