Megatherium americanum Cuvier, 1796

Verger, Kévin Le, 2023, Xenarthrans of the collection of Santiago Roth from the Pampean Region of Argentina (Pleistocene), in Zurich, Switzerland, Swiss Journal of Palaeontology (3) 142 (1), pp. 1-39 : 28

Megatherium americanum Cuvier, 1796

Referred material: Fragmented mandible: PIMUZ A/ V 478; almost complete skeleton: PIMUZ A/ V 479 ( Fig. 1 ); one tooth, one right phalanx, and two caudal vertebrae: PIMUZ A/ V 481; radius, ulna, two complete hands: PIMUZ A/ V 482 ( Fig. 7 ); one radius: PIMUZ A/ V 483

( Fig. 5 ).

Comment: Megatherium is probably the most famous giant sloth, perhaps both because of its gigantic size and for the historical context of its first descriptions by George Cuvier (1796) (Argot, 2008). According to De Iuliis (1996), only the species M. americanum lived in the Pampean Region, contrary to the supposed diversity of this genus (see Agnolin et al., 2018; and citation therein). Brandoni et al. (2008) restored the validity of the species M. gallardoi Ameghino & Kraglievich, 1921 , stating that this species is commonly recovered in the Ensenadan while M. americanum occurs primarily in the Lujanian. Te most recent work on Megatherium from the Pampean Region supports the validity of at least two species, M. americanum and Megatherium filholi Moreno, 1888 (Agnolin et al., 2018). Te distinction between the two previously cited species is primarily a difference in size, with M. filholi being much smaller (femoral length smaller than 60 cm) and more gracile than M. americanum (Agnolin et al., 2018) . Considering that the diagnoses of M. filholi is based mainly for femur characters, PIMUZ A/V 479, has a femur greater than 60 cm in height, greater trochanter higher than the femoral head, and patellar trochlea not medially extended, that suggest an attribution to M. americanum over M. filholi (Agnolin et al., 2018) . Te criterium of using size as a diagnostic feature by Agnolin et al. (2018) is problematic in the case of a small specimen with respect to ontogenetic and individual variation, or potential dimorphism as it has been reported in other sloths (e.g., Boscaini et al., 2019; Cartelle & De Iuliis, 1995). PIMUZ A/V 478, PIMUZ A/V 481, PIMUZ A/V 482, and PIMUZ A/V 483 are all large specimens similar to the subcomplete skeleton of PIMUZ A/V 479, suggesting an attribution to M. americanum .

Figures

Fig. 1 Xenarthran subcomplete specimens in right lateral view from the Roth collection at PIMUZ modified from Schulthess (1920). Above, Glyptodon munizi (PIMUZ A/V 461) associated with its carapace. Below, Megatherium americanum (PIMUZ A/V 479). Scale bar: 50 cm

Fig. 7 Plate of the most complete postcranial remains for the hand and the foot from the Roth collection at PIMUZ.For the hand of the cingulates, only an Eutatini and a glyptodont are presented and for the ground sloths, a megatherine and a scelidothere are illustrated to show associated remains. For the foot, the collection is richer with an Eutatini, several glyptodonts, a mylodont and a scelidothere represented here. Scale bar=1 cm

Fig. 5 Plate of the most complete postcranial remains of the forelimb from the Roth collection at PIMUZ and a ground sloth scapula. Among the Cingulata, the best represented clades are large-sized armadillos (Eutatini, Pampatheriinae) and glyptodonts. In the ground sloths, only the mylodonts are not represented here. Scale bar= 1 cm