The approximately 75-million-year-old nest shows 15 juvenile members of Protoceratops andrewsi—a relative of Triceratops—entombed in ancient sand dune deposits. The nest was recently discovered by Mongolian paleontologist Pagmin Narmandakh in the region's Djadokhta formation.
…the site seemed to be a popular one for dino day care, as the nests were found in at least three distinct layers of rock within the excavation, each indicating a different point in time. In fact, the researchers suggest the dinosaur moms likely returned repeatedly to the site. Also, the fact that multiple nests were found within the same layers — and thus were laid at about the same time — reveals the dinosaurs likely gathered in groups to lay their eggs, the oldest evidence of such behavior in the fossil record.