Shallow lacustrine environments from the Yecua Formation: Discovery of the giant crocodylian Mourasuchus in the Late Miocene of Bolivia.

TINEO D.E; BONA P.; PÉREZ L.M.; VERGANI G.D.

Sao Paulo

Congreso; 6th Latin American Congress of Sedimentology; 2013

Institute of Geosciences, University of São Paulo.

The La Angostura section is situated along the Pira´ı River (50 km southwest of Santa Cruz de la Sierra), which allow to recognize and describe different types of sedimentary facies, corresponding to different granulometries of the Yecua Formation (Late Miocene), exposed for approximately 480 m. There is a clear predominance of fine-size facies interbedded with thinthickness sandstone levels that contain mollusks. The mudstones are predominantly massive (argillic paleosols) and few ones with lamination product of fine-sediment decantation in lentic water bodies (lacustrine). The paleosols recognized in this sequence are indicators of high rainfall regime, in humid climate, developed in extensive floodplains in the presence of vadose zone and swampy water. The obtained information from the paleosols is consistent with the fossil remains found, which highlights the presence of Mourasuchus (Crocodylia, Caimaninae). The material corresponds to a giant specimen represented by three vertebrae (one cervical and two dorsals) and ribs founded in association with freshwaters turtles and fishes. This genus is distributed in South America from the middle Miocene (e.g M. atopus: La Venta, Colombia) to upper Miocene (e.g. M. arendsi : Urumaco, Venezuela; M. amazonensis: Acre, Brazil y M. nativus: Acre y Paran´a, Argentina). The wide distribution of these huge crocodiles in the upper Miocene of South America, and now his occurrence in Bolivia, corroborates the existence of partial connections between major watersheds, which would have favored the dispersion of freshwater vertebrates.