FIGURE 1. Geographic location and stratigraphic section of the Auca Mahuevo fossil site in the northwestern Patagonian province of Neuquén, Argentina.

The eggs and embryos were collected from the Anacleto Member of the Río Colorado Formation. Previous age assignments for the fossils from the Río Colorado Formation have ranged from Coniacian to Campanian³⁰, a wide geochronological interval extending from about 89 Myr to about 71 Myr. The eggs and embryos are restricted to a 5-m-thick interval of silty, pale reddish-brown mudstone stratigraphically located just below the midpoint of a 25-m-thick sequence of siltstones and fine- to medium-grained sandstone. These sediments seem to represent shallow palaeochannel and associated overbank deposits laid down on a low-gradient floodplain. The eggs and embryos are preserved exclusively in the finer-grained, overbank silts and muds.