918


Journal of Paleontology 92(5):911–919 As in other cavioid rodents, the configuration of the


ectal, sustentacular, and cuboid facets of the calcaneus indicates relatively high stabilization of the foot, restricting mediolateral movements and emphasizing flexion-extension. Likewise, the calcaneocuboid joint is more distally located with respect to the astragalonavicular joint, further restricting mediolateral move- ments (Candela and Picasso, 2008; Candela et al., 2017). According to García-Esponda and Candela (2016), the


hind limb of the capybara differs from that of other semiaquatic rodents (see Samuels and Van Valkenburgh, 2008) in exhibiting a lower pes index, which could be related with an increment of the out-force generated by the foot during swimming. Cardiomys displays relatively longer and slenderer ectocunei- form, metatarsal III, and phalanges with respect to those of Hydrochoerus (Fig. 4). These features indicate that Cardiomys would have had a longer and more gracile foot than extant capybaras. Therefore, the pes of Cardiomys would not have reached the degree of swimming specialization exhibited by Hydrochoerus. In turn, Cardiomyswould have had a shorter foot with respect to the cursorially adapted Dolichotis and Dasyprocta.Thisfeature suggests that running adaptations were absent in Cardiomys. In summary, in the context of the Cavioidea, the postcranial


features of Cardiomys are interpreted neither as adaptations to highly specialized cursoriality, such as those of Dolichotis (Candela and Picasso, 2008; García-Esponda and Candela, 2010), nor as specializations to an aquatic mode of life. Cardiomys exhibits several features shared with other cavioids that allow us to consider it as a generalized ambulatory species. Considering the morphology and relative size of the pes of


the extinct hydrochoerines Cardiomys and Phugatherium Ameghino, 1887 (= Protohydrochoerus Rovereto, 1914, a large cursorial hydrochoerine from the late Miocene–early Pliocene of Argentina; Kraglievich, 1940), the shortening of the pes appears to be an acquisition of the more recent genus Hydrochoerus. Therefore, the origin of a semiaquatic lifestyle in capybaras is likely a recent (Pleistocene) adaptation in the context of the evolutionary history of Hydrochoerinae. Thus, we hypothesize that the morphology of Cardiomys represents an ancestral postcranial pattern of hydrochoerines.


Conclusions and final remarks


Features of Cardiomys are more similar to Hydrochoerus than to any other Cavioidea, which seems to be compatible with the phylogenetic information provided by dental and cranial data (e.g., Pérez et al., 2017b). Our results indicate that Cardiomys can be considered an ambulatory rodent. Like the dental struc- ture, the postcranial features of these rodents show a more generalized morphology than extant capybaras. As recently proposed by García-Esponda and Candela (2016), adaptations to semiaquatic habits would have occurred more recently in the evolution of the Hydrochoerus lineage.


Acknowledgments


We gratefully thank M. Reguero (MLP), P. Teta (MACN), D. Voglino (MPS-Z), I. Olivares (MLP), D. Verzi (MLP), U. Pardiñas (CNP), D. Romero (MMPMa), E. Westwig (AMNH), and K. Zyskowski (YPM) for access to collections


under their care. Special thanks to the reviewers C.M. Deschamps and J.X. Samuels for their valuable suggestions and comments, which improve an earlier version of our manuscript.


Accessibility of supplemental data


Data available from the Dryad Digital Repository: http://doi. org/10.5061/dryad.f8h7h.


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