Abstract
Recently, dental microwear analysis has been successfully employed to xenarthran teeth. Here, we present new data on use wear features on 16 molariforms of Orophodon hapaloides and Octodontotherium grande. These taxa count among the earliest sloths and are known from the Deseadan SALMA (late Oligocene). Modern phylogenetic analyses classify Octodontotherium and Orophodon within Mylodontoidea with whom they share lobate cheek teeth with an outer layer of cementum and a thick layer of orthodentine. Similar target areas of 100μm2 were analyzed on the orthodentine surface of each tooth by stereomicroscopic microwear and by SEM microwear. Results were unlike those of extant sloths (stereomicroscopic microwear: Bradypus, Choloepus) and published data from fossil sloths (SEM microwear: Acratocnus, Megalonyx, Megatherium, Thinobadistes); thus, both approaches independently indicate a different feeding ecology for the Oligocene taxa. The unique microwear results suggest that both taxa fed on plant material with low to moderate intrinsic toughness (foliage, twigs) but also proposes intake of tougher food items (e.g., seeds). Frequent gouging of the tooth surfaces can be explained by exogenous influence on microwear, such as possible intake of abrasive grit. We suggest an unspecialized herbivorous diet for Octodontotherium and Orophodon utilizing diverse food resources of their habitat. These interpretations support the reconstruction of (1) Deseadan environments as open habitats with spreading savannas/grasslands and (2) both taxa as wide-muzzled bulk feeders at ground level.
Highlights
Orophodon and Octodontotherium are among the earliest definite sloths and are known from the Deseadan South American Land Mammal Age (Gaudin and Croft 2015)
One-way analysis of variance (ANOVA) results show that the number of coarse scratches is the only parameter being significantly different between Octodontotherium and Orophodon with the latter showing higher values
Some coarse features seen under stereomicroscopy can have widths >100 μm (Semprebon et al 2004) and the boundaries of such large features would extend beyond the boundary of field of view at 500× under SEM; they cannot be scored via SEM
Summary
Orophodon and Octodontotherium are among the earliest definite sloths and are known from the Deseadan South American Land Mammal Age (late Oligocene) (Gaudin and Croft 2015). Ameghino (1895) described two distinct species from Patagonia, the large-sized Octodontotherium grande and the smaller Orophodon hapaloides. Remains of these Oligocene sloth taxa mainly consist of a number of teeth and some few postcranial elements (Hoffstetter 1956) plus a well-preserved but edentulous skull from O. grande Vizcaíno et al (2012) indicated a body mass of 700 kg for Octodontotherium grande (on the basis of a Glossotherium skull), while Shockey and Anaya (2011) estimated a body mass half as much (based on the only Octodontotherium skull preserved).
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