Fil: Coria, Rodolfo A. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina.Fil: Coria, Rodolfo A. Provincia de Neuquen. Municipalidad de Plaza Huincul. Museo "Carmen Funes"; ArgentinaFil: Salgado, Leonardo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina.Fil: Salgado, Leonardo. Universidad Nacional de Río Negro. Sede Alto Valle; ArgentinaFil: Salgado, Leonardo. Instituto de Investigación en Biodiversidad y Medioambiente; Argentina.Fil: Salgado, Leonardo. Universidad Nacional del Comahue. Museo de Geologia y Paleontologia; ArgentinaFil: Chiappe, Luis M. Los Angeles County Museum of Natural History. Dinosaur Institute; Estados UnidostrueThe discovery of hundreds of megaloolithid type egg clutches (some including embryos of an indeterminate species of titanosaur sauropods) in several stratigraphical levels of the Late Cretaceous nesting site of Auca Mahuevo (Chiappe et al., 2005) unveiled important aspects of the reproductive behavior of sauropod dinosaurs and stimulated further work at other Patagonian dinosaur egg sites. In November of 2003, a join expedition of the National University of Comahue (Neuquén), the Museo Carmen Funes (Plaza Huincul) and the Museo de Lamarque (Lamarque) conducted fieldwork in several Late Cretaceous localities of Bajo Santa Rosa (center north Río Negro Province, Argentina) bearing the remains of dinosaur eggs and other terrestrial vertebrates (figure 1). The main focus of this expedition was to assess the diversity of dinosaur eggs, to fine tune the stratigraphy of the egg bearing layers, and to document the spatial distribution of the egg clutches. The overall results of this research were reported elsewhere (Salgado et al., 2007). In this contribution, we make focus in one of the fossil localities worked, Berthe IV, where the association of egg clutches likely belonging to different dinosaur species was detected (figure 1.2). Collected eggshells were observed with a binocular loupe ‘‘Stemi SV6 Zeiss’’, at magnifications of x1.0 and x3.2 (for macrocharacters). Microcharacters were observed in transverse thin section using a polarizing microscope (‘‘Zeiss Axioplan’’) at magnifications of x0.4 and x10. The eggshells were photographed using this polarizing microscope equipped with a digital camera. Prior to analysis, the eggshells were submitted to a process of cleaning using ultrasound (see Salgado et al., 2007 for further methodological details)