Caviomorpha

Two species of chinchillid rodents, Lagostomus (Lagostomopsis) incisus (Ameghino) and “Lagostomus (Lagostomopsis) spicatus” (Ameghino), have been recorded from the Monte Hermoso Formation (Montehermosan-Lower Chapadmalalan, early Pliocene) in southern Buenos Aires Province, eastern Argentina. L. (L.) incisus was based on skull remains, while “L. (L.) spicatus” was based on mandible remains and fragmentary skulls. Detailed study of specimens recovered from the upper section of the Monte Hermoso Formation, from the Irene “Formation” and the Chapadmalal Formation (late early-early late Pliocene, Buenos Aires Province), some of them represented by associated skull and mandible remains, indicates that L. (L.) incisus and “L. (L.) spicatus” are synonymous, with the valid name being L. (L.) incisus. The differences between both nominal species are here attributed to different ontogenetic states and sexual dimorphism. The stratigraphic provenance of the fossil material of L. (L.) incisus indicates a temporal distribution of this species restricted to the Montehermosan?-Chapadmalalan (early-early late Pliocene), instead of the Montehermosan (early Pliocene).

The genus Abrocoma Waterhouse includes eight living species of medium-sized rats with long, soft, and dense fur, large ears, and moderate to short tails. Six of these species are included within the informally called A. cinerea species complex (A. cinerea Thomas, A. budini Thomas, A. famatina Thomas, A. schistacea Thomas, A. vaccarum Thomas, and A. uspallata Braun and Mares). Based both on the study of specimens housed in biological collections and the reinterpretation of previous morphological evidence, I suggest that the current recognized species richness of the genus could be overestimated. Both qualitative and quantitative morphological approaches are consistent with the recognition of no more than two
morphotypes within the A. cinerea species complex. However, additional lines of evidence and approaches are needed to disentangle the complex taxonomy of this genus (e.g., phylogenetic analysis of DNA sequences). Until then, I prefer to not make changes in the current classification of Abrocoma, especially taking into account that any taxonomic change could have undesirable consequences in the conservation of these rodents.