Revisiting the taxonomy and biogeography of the largest sigmodontine rodent (Rodentia, Cricetidae) through ancient DNA

Abstract Megaoryzomys curioi, an extinct rodent endemic to the Galápagos Islands (Ecuador), is the largest known member of the diverse Sigmodontinae, a primarily Neotropical subfamily within the Cricetidae (Rodentia, Mammalia). Its tribal affiliation has long been debated, with past classifications assigning it to Oryzomyini, Thomasomyini, or even proposing its placement in a distinct tribe. In this study, we analyse mitochondrial sequences extracted from a topotypical specimen of M. curioi (Santa Cruz Island), within a densely sampled cricetid phylogenetic framework. Our results support its inclusion within Oryzomyini. Megaoryzomys is recovered as part of a clade that also includes the genera Aegialomys and Nesoryzomys, encompassing both Galapagoan and mainland representatives. These preliminary findings would support a scenario of two independent colonization events by oryzomyines: one ancient (∼4.3 Mya) and another more recent (∼0.3 Mya), each followed by insular diversification, to account for the known diversity of Galapagoan rodents (three genera and 11 species, including fossils and living forms). Moreover, the early origin and prolonged persistence of Megaoryzomys challenge the notion of the Galápagos as short-lived biogeographic units due to tectonic instability.