Evolutionary pattern of the genus Cercomys and its biochronological implications

Abstract

Cercomys Pascual, 1967 is a late Miocene stem Octodontidae (Rodentia) whose systematic status and taxonomic diversity remains to be reviewed. We present the results of a systematic study of 75 specimens of this genus from 15 localities of central and western Argentina. Although the molar morphology is quite conservative, the comparison among samples shows variation in size. Results of quali-quantitative analyses (variance analysis, Kruskal-Wallis, discriminant analysis) suggest the recognition of four species. The smallest species (Cercomys sp. A) is recorded in the lower late Miocene localities of Arroyo Chasicó (ca. 9.23 Ma) and Cerro La Bota (Cerro Azul Formation, central Argentina) and Ullúm (Loma de Las Tapias Formation, western Argentina). The largest species (Cercomys sp. D) is recorded in the latest Miocene–earliest Pliocene of the Cantera Vialidad locality (ca. 5.33 Ma; Cerro Azul Formation) associated to Xenodontomys ellipticus (Ctenomyidae, Rodentia). Cercomys sp. B and Cercomys sp. C are intermediate in size between the two latter, which is consistent with biochronological evidence of the sites where they are found. We interpret the Cercomys species as members of a single, undivided lineage with directional evolution marked by size increase from the late Miocene to the Mio-Pliocene transition (ca. four million years of recorded evolution). The polarity of this directional change in Cercomys is congruent with that detected for other octodontoid lineages from the same deposits, such as Chasichimys-Xenodontomys, Neophanomys (Octodontidae) and Reigechimys (Echimyidae). This pattern allows refining biochronological and biostratigraphic interpretations for the late Neogene of southern South America. In particular, Cercomys sp. D and X. ellipticus are key species for identifying the Mio-Pliocene boundary in the continental record of central and western Argentina.