FIORELLI lucas Ernesto
congresos y reuniones científicas
Ciudad de Neuquén, Argentina
Congreso; III Congreso Latinoamericano de Paleontología de Vertebrados; 2008
Institución organizadora:
Universidad Nacional del Comahue
The Sanagasta Geologic Park represent a small area –12 km2 approximately– located to 29º16´ south on the Sierra de Velasco and 25 km to northwest from La Rioja, western Argentina. During 2001 a group of paleontologists of CRILAR performed realized an exploration in the Sanagasta valley. In that opportunity, looking for remains of Tertiary fossil mammals, they surprisingly found dinosaur eggshells. In 2007, the Geosciences Department of CRILAR began a series of geopaleontological field works in the Sanagasta Geologic Park resulting in the finding of an important and extraordinary dinosaur nesting site. All the nests are located in the same stratigraphic level, corresponding to the top of the Upper Member of the Sanagasta Formation (the member was formerly named “Los Llanos Formation”; Hünicken, 2005). Although extremely variable laterally as well as vertically, the sedimentary outcrops of this formation are small, limited discordantly on the ancient relief of the Sanagasta granite. The Upper Member (2-6 mts) and bearer of the nests, represents a well-formed paleosol characterized by massive sandstones and fine conglomerates of brown-pink, grayish colorations. This member was highly affected in early stages by hydrothermalism that influenced directly on the diagenetic and pedogenenic processes and the fossilization of the eggs. As result of the initial field works, there were found to date more than 75 nests and around 15,000 eggshell fragments surrounding the nests on an area of 300,000 m2 (more than 50% of the outcrops are yet to be explored); some of these nests were already recollected. The site represents an interesting monospecific nesting site and the clutches are distributed apparently in the area with a certain pattern which possibly relates to the local restrictive paleorelief and paleoambient particularities, although the influence of diagenetic and/or erosive processes are not discarded. Neither nest building structure, architecture or clutch sedimentary rims were found. The clutches are characterized by regularly distributed eggs grouped in a complex way (i.e., in several rows). This implies that nesting behavioral and paleoethological information can be obtained. In other cases, the presence of nests and egg-clutches was determined by a dense grouping of eggshell fragments, which indicates that the nest would have been eroded. Each clutch bears from 3 to more than 35 eggs –the majority complete–, reaching a maximum surface of 3.75 m2. Nests are separated by variable distances (between 3 to 30 meters), although separations can be biased by erosive processes. In the explored area, the nests and egg-clutches have slight differences in each local site, related to the number of eggs (in each nest), to the eggshell characteristics, which is at the moment under study. Apparently, the nests are located in an inter-mountain microbasin that during the Cretaceous times would have represented a stable valley of a few kilometers furrowed by meandering river systems; that area would have been chosen by dinosaurs with gregarious social behavior to lay their eggs.