Candela et al.—Paleobiology of the basal hydrochoerine Cardiomys
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Figure 5. Mapping of six anatomical characters, as reconstructed using parsimony, onto the phylogeny of Cavioidea modified from Madozzo-Jaén and Pérez (2017). Only unambiguous character state optimizations are shown. For discrete characters (circles), numbers above branches indicate character number whereas those below are character states. For continuous characters, closed/open squares indicate an increase/decrease of the measurements (see Supplemental Data 3 for characters and character states).
Figure 4. Box plots of morphological indices of the pes in cavioid rodents. (1) Plantar process of the navicular length/navicular body length index; (2)third metatarsal length/width index. Dark grey=cursorial; middle grey=ambulatory; light gray,= swimming; black=unknown (see Candela et al., 2017).
gracile and longer Mt III and phalanges and straight caudal border of the ulna). The presence of a single radial notch of the ulna (character 40)in Cardiomys would be a reversion to the plesiomorphic condition present in Cuniculus. A phylogenetic study based on wider taxon and character sampling could test whether the common features of Cardiomys and Hydrochoerus are synapomorphies of hydrochoerines and whether postcranial characters support Cardiomys as a basal hydrochoerine.
Paleobiology based on postcranial features.—The elbow joint of Cardiomys displays several features that indicate a relatively
high stability and restricted rotational movements, as are observed in other cavioid rodents. A high humeral distal surface and a steeply angled trochlea, which contacts with the medially extended trochlear facet of the radius, restrict mediolateral mobility and increase stability at the elbow joint during flexion- extension (Argot, 2001; Sargis, 2002; Candela and Picasso, 2008; Abello and Candela, 2010). The markedly differentiated humeral capitular tail, which contacts with the laterally extended capitular tail facet of the radius during the flexion, also max- imizes the stability at the humeroradial joint. The subrectangular radial head with a flattened caudal ulnar facet restricts the arc through which the radius may be rotated and prevents supination (Taylor, 1974; Szalay and Sargis, 2001). The cranially located radial head with respect to the ulna also leads to a severe restriction of supination, indicating that the radius is more efficient in incurring loads at the humeroradial joint during locomotion (Jenkins, 1973; Argot, 2001; Sargis, 2002; Schmitt, 2003). These features would be effective for resisting medio- lateral forces at the elbow joint, maximizing the stability of the humeroradial, radioulnar, and humeroulnar joints and restricting rotational movements. A perforated fossa and a relatively long olecranon (the attachment site for the m. triceps brachii) are features compatible with a strong extension of the forearm, which could be advantageous for digging or swimming (Hildebrand, 1985; Samuels and Van Valkenburgh, 2008). The orientation of the olecranon process with respect to the ulnar shaft has been related to the effectiveness of the lever arm of the triceps muscle, the primary extensor of the elbow joint, maximizing the action of this muscle at certain joint angles (Van Valkenburgh, 1987; Drapeau, 2004; Fujiwara, 2009). Therefore, the straight caudal border of the olecranon of Cardiomys suggests that this genus would have had a lower elbow joint angle during the stance phase of locomotion than Hydrochoerus, having a more crouched position of its forelimbs (Fujiwara, 2009). In addition, the caudally oriented olecranon of Hydrochoerus may represent a consequence of its larger body size, a condition that was also observed in other groups of mammals (e.g., Van Valkenburgh, 1987; Drapeau, 2004).
Figure 3. Right feet in (1–3) dorsal and (4–6) plantar views. (1, 4) Hydrochoerus hydrochaeris (MPS-Z 142); (2, 5) Cardiomys leufuensis (MLP 29-IX-3-19); (3, 6) Dolichotis patagonum (MLP 249). a=astragalus; c=calcaneus; cu=cuboid; ec=ectocuneiform; me=mesocuneiform; Mt II–IV=second to fourth metatarsals; n=navicular; ppc=plantar process of the cuboid; ppn=plantar process of the navicular. Scale bars=10mm.
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