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Deltamys is an akodontine genus endemic to the Rioplatan area (Argentina and Uruguay) and Rio Grande do Sul (Brazil; Pardiñas & Teta 2015). It was for many years subsumed under Akodon, despite its early recognition by Thomas (1917), but today is considered a distinct genus. This status is supported by several morphological, karyological, and, to a lesser extent, molecular synapomorphies (Thomas 1917; Massoia 1964; Gentile De Fronza et al. 1981; Sbalqueiro et al. 1984; Castro et al. 1991; Bianchini & Delupi 1994; González & Massoia 1995; D’Elía et al. 2003; Smith & Patton 2007; Teta et al. 2007; Quintela et al. 2017). Furthermore, a recently described Brazilian species was added to the diversity of the genus (Deltamys araucaria; see Quintela et al. 2017; Pardiñas 2018) so that it is no longer considered monotypic. Also, strong evidence suggests the existence of still undescribed species for other populations in Brazil (Montes et al. 2008; Ventura et al. 2011; Quintela 2014). Populations from Argentinean and Uruguayan territories are referred to the type species, D. kempi, applying the subspecies D. k. kempiThomas, 1917, and D. k. langguthiGonzález and Massoia, 1995, respectively (see González & Pardiñas 2002).

In Argentina, D. kempi occurs in a very narrow coastal band from approximately 34° S to 35° S, characterized by riparian vegetation and periodic flooding (Pardiñas & Teta 2015). The southernmost previously known occurrence for this mouse was described by Udrizar Sauthier et al. (2005) at Reserva El Destino (35°08’ S; 57°23’ W; Buenos Aires province, Argentina). Here we document a new locality, listed in Pardiñas and Teta (2015:220) that substantially extends (ca. 150 km S) the southern limit to the south corner of Samborombón Bay (Fig. 1).

A single collecting trip at a small creek (Arroyo las Tijeras, Province of Buenos Aires; Fig. S1) produced 12 sigmodontine rodents identified in the field as Oxymycterus rufus (1 specimen) and Akodon azarae (11). This material was initially preserved in fluid, and in a subsequent parasitological examination by Marcela Lareschi a typical Deltamys mite (see Lareschi & Gettinger 2009) was collected from a very young specimen of those which had been referred to A. azarae. A detailed inspection of this animal (housed at Colección de Mamíferos del Centro Nacional Patagónico [CNP]; Puerto Madryn, Chubut, Argentina) confirmed its identification as D. kempi, further reinforced when the skull was removed and cleaned. Here we describe molars following Reig (1977).

The specimen (CNP 2377) is preserved as a cleaned skull and the carcass in fluid. It is a male and was collected in Arroyo las Tijeras, General Lavalle, Buenos Aires (36°22’43” S, 56°50’12” W, 1 m a.s.l.) by one of the authors (MR) on 23 October, 2008. Since the CNP 2377 is a very young individual with the third upper molar lacking wear (Fig. 2A), useful external characters to distinguish D. kempi from other small akodontines (e.g., Akodon; see Massoia 1964), such as the velvety fur or the small eye are not fully expressed. However, a set of craniodental traits permits a supported identification, including very narrow zygomatic plate, narrow parapterygoid plates, and a unique configuration of the occlusal surface of the molars. The latter are characterized by a typical “bilobed” pattern in the first lower molar, clearly present from young (Fig. 2C) to old individuals (Fig. 2E), including an ephemeral metaflexid, an early fusion between procingulum and metaconid, a very shallow anteromedian flexid, and a late emerging median murid. Differences with respect the sympatric Akodon azarae are trenchant (e.g., well-developed anteromedian flexid, large anterolabial cingulum; Fig. 2D).

Misidentification of D. kempi as A. azarae and Necromys obscurus is a recurrent topic and highlights the external resemblances among these akodontines. Massoia (1961) reported N. obscurus from northeastern Buenos Aires province based on a D. kempi specimen (see Massoia 1964). The animals referred to N. obscurus by Barlow (1969) represent largely D. kempi (see Ximénez et al. 1972:24). Barlow (1969:23) tacitly advanced the synonymy between both species, indicating “I examined the types of A. obscurus [Necromys obscurus] and A. kempi [Deltamys kempi] in the British Museum (Nat. Hist.) and found the type of the latter to be a subadult specimen of obscurus.” Here, we were “rescued” from a mistake thanks to parasitological evidence.

Contrary to the widespread occurrence of D. kempi in Uruguay (see González & Martínez Lanfranco 2010; and the references cited therein), the species in Argentina is generally rare (but see below). In fact, the range of the species in the Buenos Aires province is apparently restricted to a narrow coastal fringe (Fig. 1; Table 1). Several lines of evidence concur to advance, as a working hypothesis, that the occurrence of D. kempi in Argentina, especially from Ciudad Autónoma de Buenos Aires to the south, is a very recent event (the last century?). These are (1) D. kempi has no fossil record in Argentina, despite the existence of several Holocene assemblages in appropriate areas and the verified occurrence of other rare sigmodontines such as Bibimys (e.g., López et al. 1991; Pardiñas 1999; Teta et al. 2004, 2013). Conversely, Deltamys was recorded for a Holocene sequence in Brazil (see Stutz et al. 2020); (2) D. kempi inhabits exclusively coastal plains formed during Late Holocene as a gradual process after a sea-level peak during Middle Holocene (Parker & Marcolini 1992; Cavallotto et al. 2004); (3) the single location where D. kempi is recorded with some abundance is the Reserva Ecológica Costanera Sur (Ciudad Autónoma de Buenos Aires), described as “. . . a coastal protected area, next to the La Plata River, few blocks away from the heart of the city and government buildings, and was built by gaining land to the water using demolitions debris and silts from the river. . . ” (Teta et al. 2007:44). There, the muroid community is dominated by the cricetids D. kempi and Oligoryzomys flavescens, and the introduced murids Mus domesticus and Rattus norvegicus (Teta et al. 2007); and (4) DNA sequence analysis and phylogeographic studies show that the single Argentinean population included, sister to one Uruguayan, is well nested among Brazilian haplotypes, suggesting a recent origin (Montes et al. 2008; Quintela 2014).

Passive dispersal of several animals as a subproduct of the movement of large masses of floating vegetation (largely composed by Eichhornia spp., Panicum spp., Polygonum spp., etc.), is a recurrent event in the Río de la Plata basin (see Guerrero et al. 2017 for a review). However, almost nothing is known about the impact of this process to produce viable populations of small mammals. Could periodical vegetation “rafts” transport D. kempi individuals to several coastal points of Río de la Plata from a potential and better established source population in the Delta del Paraná area? Or even from the Uruguayan territory? To construct a solid hypothesis, more information is needed, especially from genetic markers. In any case, the unique geographic distribution of D. kempi in Buenos Aires province points to its consideration as a very recent immigrant in a potentially dynamic southern expansion.

Supplementary materials

Acknowledgments

M. Lareschi kindly informed us of the occurrence of parasites typical of Deltamys from one of the specimens from Arroyo las Tijeras. A. Castillo and his family allowed field work in Arroyo las Tijeras; J. Bogan provided crucial logistic support and S. Bogan assisted in the field; E. Cuéllar improved the English and R. Owen enriched the style and content of this contribution through a meticulous critical reading. We are very grateful to all the mentioned people.

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Información adicional

Editor asociado: M. Kittlein

Cite as: Pardiñas, U. F. J., M. de los Reyes, D. Voglino & C. A. Galliari. 2021. Southernmost occurrence of Deltamys kempi (Rodentia, Cricetidae) in Argentina: paleontological and neontological evidence to assess its current distribution. Mastozoología Neotropical, 28(1):e0525. https://doi.org/10.31687/saremMN.21.28.1.0.13

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