Riostegotherium yanei Oliveira & Bergqvist, 1998

Bergqvist, Lílian Paglarelli, Abrantes, Érika Aparecida Leite & Avilla, Leonardo dos Santos, 2004, The Xenarthra (Mammalia) of São José de Itaboraí Basin (upper Paleocene, Itaboraian), Rio de Janeiro, Brazil, Geodiversitas 26 (2), pp. 323-337 : 325-326

Riostegotherium yanei Oliveira & Bergqvist, 1998

REFERRED MATERIAL. — Buckler osteoderms (MCN- PV 1774 [holotype], 1775); movable osteoderms (MCN-PV 1776, 1778, 1779, MCT 2081 -M, MLP 75 -XII-26-1, MLP 75 -XII-26-2, UFRJ-DG 317 -M) ; caudal osteoderm (MCN-PV 1777).

DIAGNOSIS (emended from Oliveira & Bergqvist 1998). — Differs from all known early Tertiary astegotheriines in having more than 10 pits in the groove around the main figure on buckler osteoderms (25 pits on the holotype, MCN-PV 1774; and 16 pits on MCN-PV 1775).

DESCRIPTION

The former description of the osteoderms ( Oliveira & Bergqvist 1998) is partly repeated here, with additions and comments. No articulat- ed osteoderms were preserved. The buckler osteoderms are larger than the movable ones, sub-rectangular in shape and bear more than 10 pits in the groove around the main figure ( Fig. 1 ). In comparison with buckler osteoderms of other astegotheriines, Riostegotherium yanei has the largest number of foramina for the tribe. The lateral and medial borders are slightly concave and smooth. The anterior and posterior borders are irregular; the posterior has a U shaped concavity when viewed internally. The anterior border has a small, weakly defined articulation zone. The external surface is very punctated and bears fine irregularities; in combination with the presence of small depressions, these irregularities give the surface a slightly wrinkled appearance, as in Prostegotherium Ameghino, 1902 and Stegosimpsonia Vizcaíno, 1994 . The main figure has an inverted U-shape, with a subcircular anterior outline, and covers almost the whole osteoderm surface, a very characteristic feature of some genera of the tribe ( Prostegotherium , Astegotherium Ameghino, 1902 , Stegosimpsonia and Nanoastegotherium Carlini, Vizcaíno & Scillato- Yané, 1997). It is limited by a shallow groove with 16 to 25 pits. Although the anterior outline of the main figure varies slightly, it is never triangular as in some Astegotheriini . Its U-shape is very similar to the osteoderms of Prostegotherium and Nanoastegotherium . Two to four small peripheral figures are present, limited by shallow radial grooves with at least two pits, as in Prostegotherium . A well developed central keel is present on the external surface of the main figure, noted by Vizcaíno (1994) as one of the synapomorphies of Astegotheriini . The internal surface of the osteoderms is smooth and slightly concave. No foramina of the piliferous system are observed in the posterior border.

The typical movable osteoderms vary in shape, ranging from sub-quadrangular to sub-rectangular ( Fig. 2 ), which is characteristic of Dasypodidae . The anterior articular surface is poorly developed, and the external surface resembles that of the buckler osteoderms. The main figure has a subcircular anterior outline, but with a reduced number of pits (seven to 12) which is similar to Prostegotherium and Stegosimpsonia . The external surface also bears a well developed central keel. No foramina are present on the posterior border. The internal side of the posterior border is moderately inclined toward the edge of the osteoderm.

The shape of the osteoderm MCN-PV 1777 resembles caudal osteoderms of the extant dasypodid Dasypus . It differs from typical movable osteoderms in: articular surface more developed laterally, smoother external surface, pits more widely spaced in the groove limiting the main figure, and a sharp posterior border.

Figures

FIG. 1. — Riostegotherium yanei Oliveira & Bergqvist, 1998, dorsal view of buckler osteoderms; A, holotype (MCN-PV 1774); B, MCN-PV 1775; a, main figure; b, peripheral figures; c, pits. Modified from Oliveira & Bergqvist 1998. Scale bar: 1 cm.

FIG. 2. — Riostegotherium yanei Oliveira & Bergqvist, 1998, dorsal view of a movable osteoderm (UFRJ-DG 317-M). Scale bar: 1 cm.