Candela et al.—Paleobiology of the basal hydrochoerine Cardiomys


to that of Hydrochoerus, being wider and relatively shorter than that of caviines, dolichotines, and Dasyprocta (Figs. 3.4, 3.6, 4.1). The preserved portion of the cuboid indicates that, as in


other cavioids, the dorsal face of this bone is smaller than the ectocuneiform (Fig. 3.2). The calcaneo-cuboid facet of the cuboid is also similar to that of other cavioids. The plantar process of this bone is robust (Fig. 3.5), similar to that of Hydrochoerus (Fig. 3.4) and caviines but somewhat more robust than that of Dolichotis (Fig. 3.6). The ectocuneiform shows the typical plantar extension


observed in other cavioids. Its dorsal surface has a subrectan- gular outline, being relatively proximodistally longer than that of Hydrochoerus but shorter than that of Dolichotis (Fig. 3.1–3.3). The third metatarsal (Fig. 3.2) is relatively more gracile and


elongated than that of Hydrochoerus (Fig. 3.1) and Cuniculus but more robust than that of Dolichotis (Fig. 3.3), Cavia Pallas, 1766, and Galea Meyen, 1833 (Fig. 4.2). The preserved proximal portions of the second and fourth


metatarsals (Fig. 3.2) are morphologically similar but slenderer than those of Hydrochoerus (Fig. 3.1). The proximal and middle phalanges of the third digit and the middle phalanx of the forth digit are slenderer than those of Hydrochoerus and morpholo- gically similar to those of Caviodon.


Materials.—MLP 29-IX-3-19, right M3, distal ends of both humeri, proximal portions of both ulnae, right radius without its distal end, proximal portion of the left radius, left pisiform, proximal end of the left fourth metacarpal, portion of the left ischium, distal end of the right fibula, right calcaneus and partially preserved right navicular, right ectocuneiform, poorly preserved right cuboid, right Mt III, proximal portions of right Mt II and Mt IV, right proximal and middle phalanges of the third digit and middle phalanx of the fourth digit of the pes.


Remarks.—Specimen MLP 29-IX-3-19 is referred to Cardi- omys because its M3 has four prisms, which are relatively anteroposteriorly wider and present relatively more superficial labial flexi (= sulci) than those of Caviodon and Xenocardia species (Pascual et al., 1966; Vucetich et al., 2010; Vucetich et al., 2011). Therefore, we excluded assignment to Caviodon australis Ameghino, 1888 (Montehermosan, early Pliocene; Rovereto, 1914) and Caviodon pozzi Kraglievich, 1927 (Cha- padmalalan, late Pliocene) as both species have M3 with more prisms and much more penetrating fissures. Xenocardia (Chas-


icoan?) was also eliminated as it also has more penetrating labial flexi and more delicate prisms (Vucetich et al., 2010). Specimen MLP 29-IX-3-19 also differs from the type of Procardiomys martinoi Pascual, 1961 because the latter has an M3 with three lobes and a well-developed posterior projection (Pérez et al., 2014). The M3 of the type of Cardiomys ameghinorum Rover- eto, 1914 has four prims and one posterior projection (a fifth


Figure 2.


915


small lobe) that is well differentiated from the last prism by a lingual flexus. Specimen MLP 29-IX-3-19 differs from the type of this species because the fourth prism of M3 is narrower than the third and the posterior projection is continuous with the fourth prism. The M3 of the specimen MLP 29-IX-3-19 is very similar in size and general morphology (number and relative size of prisms, depth of labial flexi) to that of the type of Cardiomys leufuensis, but differs from it in the morphology of the second prism, which has two labial flexi in MLP 29-IX-3-19 and a straight labial side in C. leufuensis (Pérez et al., 2017a, fig. 6F). Besides this difference, all other shared features support the specific assignment of the MLP 29-IX-3-19 to C. leufuensis.


Discussion


Systematic considerations on the postcranial features.—The phylogeny of Cavioidea has been based on molecular, cranial, and dental data while postcranial features have not been analyzed from a systematic point of view. Our study indicates that many postcranial features of Cardiomys are shared with other analyzed cavioids (proximal portion of radius cranially located with respect to the ulna, olecranon fossa perforated, distal articular humeral surface relatively high, radial head lateromedially extended and posteriorly flattened, distal portion of calcaneus extended, dorsal face of cuboid smaller than that of ectocuneiform). Character mapping (Fig. 5) indicates that the extreme reduction of the lateral coronoid process of the ulna (character state 31), presence of a greatly differentiated capitular tail of the humerus (character state 52), shortening of the Mt III (character 1), and shortening of the plantar process of the navicular (character 2) would be potential synapomorphies of Hydrochoerinae, in the context of Cavioidea. Thus, these characters support Cardiomys as within Hydrochoerinae, in agreement with phylogenetic hypothesis based on other morphological characters. Optimization of robustness of Mt III (character 1) shows that dolichotines have a more slender Mt III than the remaining cavioids, whereas the Hydrochoerinae dis- play the most robust Mt III in the context of this group. Within the Hydrochoerinae, Mt III of Cardiomys is less robust than that of Hydrochoerus, closer to the ancestral condition of this clade. In the phylogenetic context of cavioids, the well-differentiated and craniodistally narrow capitular tail facet of the radius (character state 62) is a feature only present in Cardiomys and Hydrochoerus. However, ambiguous optimization of this feature at the node of the clade that contains Dolichotinae and Hydrochoerinae precludes us from inferring whether this feature could be a potential synapomorphy of Hydrochoerinae. Chasicoan hydrochoerines have been recognized as critical to understanding the early evolution of the group because they display dental features that anticipate the derived dentition of modern capybaras (Pérez et al., 2014). In agreement with this, some postcranial features of Cardiomys seem to be more generalized than those of modern capybaras (relatively more


(1–3) Cranial view of right humeri. (4–6) Proximal and cranial views of right radii. (7–9) Cranial and lateral views of left ulnae. (1, 4, 7)


Hydrochoerus hydrochaeris (MPS-Z 142); (2, 5, 8) Cardiomys leufuensis (MLP 29-IX-3-19); (3, 6, 9) Dolichotis patagonum (MLP 249). Views 1, 3, and 4 are mirrored. c=capitulum; ce=capitular eminence; ct=capitular tail; ctf=capitular tail facet; en=entepicondyle; f=fovea; lcp=lateral coronoid process; mcp=medial coronoid process; o=olecranon; rn=radial notch; t=trochlea; tf=trochlear facet; tn=trochlear notch. Scale bars=10mm.


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116  |  Page 117  |  Page 118  |  Page 119  |  Page 120  |  Page 121  |  Page 122  |  Page 123  |  Page 124  |  Page 125  |  Page 126  |  Page 127  |  Page 128  |  Page 129  |  Page 130  |  Page 131  |  Page 132  |  Page 133  |  Page 134  |  Page 135  |  Page 136  |  Page 137  |  Page 138  |  Page 139  |  Page 140  |  Page 141  |  Page 142  |  Page 143  |  Page 144  |  Page 145  |  Page 146  |  Page 147  |  Page 148  |  Page 149  |  Page 150  |  Page 151  |  Page 152  |  Page 153  |  Page 154  |  Page 155  |  Page 156  |  Page 157  |  Page 158  |  Page 159  |  Page 160  |  Page 161  |  Page 162  |  Page 163  |  Page 164  |  Page 165  |  Page 166  |  Page 167  |  Page 168  |  Page 169  |  Page 170  |  Page 171  |  Page 172  |  Page 173  |  Page 174  |  Page 175  |  Page 176  |  Page 177  |  Page 178  |  Page 179  |  Page 180  |  Page 181  |  Page 182  |  Page 183  |  Page 184  |  Page 185  |  Page 186  |  Page 187  |  Page 188  |  Page 189  |  Page 190  |  Page 191  |  Page 192  |  Page 193  |  Page 194  |  Page 195  |  Page 196  |  Page 197  |  Page 198  |  Page 199  |  Page 200  |  Page 201  |  Page 202  |  Page 203  |  Page 204  |  Page 205  |  Page 206  |  Page 207