A Jurassic graveyard at the Laguna Colorada Formation in Patagonia, Argentina, with numerous bones and more than a hundred fossilized eggs has revealed the earliest evidence of dinosaurs living in herds. In fact, the bones belonged to 80 Mussaurus patagonicus, a group of long-necked herbivorous sauropodomorph dinosaurs.
The large amount of eggs and bones – belonging to dinosaurs ranging in age from embryos to adults – strongly indicated that the dinosaurs lived in herds as far back as 192 million years ago. This was 40 million years earlier than previously believed. Researchers now have “...evidence of a complex social behavior structure within the herd,” as stated by Diego Pol who is a paleontologist at the Museo Paleontológico Egidio Feruglio and the National Scientific and Technical Research Council in Argentina (CONICET).
Despite the Mussaurus patagonicus dinosaurs weighing as much as 1,500 kilograms (3,300 pounds), they still laid very small eggs – about the size of a chicken egg. In an interview with Live Science, Pol explained why they would have needed the help of a herd, especially while they were still growing, “This is a time when they need to eat quite a lot to grow, but they don't have the size to be able to defend themselves and they don't have the expertise and the knowledge,” adding, “So, living in a herd actually protects you during those very vulnerable, fragile stages of your life.”
Furthermore, based on the amount of egg clusters found at the site, it would have been a popular breeding ground. The clusters of bones indicated that even though they lived in herds, dinosaurs in certain age groups typically hung out together. One cluster contained eleven juveniles less than one year of age; while another held the remains of nine almost-mature adults; and another had just two adults.
As for the nests, they contained anywhere between 8 and 30 eggs each and were basically holes that were dug so that the females could have used them for a nesting site. Based on analysis conducted by X-ray computed tomography (CT) images, the eggs, which contained embryonic bones, were set up in two or three layers.
Unfortunately, the dinosaurs probably died from a drought as Pol explained, “We know this place was seasonal, and there are indications of droughts in the sediment.” Since several of the remains were found in a resting position, it is believed that they were laying down when they died and were eventually covered up by dust being blown on top of them from the wind.
Pictures of some of the eggs and images of what their breeding grounds may have looked like can be seen here.
The study was published in the journal Scientific Reports where it can be read in full.