Biogeography of Brachyrhamdia thayeria The region where Brachyrhamdia thayeria is known to occur, situated about 40–50 meters above sea level, is typical Amazonian lowland formed by Tertiary sediments, and characterized by plan terrains drained by low-speed white-waters (Lima & Ribeiro, 2011). However, within this region, B. thayeria seems to prefer small streams not prone to flooding (“terra firme” streams), having comparatively lower sediment loads and consequently clearer (black) waters, and is associated with marginal vegetation and stream bottoms of sand or silt (Hercos et al., 2009; Queiroz & Hercos, 2009). It is interesting to note that the Leticia species of Brachyrhamdia occurs in the Laguna Yahuarcaca system, part of the lowlands of the Colombian Amazon and inhabits small black-water streams with densely forested surroundings (cf. Galvis et al., 2006), similar to habitats found in the Lago Amanã system. Although most species of Brachyrhamdia occur in the northern lowlands of South America, B. heteropleura, B. rambarrani, and an undescribed species from the Rio Tocantins basin inhabit rivers draining ancient highland, shield areas (Lima & Ribeiro, 2011). These species occur in watersheds on ancient basaltic basements that possibly suffered subsidence and were overlain by sediments, thus acquiring characteristics of lowlands and supporting a mixed highland/lowland fish fauna. For example, Brachyrhamdia heteropleura occurs in the Branco (Amazonas basin) and Essequibo (Atlantic coastal basin) draining the Takutu graben, a tectonic block that sunk, allowing the formation of a continuous sedimentary basin between the two basins (Crawford et al., 1985; Milani & Thomaz- Filho, 2000; Lima & Ribeiro, 2011; Lujan & Armbruster, 2011). Brachyrhamdia rambarrani is reportedly endemic to middle and lower reaches of the Rio Negro basin, below São Gabriel da Cachoeira, which bear characteristics of Amazonian lowlands (Latrubesse & Franzinelli, 2005; Lima & Ribeiro, 2011). The undescribed species endemic to the Araguaia-Tocantins is in a system which settled on the tectonically-developed Araguaia and Tocantins depressions (Saadi et al., 2005; Lima & Ribeiro, 2011). The genus Brachyrhamdia, as presently known, is distributed throughout the Amazon system (including the Rio Tocantins basin), part of the Orinoco River basin, and in the Essequibo and Corantijn river basins (Bockmann & Guazzelli, 2003; Slobodian, 2013). The distributional range of each species is well demarcated and nonoverlapping (Slobodian, 2013). Taking into account the broad distribution of the genus, the disjunctive distribution patterns of its species and the fact that they are weakly vagile, the present-day distribution of Brachyrhamdia is attributed predominantly to extrinsic causes, indicating that lowlands of northern South America, although recent in age, are geomorphologically dynamic.

Published as part of Slobodian, Veronica & Bockmann, Flávio Alicino, 2013, A new Brachyrhamdia (Siluriformes: Heptapteridae) from Rio Japurá basin, Brazil, with comments on its phylogenetic affinities, biogeography and mimicry in the genus, pp. 1-22 in Zootaxa 3717 (1) on page 15, DOI: 10.11646/zootaxa.3717.1.1, http://zenodo.org/record/284942