A revision of the fossil genus Phanomys Ameghino, 1887 (Rodentia, Hystricognathi, Cavioidea) from the early Miocene of Patagonia (Argentina) and the acquisition of euhypsodonty in Cavioidea sensu stricto

摘要

Cavioidea sensu stricto (Cavioidea s. s.) is one of the most divergent lineages within the South American Hystricognathi, and is currently represented by cavies, maras (Caviidae), and capybaras (Hydrochoeridae). Caviids and hydrochoerids have been interpreted as forming the crown group, whereas the stem group of Cavioidea s. s. is formed by "Eocardiidae", a paraphyletic group recorded in the late Oligocene to middle Miocene, mainly in Patagonia. One of the most interesting features of Cavioidea s. s. is the record of evolution of hypsodonty, which develops relatively slowly compared to other groups of caviomorphs, enabling this process to be followed in the fossil record. Phanomys is recorded in Patagonia during the late early Miocene, and since its description this genus has been considered close to euhypsodont genera. The objective of this paper is to: (1) report new material of Phanomys; (2) re-describe the two species of the genus; (3) determine the phylogenetic position of Phanomys among Cavioidea s. s. by morphological cladistic analysis; and (4) explore the sequence of appearance of characters related to the origin of euhypsodonty. The new materials assigned to Phanomys mixtus are the first mandibular fragments and palate known, yielding valuable morphological, ontogenetic, and phylogenetic information. Phanomysvetulus is recognized as a valid species, although better material is needed for a more robust definition. The phylogenetic analysis shows that Phanomys is the sister group of euhypsodont Cavioidea s. s. The evolutionary history of Cavioidea s. s. demonstrates that character states previously thought to correlate with the degree of hypsodonty (e. g. absence of fossettes/ids) do not all appear at the same time during the evolution of the group. Although the evolutionary trend in Cavioidea s. s. shows progressively increasing hypsodonty, the morphological changes inferred from this phylogenetic analysis indicate that hypsodonty and other character states were temporally decoupled during the evolution of the group during the Oligocene and early Miocene.