A new peculiar species of the subterranean rodent Ctenomys (Rodentia, Ctenomyidae) from the Holocene of central Argentina

Highlights

• 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

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.

Introduction

Ctenomyidae is a family of hystricomorph rodents endemic to southern South America, represented in the living fauna only by the highly polytypic subterranean genus Ctenomys (Bidau, 2015; Freitas, 2016). The oldest fossils of the family are recorded in the late Oligocene-early Miocene of southern Peru, eastern Bolivia and southern Argentina (Lavocat, 1976; Shockey et al., 2009; Verzi et al., 2014, 2016). Nevertheless, the differentiation of the clade did not take place until the late Miocene, when several extinct genera with dental morphology similar to that of extant species (i.e. rootless molars with simplified, crescent-shaped occlusal morphology) are recorded in central and western Argentina (Reig et al., 1990; Verzi et al., 2014). This stage of differentiation of the clade Ctenomyidae involves the rise of lineages with disparate specializations for digging and subterranean life (Verzi, 2008). The only survivor of these modern lineages, Ctenomys, is recognized in the fossil record since the late Pliocene, although its origin, by split from its sister group, might be previous (Verzi, 2008; Verzi et al., 2010a). Ctenomys underwent strong cladogenesis that gave rise to over 70 extant species (Azurduy, 2005; Gardner et al., 2014; Patton et al., 2015) and an undetermined number of extinct ones (Mones, 1986 and literature therein; Verzi et al., 2004, 2010a; Azurduy, 2005; Lucero et al., 2008). This high rate of speciation is unique among family-level caviomorph clades (Álvarez et al., 2017). Accordingly, the genus is not only highly diverse in the living fauna, but also rich in the fossil record, especially since the Pleistocene. Whereas the Pliocene-middle Pleistocene fossils represent extinct species (Rusconi, 1931; Verzi et al., 2004 and literature therein; Verzi et al., 2010a), those described from latest Pleistocene-Holocene paleontological and archaeological sites correspond to extant species (e.g. Bárcena et al., 1985; Pardiñas, 2001; Quintana, 2004; Chan et al., 2005; Hadler et al., 2008; De Santi et al., 2018). Although this could reflect the existence of a temporal pattern underlying the origin of living species, there could also be a bias due to poor knowledge of the paleontological and archaeological record of the genus. Indeed, at least part of the numerous materials recovered from the latest Pleistocene-Holocene, and not yet studied at the species level, could represent undescribed extinct species (e.g. Salemme and Tonni, 1983; Bonomo and Massigoge, 2004; Martín and San Román, 2010; Fernández et al., 2011, 2016; Medina et al., 2011; Salemme et al., 2012; Luna and Cruz, 2014; López et al., 2016; Tammone et al., 2017). During the Holocene, mammals experienced severe extinction due to climatic and/or anthropic causes (Turvey and Fritz, 2011; Teta et al., 2014); thus, it would be expected that Ctenomys populations, which are vulnerable due to some of their distinctive ecological features linked to the subterranean niche, may also have undergone extinction during this epoch.

In this work we present an extensive sample of Ctenomys from the archaeological site Quebrada del Real 1 (QR1), located in the hilly region of central Argentina (Rivero et al., 2008; Rivero, 2009). The materials were recovered from middle-late Holocene levels and correspond to a recently extinct population. The new materials are described and compared to an exhaustive sample of extant species in order to test whether they correspond to any of the latter or, on the contrary, they represent a new extinct species. We also discuss the evolutionary information provided by these new materials.