Pliocene sigmodontine rodents (Mammalia: Cricetidae) in northernmost South America: test of biogeographic hypotheses and revised evolutionary scenarios.

We document the first occurrence of Sigmodontinae (Mammalia, Rodentia, Cricetidae) from the Pliocene of northern South America, from the San Gregorio Formation of northwestern Venezuela. The recovered isolated molars are identified as Oligoryzomys sp. and Zygodontomys sp., two currently widespread sigmodontines in South America. These records constitute the oldest representatives of these genera, potentially new species, and the first Pliocene occurrence for Oryzomyini and the whole subfamily outside Argentina. Hypotheses on the historical biogeography of sigmodontines have been constructed almost exclusively using genetic data and the fossils we report provide a new kind of evidence. The occurrence of Oligoryzomys sp. and Zygodontomys sp. in Venezuela provides novel information for the diversification models suggested for Oligoryzomys, by supporting a potential eastern corridor of open environments from northern to southern South America. The presence of sigmodontines from the locality home of the new reports, Norte Casa Chiguaje, is consistent with the palaeoenvironmental conditions originally proposed for it based on mammals and botanical records, being characterized as mixed open grassland/forest areas surrounding permanent freshwater systems. The new sigmodontine evidence is used to discuss the putative scenarios of the ancient evolution of the subfamily in South America, favouring a model in which open areas (savannahs) to the east of the Andes played crucial role aiding or obstructing Late Miocene-Pliocene sigmodontine dispersion southwards.

Systematics and diversification of the Ichthyomyini (Cricetidae, Sigmodontinae) revisited: evidence from molecular, morphological, and combined approaches.

Ichthyomyini, a morphologically distinctive group of Neotropical cricetid rodents, lacks an integrative study of its systematics and biogeography. Since this tribe is a crucial element of the Sigmodontinae, the most speciose subfamily of the Cricetidae, we conducted a study that includes most of its recognized diversity (five genera and 19 species distributed from southern Mexico to northern Bolivia). For this report we analyzed a combined matrix composed of four molecular markers (RBP3, GHR, RAG1, Cytb) and 56 morphological traits, the latter including 15 external, 14 cranial, 19 dental, five soft-anatomical and three postcranial features. A variety of results were obtained, some of which are inconsistent with the currently accepted classification and understanding of the tribe. Ichthyomyini is retrieved as monophyletic, and it is divided into two main clades that are here recognized as subtribes: one to contain the genus Anotomys and the other composed by the remaining genera. Neusticomys (as currently recognized) was found to consist of two well supported clades, one of which corresponds to the original concept of Daptomys. Accordingly, we propose the resurrection of the latter as a valid genus to include several species from low to middle elevations and restrict Neusticomys to several highland forms. Numerous other revisions are necessary to reconcile the alpha taxonomy of ichthyomyines with our phylogenetic results, including placement of the Cajas Plateau water rat (formerly Chibchanomys orcesi) in the genus Neusticomys (sensu stricto), and the recognition of at least two new species (one in Neusticomys, one in Daptomys). Additional work is necessary to confirm other unanticipated results, such as the non-monophyletic nature of Rheomys and the presence of a possible new genus and species from Peru. Our results also suggest that ichthyomyines are one of the main Andean radiations of sigmodontine cricetids, with an evolutionary history dating to the Late Miocene and subsequent cladogenesis during the Pleistocene.

Unlocking Andean sigmodontine diversity: five new species of Chilomys (Rodentia: Cricetidae) from the montane forests of Ecuador.

The Andean cloud forests of Ecuador are home to several endemic mammals. Members of the Thomasomyini rodents are well represented in the Andes, with Thomasomys being the largest genus (47 species) of the subfamily Sigmodontinae. Within this tribe, however, there are genera that have escaped a taxonomic revision, and Chilomys Thomas, 1897, constitutes a paradigmatic example of these "forgotten" Andean cricetids. Described more than a century ago, current knowledge of this externally unmistakable montane rodent is very limited, and doubts persist as to whether or not it is monotypic. After several years of field efforts in Ecuador, a considerable quantity of specimens of Chilomys were collected from various localities representing both Andean chains. Based on an extensive genetic survey of the obtained material, we can demonstrate that what is currently treated as C. instans in Ecuador is a complex comprising at least five new species which are described in this paper. In addition, based on these noteworthy new evidence, we amend the generic diagnosis in detail, adding several key craniodental traits such as incisor procumbency and microdonty. These results indicate that Chilomys probably has a hidden additional diversity in large parts of the Colombian and Peruvian territories, inviting a necessary revision of the entire genus.

New radiometric 40Ar-39Ar dates and faunistic analyses refine evolutionary dynamics of Neogene vertebrate assemblages in southern South America.

The vertebrate fossil record of the Pampean Region of Argentina occupies an important place in South American vertebrate paleontology. An abundance of localities has long been the main basis for constructing the chronostratigraphical/geochronological scale for the late Neogene-Quaternary of South America, as well as for understanding major patterns of vertebrate evolution, including the Great American Biotic Interchange. However, few independently-derived dates are available for constraining this record. In this contribution, we present new 40Ar/39Ar dates on escorias (likely the product of meteoric impacts) from the Argentinean Atlantic coast and statistically-based biochronological analyses that help to calibrate Late Miocene-Pliocene Pampean faunal successions. For the type areas of the Montehermosan and Chapadmalalan Ages/Stages, our results delimit their age ranges to 4.7-3.7 Ma and ca. 3.74-3.04 Ma, respectively. Additionally, from Buenos Aires Province, dates of 5.17 Ma and 4.33 Ma were recovered for "Huayquerian" and Montehermosan faunas. This information helps to better calibrate important first appearances of allochthonous taxa in South America, including one of the oldest records for procyonids (7.24-5.95 Ma), cricetids (6.95-5.46 Ma), and tayassuids (> 3.74 Ma, oldest high-confidence record). These results also constrain to ca. 3 Ma the last appearances of the autochthonous sparassodonts, as well as terror birds of large/middle body size in South America. South American faunal turnover during the late Neogene, including Late Pliocene extinctions, is interpreted as a consequence of knock-on effects from global climatic changes and initiation of the icehouse climate regime.

Morphological similarity and dental homologies in two sigmodontine rodents (Mammalia, Cricetidae) from different tribes: A topological analysis to explore convergence.

We present a topological analysis of the third upper molars (M3) using the recently developed ICAMER nomenclatural system as a way to understand the dental morphological similarity in sigmodontine rodents, the most speciose subfamily of cricetids. The method is explored in Scapteromys aquaticus and Abrothrix olivacea, taxa belonging to two diverse tribes, Akodontini and Abrotrichini, respectively, which exhibit high similarity regarding several craniodental traits as well as external anatomy. Both species show morphologically similar M3 in adults characterized by cylindrification and the isolation of a large central fossette arising from the marginal fusion of the anterior and posterior lobes. The results indicate that, before the wear, these rodents have a strongly different topological pattern at the cuspal level, mostly involving production of the connection between the main cusps. The central fossette derives from the isolation of part of the metaflexus in Scapteromys, while in Abrothrix it originates from the hypoflexus. The topological analysis provides a new approach to sigmodontine systematics, including the ability to detect diagnostic characters of both tribes. More important, it constitutes a new step toward an integrative phylogeny of fossil and living cricetids.

A new genus of oryzomyine rodents (Cricetidae, Sigmodontinae) with three new species from montane cloud forests, western Andean cordillera of Colombia and Ecuador.

The Andean cloud forests of western Colombia and Ecuador are home to several endemic mammals; members of the Oryzomyini, the largest Sigmodontinae tribe, are extensively represented in the region. However, our knowledge about this diversity is still incomplete, as evidenced by several new taxa that have been described in recent years. Extensive field work in two protected areas enclosing remnants of Chocó montane forest recovered a high diversity of small mammals. Among them, a medium-sized oryzomyine is here described as a new genus having at least three new species, two of them are named and diagnosed. Although externally similar to members of the genera Nephelomys and Tanyuromys, the new genus has a unique molar pattern within the tribe, being characterized by a noticeable degree of hypsodonty, simplification, lamination, and third molar compression. A phylogeny based on a combination of molecular markers, including nuclear and mitochondrial genes, and morphological data recovered the new genus as sister to Mindomys, and sequentially to Nephelomys. The new genus seems to be another example of a sigmodontine rodent unique to the Chocó biogeographic region. Its type species inhabits cloud forest between 1,600 and 2,300 m in northernmost Ecuador (Carchi Province); a second species is restricted to lower montane forest, 1,200 m, in northern Ecuador (Imbabura Province); a third putative species, here highlighted exclusively by molecular evidence from one immature specimen, is recorded in the montane forest of Reserva Otonga, northern Ecuador (Cotopaxi Province). Finally, the new genus is also recorded in southernmost Colombia (Nariño Department), probably represented there also by a new species. These species are spatially separated by deep river canyons through Andean forests, resulting in marked environmental discontinuities. Unfortunately, Colombian and Ecuadorian Pacific cloud forests are under rapid anthropic transformation. Although the populations of the type species are moderately abundant and occur in protected areas, the other two persist in threatened forest fragments.

Corrections and emendations to the description of Deltamys araucaria Quintela et al., 2017 (Rodentia, Cricetidae).

A new species was recently described to the akodontine genus Deltamys Thomas, 1917, based on animals collected in Araucaria forest, Rio Grande do Sul state, Brazil. Quintela et al. (2017) described Deltamys araucaria on the basis of the integration of morphological, chromosomal, and molecular characters analysis. Here I correct and/or emend critical aspects of this paper, including the type locality, diagnostic characters, and the appropriate recognition of earlier literature on this same taxon. My aim in this note is not to criticize this important new contribution but to facilitate future communication among researchers.

Phylogeny of the tribe Abrotrichini (Cricetidae, Sigmodontinae): integrating morphological and molecular evidence into a new classification.

The tribe Abrotrichini (five genera and 14 living species) is a small clade within the speciose subfamily Sigmodontinae (Rodentia, Cricetidae), representing one of the extant successful radiations of mammals at southern high latitudes of the Neotropics. Its distribution is mostly Andean, reaching its greatest diversity in southern Argentina and Chile. We evaluate the phylogenetic relationships within this tribe through parsimony and Bayesian approaches based on 99 morphological characters (including 19 integumental characters, 38 skull characters, 31 dental characters, three postcranial skeletal characters, seven from the male accessory glands and phallus and one from the digestive system) and six molecular markers (one mitochondrial and five nuclear). We include representatives of all, except one, of the currently recognized species of living Abrotrichini plus one fossil form. Based on total evidence, we recovered a primary division between the genus Abrothrix and a group including the long-clawed Abrotrichini, Chelemys, Geoxus, Notiomys and Pearsonomys. Both clades are recognized and named here as subtribes. The large degree of morphological variation observed within Abrothrix suggests that species in the genus fall into four groups, which we recognize as subgenera. In addition, the two known species of Chelemys do not form a monophyletic group, and Geoxus was recovered as paraphyletic with respect to Pearsonomys. To reconcile classification and phylogenetics, we describe a new genus for Chelemys macronyx and include Pearsonomys as a junior synonym of Geoxus. Our results highlight the importance of both morphology and molecules in resolving the phylogenetic relationships within this tribe. Based on biogeographical analyses, we hypothesize that Abrotrichini originated in south-western South America by vicariance and then diversified mostly by successive dispersal events.

Regional Extinctions and Quaternary Shifts in the Geographic Range of Lestodelphys halli, the Southernmost Living Marsupial: Clues for Its Conservation.

The Patagonian opossum (Lestodelphys halli), the southernmost living marsupial, inhabits dry and open environments, mainly in the Patagonian steppe (between ~32 °S and ~49 °S). Its rich fossil record shows its occurrence further north in Central Argentina during the Quaternary. The paleoenvironmental meaning of the past distribution of L. halli has been mostly addressed in a subjective framework without an explicit connection with the climatic "space" currently occupied by this animal. Here, we assessed the potential distribution of this species and the changes occurred in its geographic range during late Pleistocene-Holocene times and linked the results obtained with conservation issues. To this end, we generated three potential distribution models with fossil records and three with current ones, using MaxEnt software. These models showed a decrease in the suitable habitat conditions for the species, highlighting a range shift from Central-Eastern to South-Western Argentina. Our results support that the presence of L. halli in the Pampean region during the Pleistocene-Holocene can be related to precipitation and temperature variables and that its current presence in Patagonia is more related to temperature and dominant soils. The models obtained suggest that the species has been experiencing a reduction in its geographic range since the middle Holocene, a process that is in accordance with a general increase in moisture and temperature in Central Argentina. Considering the findings of our work and the future scenario of global warming projected for Patagonia, we might expect a harsh impact on the distribution range of this opossum in the near future.

Dating an impressive Neotropical radiation: Molecular time estimates for the Sigmodontinae (Rodentia) provide insights into its historical biogeography.

With about 400 living species and 82 genera, rodents of the subfamily Sigmodontinae comprise one of the most diverse and more broadly distributed Neotropical mammalian clades. There has been much debate on the origin of the lineage or the lineages of sigmodontines that entered South America, the timing of entrance and different aspects of further diversification within South America. The ages of divergence of the main lineages and the crown age of the subfamily were estimated by using sequences of the interphotoreceptor retinoid binding protein and cytochrome b genes for a dense sigmodontine and muroid sampling. Bayesian inference using three fossil calibration points and a relaxed molecular clock estimated a middle Miocene origin for Sigmodontinae (∼12Ma), with most tribes diversifying throughout the Late Miocene (6.9-9.4Ma). These estimates together results of analyses of ancestral area reconstructions suggest a distribution for the most recent common ancestor of Sigmodontinae in Central-South America and a South American distribution for the most recent common ancestor of Oryzomyalia.

Genetic footprints of late Quaternary climate change in the diversity of Patagonian-Fueguian rodents.

Species are impacted by climate change at both ecological and evolutionary time scales. Studies in northern continents have provided abundant evidence of dramatic shifts in distributions of species subsequent to the last glacial maximum (LGM), particularly at high latitudes. However, little is known about the history of southern continents, especially at high latitudes. South America is the only continent, other than Antarctica, that extends beyond 40 degrees S. Genetic studies of a few Patagonian species have provided seemingly conflicting results, indicating either postglacial colonization from restricted glacial refugia or persistence through glacial cycles and in situ differentiation. Using mitochondrial DNA sequences of 14 species of sigmodontine rodents, a major faunal ensemble of Patagonia and Tierra del Fuego, we show that at least nine of these species bear genetic footprints of demographic expansion from single restricted sources. However, timing of demographic expansion precedes the LGM in most of these species. Four species are fragmented phylogeographically within the region. Our results indicate that (i) demographic instability in response to historical climate change has been widespread in the Patagonian-Fueguian region, and is generally more pronounced at high latitudes in both southern and northern continents; (ii) colonization from lower latitudes is an important component of current Patagonian-Fueguian diversity; but (iii) in situ differentiation has also contributed to species diversity.

A role for suppressed incisor cuspal morphogenesis in the evolution of mammalian heterodont dentition.

Changes in tooth shape have played a major role in vertebrate evolution with modification of dentition allowing an organism to adapt to new feeding strategies. The current view is that molar teeth evolved from simple conical teeth, similar to canines, by progressive addition of extra "cones" to form progressively complex multicuspid crowns. Mammalian incisors, however, are neither conical nor multicuspid, and their evolution is unclear. We show that hypomorphic mutation of a cell surface receptor, Lrp4, which modulates multiple signaling pathways, produces incisors with grooved enamel surfaces that exhibit the same molecular characteristics as the tips of molar cusps. Mice with a null mutation of Lrp4 develop extra cusps on molars and have incisors that exhibit clear molar-like cusp and root morphologies. Molecular analysis identifies misregulation of Shh and Bmp signaling in the mutant incisors and suggests an uncoupling of the processes of tooth shape determination and morphogenesis. Incisors thus possess a developmentally suppressed, cuspid crown-like morphogenesis program similar to that in molars that is revealed by loss of Lrp4 activity. Several mammalian species naturally possess multicuspid incisors, suggesting that mammals have the capacity to form multicuspid teeth regardless of location in the oral jaw. Localized loss of enamel may thus have been an intermediary step in the evolution of cusps, both of which use Lrp4-mediated signaling.