Glossotherium wegneri

Glossotherium wegneri (Spillmann, 1931) NEOTYPE. — EPN V. 120. Skull without mandible, atlas, right scapula, elements of the third digit of the left manus (third metacarpal, proximal and distal phalanges, lateral sesamoid), fragment of pelvis, left navicular, and dermal ossicles. La Cocha, close to Alangasí, late Pleistocene (Montellano-Ballesteros & Román-Carrión 2011: fig. 2) (Fig. 1). REFERRED MATERIAL. — See Figures 2-4 and Appendices 1-4. REVISED DIAGNOSIS. — Cf1 is intermediate in size (neither the largest nor the smallest tooth of the upper tooth row); there is pronounced separation of Cf1 from the anterior edge of maxilla; the mandibular symphysis is short and ends anteriorly to cf1; the upper tooth rows are markedly divergent, particularly due to the lateral position of the Cf1s; an internasal element, anterior to the nasals and resting on the vomer, is present and the nasals diverge anteriorly at the midline; in ventral view, the maxilla extends both laterally and anteriorly well beyond the position of the teeth; the braincase width is relatively large; the rostrum is anteroposteriorly elongated; the anterior part of the rostrum is markedly expanded transversely and dorsoventrally; the premaxilla is tightly sutured to the skull; the palate is not conspicuously extended posteriorly; the zygomatic process of the squamosal is greatly elongated; the hypoglossal foramen is relatively enlarged; the ventral margin of the dentary is markedly concave; the mandibular spout is strongly expanded transversely; the proximal epiphysis of the radius is subcircular in shape; in the ulna, the angle between the olecranon and the posterior border is approximately orthogonal. COMPARATIVE DESCRIPTION The skull of G. wegneri (Fig. 2) is particularly distinct from that of other Glossotherium species in possessing an internasal element, which is not present in G. robustum, G. tropicorum, and G. phoenesis (e.g. Owen 1842; Pitana et al. 2013; De Iuliis et al. 2017; Cartelle et al. 2019), and in the dorsoventral expansion of the rostrum anteriorly, which is reflected in the rising dorsal profile of the rostrum in lateral view (Fig. 2B). An anterior ossification is present in Mylodon darwinii Owen, 1839 (Hoffstetter 1952) but is not homologous to the internasal described for G. wegneri; in the former the ossification results from dorsal extension of the premaxillae to contact the nasals and forms a characteristic bony arch that is absent in the latter. The dorsal profile is either nearly horizontal for most of its length and declines only at its anteriormost extremity in G. robustum (e.g. Owen 1842) or descends gradually anteriorly in G. tropicorum and G. phoenesis (De Iuliis et al. 2017; Cartelle et al. 2019). Paramylodon harlani Owen, 1839 resembles more the condition in G. robustum (Stock 1925). The dorsal profile of M. darwinii bears a convexity approximately at about the midpoint of the rostrum, but it descends farther anteriorly (Hoffstetter 1952). Thus, a portion of the rostrum does rise, but this condition is distinct from that in G. wegneri, in which the rostrum does not decline anteriorly in lateral view (Fig. 2B; Appendices 2-4). The condition in MNHN.F.PUN220 (Appendix 4), with the anteriormost part of the rostrum declining, represents an exception, and is probably due to an extended ossification of the cartilage at the tip of the nasals, variably observed in other mylodontids (e.g. Simomylodon uccasamamensis Saint-André, Pujos, Cartelle, De Iuliis, Gaudin, McDonald, and Mamani Quispe, 2010; Boscaini et al. 2019a). In ventral view, the upper tooth rows diverge in all Glossotherium species and P. harlani (Owen 1842; Stock 1925; McAfee 2009; Pitana et al. 2013; De Iuliis et al. 2017; Cartelle et al. 2019), but is most marked in G. wegneri (Fig. 2C, E) and least marked in P. harlani, with the differences reflected in the curvature of the tooth rows. Glossotherium wegneri differs notably from the other species so far noted in the degree of transverse expansion of the maxillae, particularly anteriorly near the level of the caniniform teeth (Gaudin 2004). In G. wegneri (Fig. 2C, E), the maxilla extends laterally and anteriorly to a greater extent than in the other Glossotherium species (Owen 1842; Pitana et al. 2013; De Iuliis et al. 2017; Cartelle et al. 2019). The degree of this expansion imparts a strongly and abruptly widened palatal region, especially as compared to the postorbital constriction of the skull, whereas in the other species, particularly P. harlani, the palatal region is less expanded and the skull overall appears more elongated and narrow (Stock 1925; McAfee 2009). This is particularly true in M. darwinii, in which the rostrum is long and narrow, and the tooth rows are essentially parallel (Hoffstetter 1952). This latter species also differs from the Glossotherium species in the absence of a caniniform tooth, so that only four upper teeth are present on each side (although four lower teeth are present in each dentary, as in Glossotherium species and P. harlani). Stock (1925) noted the occasionally absence of the first upper tooth in P. harlani, although this loss is restricted to late Pleistocene (Rancholabrean NALMA) specimens and occurs at a frequency of nearly 50% (McAfee 2015; McDonald 1995). The occiput of Glossotherium robustum in posterior view was characterized as low and wide among mylodontids by Brambilla & Ibarra (2018), but this morphology is not clearly diagnostic generically for Glossotherium. Indeed, the occiput of G. wegneri (Hoffstetter 1952) is relatively higher, and it is progressively higher in G. tropicorum (De Iuliis et al. 2017) and G. phoenesis (Cartelle et al. 2019), so that in the latter it is nearly semicircular, and thus more closely resembles the occiput of Mylodon darwinii, as illustrated in Brambilla & Ibarra (2018). In lateral view, the ventral margin of the dentary beneath the tooth row is concave in G. wegneri (Fig. 3), in contrast to the nearly rectilinear or slightly convex margin in the other mylodontines (Gaudin 2004). The depth of the horizontal ramus tends to be nearly constant below the tooth row in G. robustum, G. tropicorum, G. phoenesis, and P. harlani (Owen 1842; Pitana et al. 2013; De Iuliis et al. 2017; Cartelle et al. 2019). Although this is not the case in G. wegneri, owing to the concave ventral margin, the anterior and posterior portions of the dentary are approximately of the same height (Fig. 3 I-K). In M. darwinii, however, the height decreases anteriorly, so that the ramus tapers (Hoffstetter 1958). The mandibular spout of G. wegneri is anteriorly flat in occlusal view (Fig. 3D), whereas it is anteriorly rounded in G. tropicorum and G. phoenesis (De Iuliis et al. 2017; Cartelle et al. 2019). As in G. tropicorum (De Iuliis et al. 2017), the coronoid process of the mandible is not hooked posteriorly, and the condyloid process is relatively shorter, in comparison to all the other Glossotherium species (e.g. Pitana et al. 2013; Cartelle et al. 2019). In the postcranial elements, we observed a general resemblance between G. wegneri and the remains attributed to the other Mylodontinae species (Boscaini et al. 2019c). Only two postcranial features are worth mentioning, as they appeared to be autapomorphies of G. wegneri: the subcircular shape of the proximal articulation of the radius, and the roughly orthogonal angle between the ulnar olecranon and posterior border in lateral view. Both features are unusual among Mylodontini and resemble more the conditions observed in scelidotheres and some lestodontine sloths (Boscaini et al. 2019c).

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