A new rodent (Cavioidea, Hystricognathi) from the middle Miocene of Patagonia, mandibular homologies, and the origin of the crown group Cavioidea sensu stricto

ABSTRACT

Cavioidea sensu stricto groups three traditionally recognized families that are characterized by hypsodont, double-heart-shaped cheek teeth and moderate hystricognathy: Eocardiidae, Caviidae, and Hydrochoeridae. Eocardiidae was erected to include a diverse assemblage of extinct and plesiomorphic taxa, whereas Caviidae and Hydrochoeridae (the crown group of Cavioidea) include the lineages with extant representatives (cuyes, maras, and capybaras). A new genus and species of Cavioidea sensu stricto, Guiomys unica, is described here from the middle Miocene of west central Patagonia (Argentina). The new taxon is known from mandibular and maxillary fragments with molars, and isolated cheek teeth. A phylogenetic analysis of Cavioidea sensu stricto shows G. unica as the sister taxon of the clade formed by the crown group of Cavioidea sensu stricto (‘medialis series’). The new species displays characters states intermediate between eocardiids and the ‘medialis series.’ The most outstanding character of Guiomys unica is the autapomorphic position of the notch for the insertion of the masseter medialis pars infraorbitalis muscle, isolated from both the masseteric and horizontal crests. This notch is located at the anterior end of the masseteric crest in eocardiids and represents the plesiomorphic state for Cavioidea sensu stricto. In caviids and hydrochoerids, the notch is located at the anterior end of the horizontal crest, the derived state for this character. G. unica allows reinterpretation of the homologies of the mandibular crests in basal cavioids and clarifies the evolutionary origins of crown-group cavioids (Caviidae + Hydrochoeridae).

ACKNOWLEDGMENTS

I thank G. Vucetich (MLP) and D. Pol (MEF) for their valuable comments about this research, C. Deschamps (MLP) for improving the English version, P. Puerta (MEF) for his technical fieldwork, L. Reiner (MEF) for laboratory work. Access to SEM lab was possible thanks to ALUAR Aluminio Argentino SAIC and the help of Mr. J. Groizard. I thank D. Verzi, A. Álvarez, and I. Olivares for allowing me to participate in the dissection of caviids, and also the curators E. Ruigómez (MPEF-PV), M. Reguero (MLP-PV), A. Kramarz (MACN-PV), D. Verzi (MLP), D. Romero (MMP), J. Flynn and J. Meng (AMNH), Christopher Norris and D. Brinkman (YPM PU), and K. Wellspring (ACM) for access to material under their care. I am especially thankful for A. Walton's valuable comments about Dolichotinae of La Venta (Colombia), to D. Pol for his aid in the phylogenetic analysis, and to C. Vieytes and A. Kramarz for their valuable comments about definitions of characters. Fieldwork at Río Chico was supported by grants from the National Science Foundation to R. Kay. Fieldwork at El Petiso was funded by Fundación Egidio Feruglio, Ministerio de Educación (Chubut, Argentina). Comparisons with cavioid taxa for the phylogenetic analysis was made possible thanks to the Collection Study Grant (AMNH), and Ostrum Fund Grant (YPM, USA), PICT 38112 (D. Verzi and G. Vucetich), and the Fundación Egidio Feruglio.