• The material was collected at the Araras locality, Nova Mamore ́ Municipality, Rondonia State, Rio Madeira Formation (Fig. 1). The outcrops of the Rio Madeira Formation are exposed along the margins and beds of the Madeira River in the Abuna ̃ Basin. This formation is subdivided into 2 units (Rizzotto et al. 2007). The lower one is composed of solid argillaceous sediments, locally bioturbated, with carbonaceous material and plant remains. The upper unit is composed of fluvial sediments, including basal conglom- erates with a sandy or calcareous matrix. Associated portions of coarse stratified sand, ferruginous and cemented by iron oxide and hydroxide, commonly occur. All the remains of vertebrates are found at this level. Above these conglomerates, in the top of the sequence, is a unit of yellow, silty clay, massive and bioturbated with levels hardened by iron oxides and hydroxides (Rizzotto et al. 2007). The sediments of the Rio Madeira Formation were accumulated in a meandering fluvial system and present facies of channel bars and evidence of fluvial floodplains.