Museomics and new topotypes resolve the taxonomy and nomenclature of a widespread Patagonian <i>Ctenomys</i> (Rodentia, Ctenomyidae)

Abstract Historical specimens are critical for assessing systematics and fixing nomenclatural issues in vertebrates characterized by complex taxonomic histories. Between the late 19th and early 20th centuries, the alpha taxonomy of the subterranean caviomorph Ctenomys flourished thanks to extensive collections conducted in southern South America and fueled by the renowned mammalogist Oldfield Thomas. Ctenomys haigi and C. haigi lentulus were described by Thomas in 1919 for northwestern Patagonia, with lentulus later considered indistinguishable from typical haigi. Phylogenetic studies based on the cytochrome b gene subsequently revealed C. haigi as paraphyletic, with the only available topotypic sequence falling outside the clade formed by other supposedly conspecific sequences. These preliminary results favored the proposal of lentulus as a distinct species, a hypothesis that is addressed here through morphological and molecular analyses of museum specimens as well as examination of new topotypes and additional material for both nominal forms. Two species are recorded at El Maitén—the type locality of C. haigi—seemingly delimited by the Río Chubut. Populations northeast of the river group with topotypes of lentulus, while specimens southwest of this river form a distinct clade including historical type material for C. haigi. Amended characterizations, including diagnoses, type localities, and geographic ranges are provided for both lentulus and haigi.