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A new tribe of sigmodontine rodents is formally defined and diagnosed. The tribe contains the extant genera Abrothrix (including Chroeomys), Chelemys, Geoxus, Pearsonomys, and Notiomys, a group of taxa distributed in the central and southern Andes and lowlands both east and west of the cordillera. The new tribe presents a unique combination of characters including nasals and premaxilla usually projected anterior to the incisors, moderately trumpeted; zygomatic plate with upper free border reduced or obsolete; and third upper molar reduced and sub-cylindrical in outline with an internal ring-like enamel fossette. Molecular phylogenetic analyses indicate that Abrothrix is sister to a clade containing the remaining genera of the new tribe.

The last 500 years of the historical era, an interval that corresponds to the concerted dispersal of European explorers, traders, and colonists around the globe, has witnessed the global disappearance of ~90 mammal species. Besides the known cases of biological extinctions, this time period was also characterized by the regional extirpations of specialized taxa and by the expansion and population growth of some opportunistic species. Several lines of evidence suggest that the current configuration of small mammal communities-i. e. richness (number of species) and diversity (distribution of species abundance)-in southern South America would have been generated in the period after the arrival of Europeans ca. 1500 AD. In this study, we reviewed the fossil record for small rodents and marsupials during the last 500 years, with emphasis on the Pampean and Patagonian regions. Based on these findings, we offer some considerations concerning the biogeography and conservation of these species. Methods: Micromammals from archaeological and paleontological sites have provided considerable information on environmental conditions during the Quaternary in South America. In this work, we reviewed several micromammal fossil samples, mostly generated by the trophic activity of owls. The use of this kind of data involves some extrinsic and intrinsic biases that must be considered at the time to study the diversity of past communities, such as the bird involved on the accumulations, its hunting techniques, the time of the year, the size and behavior of the prey species, etc. For this work, we compared the fossil samples with more than 700 owl pellet assemblages of the same geographical areas. Results: Richness and diversity of small mammal communities was higher prior to the deepest human impact (> 0.5 ka), showing a pronounced drop in both parameters to the present. The regional extinction of some cricetid rodents and small marsupials was recorded, both in the Pampean (e. g. Bibimys torresi, Pseudoryzomys simplex) and in the Patagonian regions (e. g. Euneomys mordax, Lestodelphys halli, Tympanoctomys kirchnerorum), as well as the biological extinctions of the bat Desmodus cf. D. draculae and the cavy Galea tixiensis in the Pampas. At this same time, opportunistic species (e. g. Calomys spp., Oligoryzomys longicaudatus) showed a dramatic increase of their populations, especially in the most extensively disturbed areas. Discussion and conclusions: In the last 500 years, significant variations occurred in the micromammal assemblages of southern South America. These changes included the dispersion and increase (in some cases extensive) of some specialized species and the regional extinctions of others, involving hundreds to thousands of kilometers of distributional extent. We suggest that the changes produced by livestock and agriculture have caused extensive habitat uniformity which was beneficial for some opportunistic taxa (e. g. Calomys spp., Oligoryzomys longicaudatus), facilitating their dispersion and allowing the increase of ...

The Patagonian and Fuegian regions host a rich assemblage of sigmodontine rodents. In this article, we present a synthesis of our current understanding of their late Neogene diversification. Sigmodontine diversity in this area encompasses 16 genera and about 24 species; however, most belong to the tribes Abrotrichini and Phyllotini. Several abrotrichine genera are endemic to the region, whereas phyllotines are represented mainly by species widely distributed outside Patagonia. Two main ecogeographical assemblages of sigmodontines can be recognized: a lowland northeastern group with species mostly associated with shrub formations of the Monte Desert, and a medium to highland southwestern group of typical Patagonian elements, including several abrotrichines and a diversity of Andean forms. The pattern of latitudinal decline in species' number in sigmodontine rodents is more complex than the traditionally envisioned north to south pauperization; southern mainland Patagonia is as poor in species' number as is northern Tierra del Fuego (six species), suggesting that insularity is insufficient to explain the island assemblage. Glacial cycles may have had a major role in the control of sigmodontine richness. The fossil Fuego–Patagonian sigmodontine record is restricted to late Pleistocene–Holocene times. Remarkable events include regional extinctions of several widespread southwestern species during the latest Holocene and a reworking of micromammal assemblages, probably as a result of anthropogenic environmental changes. Two main phylogeographical patterns can be broadly associated with the northeastern and southwestern groups. The northeastern assemblage basically comprises species without phylogeographical structure, whereas the southwestern group involves several lineages with deep phylogeographical breaks. The current Fuego–Patagonian sigmodontine assemblage is mainly composed of species that colonized the area from lower latitudes and by others that have differentiated in situ. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103, 495–513. La región Patagónico‐Fueguina comporta un rico ensamble de roedores sigmodontinos. En este trabajo presentamos una síntesis del conocimiento sobre los procesos de diversificación del grupo durante el Neógeno tardío. La diversidad de sigmodontinos comprende 16 géneros y cerca de 24 especies; sin embargo, la mayor parte de las mismas pertenecen a las tribus Abrotrichini y Phyllotini. Varios géneros de abrotriquinos son endémicos de la región, mientras que los filotinos están en general representados por especies de amplia distribución fuera de Patagonia. Se pueden reconocer dos grandes ensambles eco‐geográficos de sigmodontinos: un grupo nor‐oriental de tierras bajas, con especies mayoritariamente asociadas a las formaciones vegetales arbustivas del Monte, y otro sud‐occidental de tierras de mediana a alta elevación, que agrupa típicos elementos patagónicos, incluyendo abrotriquinos y una diversidad de formas andinas. El patrón de disminución ...

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