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The South American Ctenomys is the most speciose genus among both hystricomorphs and subterranean rodents of the world. Here, we present the most exhaustive phylogenies and timetree of living and extinct Ctenomys attempted thus far. We describe Ctenomys rusconii sp. nov., a small-sized species from the upper Early Pleistocene of central Argentina. We analyse its cranial and mandibular shape as well as its phylogenetic position in the context of other extinct Ctenomys and a wide sample of living species. A parsimony analysis shows that Ctenomys rusconii sp. nov. integrates the stem group of Ctenomys together with late Pliocene to Middle Pleistocene species, while the crown group comprises only Middle Pleistocene to recent representatives. Within the crown Ctenomys, nine well-supported major clades were recovered, eight of which are consistent with previously recognized extant species groups. A Bayesian tip-dating analysis provided divergence age estimates of 3.8 Ma and 1.3 Ma for the origin of the genus and the crown clade, respectively. Remarkably, the extinct species recovered as members of the crown clade, i.e. C. dasseni, C. kraglievichi, C. subassentiens and C. viarapaensis, were clustered into the earliest diverging clade corresponding to the frater species group. Age estimates for the divergence of the crown and its major clades are markedly younger than what has been generally considered so far, which implies a new view on the timing of taxonomic, ecological and geographical diversification of the genus. Even considering that this interpretation is affected by biases inherent to the fossil record, the phylogenetic delimitation of the crown clade as restricted to species recorded since the Middle Pleistocene seems to configure a pattern underlain by a fast and late cladogenesis. [ABSTRACT FROM AUTHOR]

Octodontoidea is the most species-rich clade among hystricomorph rodents, and has a fossil record going back to at least the late Oligocene. Affinities of fossils previous to the late Miocene differentiation of the extant families Abrocomidae, Echimyidae and Octodontidae are controversial, essentially because these fossils may share few apomorphies with modern species. In fact, pre-late Miocene representatives of Abrocomidae had not been recognised until very recently. Here we revise the early Miocene genusAcarechimys, originally assigned to Echimyidae, and alternatively to stem Octodontoidea or to Octodontidae. A systematic and parsimony-based phylogenetic analysis of the species traditionally included inAcarechimysshowed that this genus is part of stem Abrocomidae. These results are primarily supported by morphology of the mandible and lower molars.Acarechimysis here restricted to three species,A. minutus,A. pulchellusandAcarechimys pascualisp. nov., while another species,A. constans, is here transferred to a new abrocomid genus. The remaining species were nested within Octodontidae. According to these results, Abrocomidae might have been as diverse as its sister clade Octodontidae-Echimyidae during the late Oligocene–early Miocene. Extinction of this diversity would have resulted in marked loss of evolutionary history, with extant abrocomids being currently restricted to late-diverged euhypsodont representatives. [ABSTRACT FROM PUBLISHER]

Octodontoidea is the most diverse clade of hystricognath rodents, and is richly recorded in South America since at least the Oligocene. A parsimony-based morphological phylogenetic analysis of a wide range of extant and extinct octodontoids recovered three major clades, here recognised as Echimyidae, Octodontidae, and Abrocomidae. Taxa previously assigned to Echimyidae or Octodontoidea incertae sedis are here interpreted for the first time as early representatives of Ctenomyinae (Octodontidae), Octodontinae or Abrocomidae. Based on our results, we estimate the divergence of octodontoid families and subfamilies to have occurred during the Late Oligocene, which is consistent with molecular estimates, but older than previous inferences based on the fossil record. Contrary to previous suggestions, we show the first appearances of modern members of Abrocomidae, Octodontinae and Ctenomyinae to be distinctly decoupled from the origin of these clades, with different stages in the evolutionary history of octodontoids seemingly following distinct phases of palaeoenvironmental change. Depending on the phylogenetic pattern, fossils from the stage of differentiation bear evolutionary information that may not be provided by crown groups, thus highlighting the unique and important contribution of fossils to our understanding of macroevolutionary patterns. [ABSTRACT FROM AUTHOR]

Abstract: The scapula of the ecomorphologically diverse South American caviomorph rodents was studied through geometric morphometric techniques, using landmarks and semilandmarks to capture the shape of this complex morphological structure. Representatives of 33 species from all caviomorph superfamilies, as well as Hystrix cristata for comparisons, were analyzed. Marked differences in scapular shape were found among the major caviomorph lineages analyzed, particularly in the shape and length of the scapular spine and development of the great scapular notch. Shape differences were not influenced by body size, and only partially influenced by locomotor mode. Thus, at this scale of analysis, phylogenetic history seems to be the strongest factor influencing scapular shape. The scapular shape of erethizontids, chinchillids and Cuniculus paca could represent the less specialized state with respect to the highly differentiated scapula of octodontoids and most cavioids. In this sense, the characteristic scapular morphologies of octodontoids and cavioids could reflect particular functional capabilities and constraints associated with the evolution of prevalent locomotor modes within each lineage. [Copyright &y& Elsevier]

The aim of this study was to analyse the morphofunctional and adaptive significance of variation in the upper incisor enamel microstructure of South American burrowing ctenomyids and other octodontoid taxa. We studied the specialized subterranean tooth-digger † Eucelophorus chapalmalensis (Pliocene – Middle Pleistocene), and compared it with other fossil and living ctenomyids with disparate digging adaptations, two fossorial octodontids and one arboreal echimyid. Morphofunctionally significant enamel traits were quite similar among the species studied despite their marked differences in habits, digging behaviour and substrates occupied, suggesting a possible phylogenetic constraint for the Octodontoidea. In this context of relative similarity, the inclination of Hunter–Schreger bands, relative thickness of external index (EI) and prismless enamel zone were highest in † Eucelophorus, in agreement with its outstanding craniomandibular tooth-digging specialization. Higher inclination of Hunter–Schreger bands reinforces enamel to withstand high tension forces, while high external index provides greater resistance to wear. Results suggest increased frequency of incisor use for digging in † Eucelophorus, which could be related to a more extreme tooth-digging strategy and/or occupancy of hard soils. Higher external index values as recurring patterns in distant clades of tooth-digging rodents support an adaptive significance of this enamel trait. [ABSTRACT FROM AUTHOR]

Abstract: A new species of Ctenomyidae from the late Pliocene of Uquía Formation (northwestern Argentina) is described. The new remains consist of a fragmentary rostrum, and a left mandible with partial lower dentition. Its phylogenetic affinity and morphological specializations for tooth-digging support its assignation to the South American rodent genus Ctenomys. In this context, we highlight the importance of unique morphological specializations for the delimitation of genera within an intrafamilial clade in which similar adaptive strategies could have evolved more than once. The new materials are the oldest fossils for the genus (ca. 3.5Ma), and their finding in the central Andes agrees with previous hypotheses about the possible area of origin of Ctenomys. They precede by about one million years the presence of Ctenomys chapalmalensis in the Pliocene of the Pampean region of central Argentina, the oldest record previously known for the genus. Nevertheless, the new species does not contribute key information about ancestral character states for the genus beyond those already known through C. chapalmalensis. The phylogenetic, adaptive and even chronological information supplied by these new materials would be linked to the differentiation of the genus rather than to its origin. [Copyright &y& Elsevier]

The subterranean rodent Ctenomys is the single living representative of the family Ctenomyidae, and the most diverse genus within Hystricomorpha. Its fossil record begins in the late Pliocene and shows an increase in diversity since the Pleistocene. Here we describe a new middle-sized Ctenomys species from a large cranial sample collected at the archaeological site Quebrada del Real 1 in the high plains of the Pampa de Achala (Córdoba, Argentina). For this goal, we analyzed the cranial and mandibular shape variation through a geometric morphometric approach, and performed a comparative morphological analysis in the context of the living Ctenomys. The new species is characterized by having a wide rostrum with a deep rostral fossa, strong temporal fossa in the frontal, upper incisors strongly procumbent and with grooved enamel, and mandible with low corpus, long procumbent diastema and descending masseteric crest. A phylogenetic analysis demonstrated its close affinity with Ctenomys osvaldoreigi , an extant species from a nearby area. The new species represents the first notice of an extinct Ctenomys species from the Holocene. Given its peculiar morphology, the extinction of this species would have resulted in significant loss of morphological diversity, thus constraining the current boundaries of variation of the genus. The last record of the new species occurs in a period characterized by marked extinction of small mammals in southern South America mainly due to anthropogenic causes. • A new species of the rodent Ctenomys from the Holocene of Pampa de Achala, central Argentina, is described. • Phylogenetic analysis shows close relationship between the new species and the living Ctenomys osvaldoreigi. • This is the first report of extinction of a Ctenomys species in the Holocene. [ABSTRACT FROM AUTHOR]