Feeding Ecology in Oligocene Mylodontoid Sloths (Mammalia, Xenarthra) as Revealed by Orthodentine Microwear Analysis

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.Electronic supplementary materialThe online version of this article (doi:10.1007/s10914-017-9405-x) contains supplementary material, which is available to authorized users.

The late Oligocene xenarthran fauna of Quebrada Fiera (Mendoza, Argentina) and its implications for sloth origins and the diversity of Palaeogene cingulates

“…The described Mf1 (Fig. 4) shows the same peculiar structure as in other Deseadan Mylodontidae (Kalthoff 2011; Shockey & Anaya 2011; Kalthoff & Green 2018), with a thin peripheral wall of cementum, a thick orthodentine layer, and an extremely reduced central core of vasodentine. Instead, Neogene and Quaternary sloths exhibit a central nucleus of vasodentine (Kalthoff 2011).…”

“…Concerning palaeobiological aspects, the teeth of Octodontotherium , as well as those of Orophodon , reflect an unspecialized herbivorous diet as suggested by Kalthoff & Green (2018), which contradicts the myrmecophagous ancestral condition in sloths suggested by Gaudin & Croft (2015). Furthermore, Octodontotherium is graviportal and plantigrade, whereas Similhapalops is more agile and perhaps arboreal, as is Hapalops (Toledo 2016).…”

“…femur, tibia, and astragalus) from La Flecha and now Quebrada Fiera. However, other than the pioneering works of Hoffstetter (1954, 1956) and, more recently, those on its dental structure (Kalthoff 2011; Kalthoff & Green 2018), few studies have focused on this stem‐mylodontid. The described Mf1 (Fig.…”

“…4A). This peculiar dental structure is typical of the Deseadan Orophodon and Octodontotherium (Pujos & De Iuliis 2007; Kalthoff 2011; Kalthoff & Green 2018), rather than Neogene sloths, which generally exhibit a prominent central nucleus of vasodentine (Kalthoff 2011). Late Palaeogene mylodontids such as Octodontotherium (Hoffstetter 1956) and Paroctodontotherium (Shockey & Anaya 2011) have a dental formula of 1/1Cf‐4/3Mf.…”

Complementary approaches to tooth wear analysis in Tritylodontidae (Synapsida, Mammaliamorpha) reveal a generalist diet

“…A recent study [43] comparing tooth microwear patterns using stereomicroscopy and scanning electron microscopy (SEM) showed that the combination of the two approaches leads to a more robust and objective interpretation of feeding adaptations, especially in extinct taxa. Here, we applied a different combination of methods to analyze microwear (stereomicroscopy and 3D surface texture) for a sample of ten tritylodontid taxa spanning the Upper Triassic to Lower Cretaceous.…”