Estratigrafía y análisis tafonómico de Bonitasaura salgadoi Apesteguía, en el sitio «La Bonita» (Cretácico superior, Río Negro, Argentina)

Resumen

Stratigraphic and taphonomic analysis of Bonitasaura salgadoi Apesteguía (Sauropoda,
Titanosauria) at «La Bonita» site (Upper Cretaceous, Río Negro Province, Argentina). «La Bonita» is
a fossiliferous locality close to the town of Cerro Policía, Río Negro Province, Argentina. At the quarry are
exposed siliceous rocks belonging to the Bajo de la Carpa and Anacleto formations (Santonian-Campanian),
separated by an unconformity from the Rentería Formation (Upper Pliocene-Pleistocene). The Bajo de la Carpa
Formation is composed of sandstones, fine beds of conglomerates, alternate with pelites and wackes beds. The
Anacleto Formation is composed of sandy to pelitic beds. The Rentería Formation is dominated by conglomerates
with clasts that reach 15 cm in length, included in a sabullitic matrix. The facies analysis suggests a fluvial
regime, with a subacuatic control with changes from mid to low energy. The lenticular-shaped sandy channels
with crossed stratification point to an unidirectional flow in the transporting agent and bioclasts. The
taphonomical analysis suggested that the dinosaur dies close to the river margins, being rapidly incorporated to
the fluvial sediments that sepulted the specimen in successive events. The spatial distribution and bone relations
suggest a very short transport from the source area, with a slight northward movement. The presence of two
caudal vertebrae series with an opisthotonous articulation pattern, suggests that the animal was exposed long
enough to permitt the rigor mortis and dissecation. The disarticulation degree also point to some amount of
weathering of the skeleton before being sepulted. However, the exceptional periostium preservation in many
cases suggests a very short subaerial exposure.