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The idea of an area of endemism implies that different groups of plants and animals should have largely coincident distributions. This paper analyses an area of 1152 000 km2, between parallels 21 and 32°S and meridians 70 and 53°W to examine whether a large and taxonomically diverse data set actually displays areas supported by different groups. The data set includes the distribution of 805 species of plants (45 families), mammals (25 families), reptiles (six families), amphibians (five families), birds (18 families), and insects (30 families), and is analysed with the optimality criterion (based on the notion of endemism) implemented in the program NDM/VNDM. Almost 50% of the areas obtained are supported by three or more major groups; areas supported by fewer major groups generally contain species from different genera, families, or orders. © The Willi Hennig Society 2011.

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Aim The main drainages of the Plata Basin -the Paraná, Paraguay and Uruguay rivers -begin in tropical latitudes and run in a north-south direction into subtropical-temperate latitudes. Consequently, the biota of these rivers has tropical elements that contrast with temperate biomes through which the rivers run. We apply a panbiogeographical approach, to test whether the large rivers of the Plata Basin have a differential influence on distributional patterns of tropical snakes in subtropical and temperate latitudes of South America.Location Subtropical and temperate sections of the major Plata Basin rivers, South America.Methods We compared the individual tracks of 94 snake taxa. The track analysis consisted of: (1) plotting the localities of each taxon on maps, (2) connecting the localities of each taxon using a minimal geographical proximity determinant of the 'individual tracks', and (3) superimposing the individual tracks to determine generalized tracks. To detect tropical snakes that reach higher latitudes through the rivers we used the preferential direction of distribution concept. For each taxon we measured the angular deviations between the line of its individual track and the course of the rivers in a 100 · 100 km scaled grid. Average angular values < 45°indicated a positive association with the rivers.Results Thirty-five of 94 taxa showed distributions associated with the major rivers of the Plata Basin, including fauna from distinct biogeographical lineages, supported by the occurrence of five generalized tracks as follows: (1) the Paraguay-Middle Paraná, (2) the Paraguay-Paraná fluvial axis, Upper Paraná and Middle Paraná to Upper Delta, (3) the Lower Paraguay, Paraná and Uruguay rivers, excluding the sectors High Paraná and High Uruguay, (4) the Uruguay River and Upper Paraná, and (5) the High Paraná. The Atlantic species occurred with significantly higher frequency in the Uruguay River and High Paraná river sections, the Amazon species were found with significantly higher frequency in the Paraguay and Middle Paraná sections, and the species with a Pantanal distribution were found in all sections. Main conclusionsThe observed distributional patterns may be explained by the interaction of ecological, geographical and historical factors. Previous authors have developed ecological (hydrological or environmental similarity) or dispersalist (effect of rivers as migration routes) explanations. The coincidence between generalized tracks and past geomorphological events that caused displacements and changed relationships between the Paraguay, Paraná and Uruguay river sections supports hypotheses involving the strong influence of historical factors in the present configuration of tropical snake distribution in temperate latitudes.