Constraints for the Age of Sauropod Embryo- Bearing Rocks in the Neuquén Group

The stratigraphy and age of a sauropod nesting ground containing the first definitive em-bryonic remains of sauropods preserved inside their eggs is analyzed. The fossil locality, called Auca Mahuevo, occurs in the Anacleto Member of the Rı́o Colorado Formation in Neuquén Province, Argentina. The 5 m thick interval of overbank mudstones containing the fossilized eggs and embryos occurs near the middle of a 35 m sequence of thin, fluvial, concretionary sandstones and thicker units of silty sandstone. Flooding of shallow stream channels deposited overbank silt and mud on the eggs, killing the embryos and initiating the process of fossil-ization. Egg fragments containing patches of fossilized integument were found as float weath-ering out of the mudstone on local flats. Complete eggs containing embryonic bones and teeth were quarried from a steep ridge where the mudstone was exposed. Twelve paleomagnetic samples collected throughout the lower 30 m of the section establish the presence of a Reverse geomagnetic polarity interval. This constitutes the first magneto-stratigraphic characterization for this part of the Rı́o Colorado Formation and for the late Cretaceous sequence of formations that comprise the Neuquén Group. Biochronologic ag