Evolutionary pattern ofMetacaremysgen. nov. (Rodentia, Octodontidae) and its biochronological implications for the late Miocene and early Pliocene of southern South America

Abstract

AbstractWe analyse the taxonomic status and diversity of the late Miocene Octodontoidea (Hystricognathi)Cercomys primitivaand related samples, a taxon previously known only from the holotype specimen. New findings associated with an extensive review of late Miocene and early Pliocene rodents have allowed us to recognize the occurrence of this and other related species in several localities of central and western Argentina, and in south‐central Bolivia. We discuss the invalidity ofCercomysand propose the new genusMetacaremys, which includes the type speciesMetacaremys primitivacomb. nov. and two new species,Metacaremys calfucalelsp. nov. andMetacaremys dimisp. nov. Osteological, brain and dental morphology show that the new genus is not related to Brazilian Echimyidae, as previously considered, but to the southern family Octodontidae. Although the molar morphology of this genus is quite conservative, comparison of the samples shows a variation in size. We discuss the plausible evolutionary pattern explaining this variation and the implied biochronological and biostratigraphical information. It is recognized as an anagenetic lineage in which an increase in size occurs from the oldest species,M. primitivacomb. nov. (early late Miocene,c. 9.23 Ma), to the youngest species,M. dimisp. nov. (Miocene–Pliocene boundary,c. 5.28 Ma).Metacaremys calfucalelis intermediate in size and age between these two species. The polarity of this pattern of change is consistent with that shown by other partially synchronous independent lineages of octodontoids, thus providing new evidence for the biochronological and biostratigraphic scheme of the late Miocene and early Pliocene of southern South America.