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Journal of Paleontology 92(5):911–919
from the Chasicó Formation (late Miocene, Buenos Aires Province, Argentina) and represents the ‘cardiomyine’ with the most completely preserved postcranium known. We evaluate it from a paleobiological point of view and discus the systematic implications of its postcranial features.
Materials and methods
The new specimen of Cardiomys (MLP 29-IX-3-19) described here is represented by an isolated right M3 and associated postcranial remains. Molar morphology of this specimen was compared with specimens of Caviodon, Cardiomys, Xenocardia, and Procardiomys by direct observation or by comparison with published data (Ameghino, 1885; Rovereto, 1914; Kraglievich, 1930b; Pascual, 1961; Pascual et al., 1966; Vucetich et al., 2011; Pérez et al., 2014). The postcranial elements of specimen MLP 29-IX-3-19 were compared with the postcranium of the type specimen of Caviodon cuyano from the Aisol Formation (Pliocene, Mendoza Province), based on descriptions and illustrations provided by Vucetich et al. (2011). Wealso compared the postcranial remains with extant species of all main lineages of Cavioidea (Supplemental Data 1). Dental nomenclature follows that of Pérez et al. (2014). The osteolo- gical nomenclature follows that used by Candela and Picasso (2008) and the International Committee on Veterinary Gross Anatomical Nomenclature (2005). Most of the postcranial characters of cavioids used in the comparisons have been previously discussed by Candela and Picasso (2008) and García-Esponda and Candela (2016). The myological nomen- clature and muscular system are based on Woods (1972) and García-Esponda and Candela (2010). Locomotor habits and substrate preferences of present-day species follow Candela et al. (2017). Four linear measurements were taken from photographs of
the studied material using ImageJ 1.50i software (Schneider et al., 2012). The variables measured were navicular body length, plantar process of the navicular length, third metatarsal (Mt III) length, and Mt III width. Two indices were calculated: (1) plantar process of the navicular length/navicular body length, and (2) Mt III length/width. Each box plot was created in R software 3.1.5 (R Development Core Team, 2015). Part of the morphological variation observed among
cavioids was coded in six characters. Only those characters considered informative in the context of our taxon sample were included in the character mapping. Character state definitions are provided in Supplemental Data 2, and the resultant data matrix is provided as a TNT script in Supplemental Data 3. The evolution of these characters was mapped on the composite molecular-morphological phylogeny of Cavioidea provided by Madozzo-Jaén and Pérez (2017), which was simplified to living taxa and the extinct Cardiomys. Cladistic mapping was done with TNT 1.5 (Goloboff and Catalano, 2016). Four discrete character states were considered unordered whereas the remaining two characters were coded as continuous (Goloboff et al., 2006), using the mean value for each terminal taxa.
Repositories and institutional abbreviations.—The studied specimens are housed in the following collections: Museo de La Plata (MLP), La Plata, Argentina; Museo de Ciencias Naturales
‘Bernardino Rivadavia’ (MACN), Buenos, Aires, Argentina; zoological collection of Museo de Ciencias Naturales ‘P. Antonio Scasso’ (MPS-Z), San Nicolás, Argentina; Centro Nacional Patagónico (CNP), Puerto Madryn, Argentina; mam- mal collection of Museo Municipal de Ciencias Naturales ‘Lorenzo Scaglia’ (MMPMa), Mar del Plata, Argentina; American Museum of Natural History (AMNH), New York, New York, USA; Yale Peabody Museum of Natural History (YPM), New Haven, Connecticut, USA.
Systematic paleontology Order Rodentia Bowditch, 1821
Suborder Hystricomorpha Brandt, 1855
Superfamily Cavioidea (Fischer von Waldheim, 1817) Kraglievich, 1930a
Family Caviidae Fischer von Waldheim, 1817
Subfamily Hydrochoerinae (Gray, 1825) Gill, 1872; Weber, 1928 sensu Kraglievich, 1930a Genus Cardiomys Ameghino, 1885
Type species.—Cardiomys cavinus Ameghino, 1885, ‘Mesopotamiense’ (lower member of Ituzaingó Formation, late Miocene), Entre Ríos Province, northeast Argentina.
Cardiomys leufuensis Pérez, Deschamps, and Vucetich, 2017
Occurrence.—Chasicó Locality, southwest Buenos Aires Province (Argentina); Arroyo Chasicó Formation, Chasicoan Stage/Age, late Miocene (see Tonni et al., 1998; Cione and Tonni, 2005; Zárate et al., 2007).
Description.—The M3 has four prisms separated by deep lingual flexi (Fig. 1). The first prism is the most anteroposteriorly compressed of all prisms; it displays a convex anterior border and a labial superficial flexus (sulcus).The second and third prisms are heart-shaped. The second prism has two labial flexi, somewhat deeper than that of the first prism. The third prism is ante-
roposteriorly wider than the second and displays a labial super- ficial sulcus on its anterior labial border. The fourth prism is labiolingually narrower than the other prisms and shows a pos- terior prolongation. Enamel is thicker on the lingual border of the prisms than on the labial side. This tooth would correspond to an adult individual because the diameters of its base and occlusal surfaces are approximately equal (Vucetich et al., 2011). In general appearance, the distal portion of the humerus
(Fig. 2.2) is similar to that of other cavioids. The olecranon fossa is perforated. The entepicondyle is moderately developed, showing an expansion similar to that of Hydrochoerus (Fig. 2.1) and Dasyprocta Illiger, 1811, but relatively larger than in caviines and dolichotines (Fig. 2.3) and smaller than that of Cuniculus Brisson, 1762. The distal articular surface of the humerus is relatively higher proximodistally than that of Cuniculus, similar to that of Hydrochoerus, and lower than that of Caviinae, Dolichotinae, and Dasyprocta. The capitular tail has a degree of differentiation similar to that of Hydrochoerus; compared to other cavioids, it is more lateromedially extended and differentiated from the capitulum. The trochlea is steeply
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