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Caviomorph rodents are endemic to South America and are one of the most adaptively diverse radiations of rodents today. Although their origin and diversification have been intensively studied, questions still remain about many of the details of where, when and how the group radiated. One area of continuing debate relates to the evolution of their neurosensory system. Modern caviomorphs exhibit a rich brain shape and size diversity. So far the oldest species for which endocranial data are known is dated to the Early Miocene. Here, we describe the virtual brain endocast of the late Oligocene stem chinchillid Incamys bolivianus from Bolivia and provide new hypotheses regarding the ancestral brain of Chinchillidae and Caviomorpha more broadly. Caviomorph rodents, independent from other rodent clades, acquired an expanded neocortex and their common ancestor was probably lissencephalic or had few sulci. Incamys uniquely combines extended neocortical temporal lobes and exposed caudal colliculi, which have roles in audition and vocalization processing. We interpret this morphology as evidence for enhanced auditory acuity, vocalization processing and potentially group‐living in Incamys, which is known in modern members of the Chinchillidae family. No temporal effect was found on relative brain size in South American mammals; however, our sample is limited to available brain endocasts and as such remains small and unevenly distributed taxonomically and temporally. Incamys provides crucial insight into the evolution of the caviomorph brain and shows that we still have much to explore regarding how these small mammals achieved one the most impressive adaptive radiations of the Cenozoic. [ABSTRACT FROM AUTHOR]

The Neogene strata of the Solimões Formation (northern Brazil) have revealed an astonishing diversity of extinct caviomorph rodents. The main exposures of this formation are found along the Juruá, Acre, and Purus rivers, and some road cuts in the State of Acre. In this paper, we reported a new locality containing a rodent assemblage. The outcrop (Coqueiro locality) is located in the Envira River, central region of State of Acre, Brazil. During fieldwork in 2010 and 2019, several isolated cheek teeth and bones of mammals were recovered in conglomeratic levels from this locality. Here, we described specimens assigned to Cardiatherium sp. (Caviidae: Hydrochoerinae), Potamarchus sp. and cf. Pseudopotamarchus (Dinomyidae: Potamarchinae), and Neoepiblema sp. (Neoepiblemidae). The locality is not yet dated by radiometric methods but based on the biostratigraphic information of the taxa here reported, we suggest that the fossil-bearing levels of this locality were deposited during the Late Miocene. [ABSTRACT FROM AUTHOR]

The Amazonian region covers a significant part of the South American continent and harbors outstanding biodiversity. However, much of its history is still unknown. This situation has begun to change with paleontological field efforts over the last decades, which have been proving that fossils can be common in this region. Despite their great current species richness and abundance in the area today, marsupials have a sparse fossil record, restricted to a few specimens from handful Cenozoic Amazonian localities. Here we present new records of fossil marsupial teeth from the Solimões Formation (lower Eocene–Pliocene), on the Juruá and Envira riverbanks (Acre, Northwestern Brazil). The localities investigated yield at least four distinct didelphid didelphimorphians at PRE 06 (Ponto Rio Envira: Marmosini ?Marmosa sp., Didelphis cf. D. solimoensis, Thylamys? colombianus, plus unidentified didelphids), and two paucituberculatans from the Juruá River localities (Ponto Rio Juruá: the palaeothentid Palaeothentinae indet. at PRJ 25 and PRJ 33’, and Abderitidae indet. from PRJ 33). In agreement with the associated mammalian faunas, most of the didelphids, except for Thylamys? colombianus from PRE 06, indicate a (?early) Late Miocene age for this locality. Conversely, the abderitid specimens found in situ at PRJ 33 would match a Middle Miocene age. The palaeothentids found at PRJ 25 and PRJ 33’ localities cannot be considered for biostratigraphic inferences, since they were found outside a stratigraphic context. Nevertheless, these paucituberculatans considerably add to our knowledge, as they are the first ever recorded in Brazilian Amazonia. [ABSTRACT FROM AUTHOR]