XENARTHRA

Benton, MJ, Donoghue, PCJ, Vinther, J, Asher, RJ, Friedman, M & Near, TJ, 2015, Constraints on the timescale of animal evolutionary history, Palaeontologia Electronica (Florence, Italy) 15 (1), pp. 1-107 : 63

CROWN XENARTHRA (62)

Node Calibrated. Divergence of cingulates (armadillos) from Pilosa (i.e., sloths and anteaters).

Fossil Taxon and Specimen. Riostegotherium yanei from Itaboraí, Brasil (Scillato-Yané 1976). The holotype of this species is an osteoderm in the collections of the Museu de Ciências Naturais, Fundação Zoobotânica do Rio Grande do Sul, Porto Alegre : MCN-PV 1774 (Bergqvist et al., 2004: figure 1) .

Phylogenetic Justification. Riostegotherium belongs to crown Xenarthra based on the diagnostically cingulate osteoderms (e.g., the type mentioned above). Other skeletal elements (e.g., astragali) with a close resemblance to cingulates are also known from Itaboraí (Bergqvist et al., 2004).

Minimum Age. 47.6 Ma

Soft Maximum Age. 164.6 Ma

Age Justification. Until recently, Itaboraí was regarded as upper Paleocene in age, but Woodburne et al. (2014) suggest that it is early Eocene, which would lead to a paleontological minimum estimate for crown Xenarthra corresponding to the the top of the Ypresian marine stage, 47.8 Ma ± 0.2 Myr = 47.6 Ma ( Gradstein et al., 2012).

The soft maximum date is based on the divergence of Eutheria from other mammals in the late Jurassic, represented by Juramaia (Luo et al., 2011) . This taxon is represented in the Daxigou site of the Tiaojishan Formation, Liaoning Province, Northeastern China, and has been constrained by radiometric dates to derive from deposits of just over 160 Ma in age (Luo et al., 2011). The equivalent marine stage is the Oxfordian ( Gradstein et al., 2012), with a lower boundary of 163.5 Ma ± 1.1 Myr, and thus 164.6 Ma is a soft maximum.