Interplay Between River Formation and Historical Climatic Changes Drove Diversification of the Thamnomanes caesius and T. schistogynus (Passeriformes, Thamnophilidae) Species Complex Across Amazonian and Atlantic Forests

The Amazon and Atlantic forests harbour immense biodiversity shaped by complex evolutionary history with diverse processes leading to species build‐up. We investigated the Cinereous Antshrike species complex ( Thamnomanes caesius and T. schistogynus ) to disentangle the relative contributions of riverine barriers and historical climatic fluctuations to diversification, using large specimen sampling across their entire distribution and a multilocus dataset. Our results revealed hidden genetic diversity, with 13 geographically structured phylogroups largely concordant with Amazonian interfluves and areas of endemism. Species delimitation analyses supported eight of these lineages as independently evolving units, unveiling cryptic diversity within T. caesius . Divergence time estimates indicate that diversification took place during the Pliocene to Late Pleistocene, with signatures of demographic expansion consistent with climate‐driven range dynamics. Our findings underscore a strong association between phylogroups and major river systems, revealing their role as primary drivers of lineage diversification, later modulated by demographic shifts associated with climatic oscillations. Our findings highlight an interplay between geomorphological and climatic factors in shaping avian diversity across the Amazon basin, while also pointing to underestimated taxonomic richness within the Thamnomanes caesius /schistogynus species complex.