Candela et al.—Paleobiology of the basal hydrochoerine Cardiomys


913


is well differentiated from the fovea and to a greater degree than in caviines (in Dolichotinae, this facet is not differentiated from the fovea). In addition, the most lateral portion of the radial head is craniocaudally shorter with respect to the rest of the articular surface. This narrowing is similar to, although not as pronounced as, that of Hydrochoerus but greater than that of Caviinae. Although the ulnar facet is poorly preserved, it is relatively flat. The ulna (Fig. 2.8) is not completely preserved, but it can


Figure 1. MLP 29-IX-3-19, right upper third molar of Cardiomys leufuensis in occlusal view. Scale bar=2mm.


angled. The lateral lip of the trochlea is well developed on the caudal facet of the articular surface, but cranially it ends abruptly. As in other cavioids, the proximal extremity of the radius


(Fig. 2.5) articulates cranially with respect to the ulna. The proximal articular surface of the radius is subquadrangular, being wide lateromedially and short craniocaudally. Among cavioids, this configuration is also seen in Cuniculus, Caviinae, and Hydrochoerus (Fig. 2.4), whereas in Dolichotinae (Fig. 2.6) and Dasyprocta, this articular surface is less lateromedially extended. The proximal articular surface has three distinct facets: a central facet, the fovea; a medial facet, the trochlear facet; and a lateral facet, the capitular tail facet. The fovea is the main facet of the radial head, articulating with the capitulum of the humerus. It has a somewhat triangular outline, resembling that of Hydrochoerus. The trochlear facet has a steep inclination from lateral to medial, showing a similar development to that of Caviinae and Dolichotinae but greater than that of Cuniculus and Dasyprocta. The fovea and the trochlear facet are separated by a crest, which ends in a spine, the capitular eminence, on the cranial border of the head of the radius. The capitular tail facet lies on the lateral side of the radial head, articulating with the capitular tail of the humerus when the elbow is flexed. This facet has an inclination from medial to lateral and is separated from the fovea by a crest. As in Hydrochoerus, the capitular tail facet


be noticed that the olecranon process is as long as the trochlear notch, a condition also observed in other cavioids (with the exception of Dasyprocta, in which the olecranon process is relatively shorter). The medial coronoid process is well differentiated, but the lateral coronoid process is very reduced (Fig. 2.8). The latter condition is close to that of Hydrochoerus and Cuniculus, in which this process is absent, and differs from that of caviines, dolichotines (Fig. 2.9), and Dasyprocta, all of which have a well-developed lateral coronoid process. The radial notch is wide, as in other cavioids, reflecting a broad anterior contact with the radius. As in Cuniculus and Dasyprocta, this notch is represented by a single facet. On the contrary, in Hydrochoerus, dolichotines, and caviines, the radial notch is composed of two separated facets for articulation with the radius. As in most Cavioidea, the caudal border of the proximal portion of the ulna is straight. This contrasts with the condition observed in Hydrochoerus, in which the caudal border is more concave, as a consequence of its caudally oriented olecranon. As observed in most cavioids, the lateral surface of the ulnar shaft has a shallow fossa (area of origin of the m. abductor pollicis longus), which extends proximally to the level of the radial notch. Bones of the left manus are only represented by the


pisiform (which is similar in shape to that of the other cavioids) and a fragmentary proximal end of the fourth metacarpal. Only a small portion of the left ischium is preserved. This


portion comprises the cranial part of the body of the ischium, including part of the acetabulum. It also includes the cranial portion of the lesser sciatic notch. The ischiadic spine and the pulley where the tendon of m. obturator internus slides are similar to those of Hydrochoerus. Only the distal portion of the right fibula is preserved. The


articular facet for the tibia and astragalus, and the sulci for the mm. peronei are similar to those of Hydrochoerus. The ectal facet of the calcaneus (Fig. 3.2) is somewhat obliquely oriented with respect to the longitudinal axis of the


bone, facing distally. The sustentaculum bears a subcircular and flat facet that is obliquely oriented and faces dorsodistally. The tuber calcanei is not preserved. The peroneal tubercle is short and indistinct, as in other cavioids. The cuboid facet is concave in dorsoventral direction, facing medially, similar to that of Hydrochoerus. As in other cavioids, this facet is far distally located with respect to the sustentaculum. The navicular is poorly preserved, represented by two


portions: a part of the navicular body (Fig. 3.2) and the plantar process (Fig. 3.5). The facet for the astragalar head is less lateromedially extended than in Hydrochoerus (Fig. 3.1), similar to that of Dolichotis Desmarest, 1820 (Fig. 3.3). The preserved portion of the body is relatively proximodistally longer than that of Hydrochoerus. The plantar process is similar


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