Mylodon darwinii Owen, 1839a

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 : 24-25

Mylodon darwinii Owen, 1839a

Referred material: Left fourth lower molariform: PIMUZ A/ V 495; cranial vault and fragments: PIMUZ A/ V 496 ( Fig. 10 ); incomplete left mandible: PIMUZ A/ V 504 ( Fig. 11 ); incomplete left mandible: PIMUZ A/ V 505; right tibia: PIMUZ A/ V 4147 ( Fig. 6 ).

Comment: Te genus Mylodon is considered as monospecific since the work of Esteban (1996) although a recent study suggests that a Patagonian species attributed to this genus probably existed (Brambilla & Haro, 2022). Te only cranial remains belonging to M. darwinii in the PIMUZ Santiago Roth Collection correspond to the cranial vault and the dorsal part of the occiput. Tese cranial regions are frequently found in ground sloths, which led Brambilla and Ibarra (2018b) to produce a study focused on the occiput. While occipital shape has been shown to be a complex character within mylodonts (see Boscaini et al., 2022; De Iuliis et al., 2020;), PIMUZ A/V 496 does not exhibit the occiput enlargement known in Gl. robustum compared to other sloths (Brambilla & Ibarra, 2018b). Although the occiput is not complete, a lateromedially subelliptical rather than subcircular shape is observed in the specimen, which would favor an attribution to Gl. robustum (Brambilla & Ibarra, 2018b) . Te specimen is relatively small in size for each of the potential species and the nuchal crests are only weakly developed. In particular, Brambilla and Ibarra (2018b) proposed that the weak development of the nuchal crests corresponds to an ontogenetic character suggesting that their protrusion occurred towards the end of ontogeny. Tese same authors explain that the occiput of M. darwinii also tends to become more subcircular towards the end of growth and, therefore, that subadult specimens can be confused with Gl. robustum if one does not consider variation associated with ontogenetic change. Terefore, I suggest that PIMUZ A/V 496 could correspond to a subadult representative of M. darwinii . PIMUZ A/V 495 corresponds to the anterior lobe of an mf3 with a particularly narrow connection to the posterior lobe, in contrast to Gl. robustum (Bargo & Vizcaíno, 2008) . Although the connection between the two lobes of the mf3 is not a sufficiently diagnostic element to discern between the two species ( McAfee, 2009), I suggest to keep the attribution to the genus Mylodon and thus reassign PIMUZ A/V 495 to M. darwinii . Te same shape for mf3 is observed in PIMUZ A/V 504, but identification of the specimen is limited by the quality of preservation of the mandible and teeth. Te shape of the predental spout differs between Gl. robustum and M. darwinii (see above) but this anterior region is not preserved in PIMUZ A/V 504 and PIMUZ A/V 505. For these two specimens, the general shape of the dental alveoli does not allow to reject an attribution to the genus Mylodon , suggesting an assignment to M. darwinii . I note, however, that PIMUZ A/V 505 is much smaller than PIMUZ A/V 504 and that the lobation of the teeth is only slightly advanced, suggesting that PIMUZ A/V 505 is a juvenile or subadult specimen. Finally, PIMUZ A/V 4147 is a subcomplete tibia in which the lateral portion for articulation with the distal fibula is almost not preserved. Te contact of this portion with the medial portion of the astragalar articulation is nevertheless visible and forms an obtuse edge as in M. darwinii ( McAfee, 2016) , although the medial portion of the astragalar articulation is not particularly shallower than the specimen previously identified as Gl. robustum , i.e., PIMUZ A/V 501. I therefore propose a reassignment of PIMUZ A/V 4147 to M. darwinii . Darwin’s Mylodon is poorly represented in Roth collection at PIMUZ, but the species has the particularity of including two juvenile representatives, allowing to measure the ontogenetic variation on the mandible. Te presence of a well-preserved tibia of good preservation quality is also noteworthy, and is potentially useful for functional studies (e.g., Toledo et al., 2012).

Figures

Fig. 10 Plate of crania belonging to the Pilosa from the Roth collection at PIMUZ. Each‘subfamily’ presents cranial remains among which the most complete belong to the Mylodontinae, the Scelidotheriinae and the Nothrotheriinae. Scale bar= 10 cm

Fig. 11 Plate of mandibles/hemimandibles belonging to the Pilosa from the Roth collection at PIMUZ.The most complete remains illustrated here correspond to mylodonts and scelidotheres.Scale bar =10 cm

Fig. 6 Plate of the most complete postcranial remains of the hindlimb from the Roth collection at PIMUZ and a glyptodont pelvic girdle. Among the Cingulata, the best represented clades are large-sized armadillos (Eutatini, Pampatheriinae) and glyptodonts. In the ground sloths, only the megatherines are not represented here. Scale bar= 1 cm