Evolution of tarsal Morphology and weight support in glyptodonts (Mammalia, Xenarthra, Cingulata)

FERNICOLA, J. C.; TOLEDO, N.; BARGO, M. S.; VIZCAÍNO, S. F.

Mendoza

Congreso; 4th International Palaeontological Congress; 2014

Glyptodonts (middle Eocene-late Pleistocene/early Holocene) are a unique group of placental mammals that evolved in America, characterized by a dorsal carapace and elephantine hind-feet, among other features. During the late Pleistocene, these herbivorous animals reached gigantic sizes, varying between 500 kg and two tons. In the early twentieth century, Scott described the foot of the Miocene Propalaehoplophorus (~ 80 kg), emphasizing that the relationships among the tarsal elements were different from those observed in the larger Plio-Pleistocene glyptodonts. A reexamination of the relationships among the tarsal elements of glyptodonts, their sister-group pampatheres, and armadillos is presented here. Within glyptodonts, the tarsal elements of Propalaehoplophorus, and the Pleistocene Glyptodon (800-2000 kg), Neosclerocalyptus (600 kg), Doedicurus (1500 kg) and Panochthus (1000 kg) show a unique articular pattern, where the distal facet of the lateral cuneiform articulates with both metatarsals III and IV. In the Pleistocene pampathere Holmesina (180 kg), the extinct armadillos Proeutatus (Miocene; 15 kg) and Eutatus (Pleistocene; 50 kg), and the extant Chaetophractus (4 kg), Priodontes (45 kg) and Dasypus (2.5 kg), this facet articulates only with metatarsal III. The arrangement in glyptodonts allowed a greater interlocking of the midtarsal segment of the foot, and would enable a more widespread distribution of the weight loads from astragalus to I, II, III and IV digits. The digits IV and V, also receive loads indirectly from calcaneum via the cuboid. In pampatheres and armadillos, the weight loads from talus distribute via the navicular and cuneiforms only to digits I, II and III, while digits IV and V receive loads indirectly from calcaneum via the cuboid. At a first glance, this pattern appears as independent from body size because Holmesina is larger than the basal glyptodont Propalaehoplophorus. However, this articular pattern shows variation throughout the glyptodont clade, concurrent with the increase in body size evidenced by this group from the Miocene to the Pleistocene. The articular contact between the ectocuneiform and the metatarsal IV increases from 1/5 of the distal facet of the cuneiform in the smaller Miocene taxa, to 1/2 of the facet in more derived and larger Pleistocene genera (Glyptodon, Neosclerocalyptus, Doedicurus and Panochthus). Although it seems clear that this arrangement was not to support a large body size in Miocene glyptodonts, it enabled the developing of giant body sizes along the evolution of the group.