The fossil-bearing stratigraphic sections of the Solimões Formation (southwestern Brazilian Amazonia) are exposed mainly along the Juruá, Purus, and Acre rivers, and in road cuts. These deposits have provided fossils of the four main lineages of Caviomorpha -Cavioidea, Erethizontoidea, Octodontoidea, and Chinchilloidea, contributing to the understanding on the evolution of tropical Neogene rodents. Herein, our knowledge about fossil rodents from this region is reviewed. New specimens are recorded, including taxa mentioned for this region for the first time, such as basal cavioids, Dolichotinae, Caviodon (Hydrochoeridae), and Drytomomys (Dinomyidae). Unfortunately, the deposits have no absolute ages, and based on palynological data and the biochronology of several taxa (mainly mammals), the encompassed fauna has been constrained to the late Miocene. However, some rodent lineages recorded here seem to be more related to older faunas, from the middle Miocene and Paleogene. Regarding the biogeographic and paleoenvironmental affinities, most of the Neogene rodents from the Acre region show more similarities to those from the Entre Rios, Argentina, and Urumaco, Venezuela, where wet environments were present during Neogene times. An increase in prospecting along southwestern Amazonian rivers looking for rodents (among other vertebrates) associated with methods to better constrain the ages of these faunal assemblages will contribute to a better understanding of the evolution of the tropical rodents as well as the stratigraphy and age of that portion of the basin.

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In this work, we analyse cheek teeth crown complexity through the calculation of fractal dimension in the giant caviomorph rodent Eumegamys paranensis (Late Miocene of Argentina) and evaluate its functional significance. Our results indicate that, in all teeth of Eumegamys paranensis, the fractal dimension was around 1.5, similar to the Koch quadratic curve type two. The anterior portions of the molars, with the highest values of fractal dimension, are interpreted as areas that supported greater occlusal pressures. Crown complexity in E. paranensis is related to the increased mechanical capacity to process relatively demanding food items and to allow more food to be divided in each masticatory cycle. Eumegamys paranensis would have been a mixed feeder, consuming a variable diet obtained close to the ground. This feeding behaviour is compatible with the heterogeneous environment inferred for the Mesopotamic area during the Late Miocene. E. paranensis was probably a wide ranging species, being able to eat close to water bodies and in gallery forests that occurred in the surrounding of the pre‐Paraná river system.

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