Researchers from MIT, Argentina, and South Africa explain their finding of an unusually preserved group of early dinosaurs that exhibit evidence of sophisticated herd behavior as early as 193 million years ago, 40 million years earlier than prior records of dinosaurs herding. Since 2013, members of the team have discovered more than 100 dinosaur eggs and the fragmentary skeletons of 80 young and adult dinosaurs from a rich fossil bed.
They were able to examine the eggs’ contents without breaking them apart and discovered preserved embryos inside. It helped to confirm that the fossils were all members of Mussaurus patagonicus, a plant-eating dinosaur that lived in the early Jurassic period and is classified as a sauropodomorph.
Surprisingly, the researchers discovered that the fossils were organized by age: dinosaur eggs and hatchlings were found in one region, while juvenile skeletons were located in another. Adult dinosaur skeletons were discovered alone or in pairs across the field site.
According to the researchers, this age segregation is a significant indicator of a sophisticated, herd-like social structure. The dinosaurs most likely worked together as a group, depositing their eggs in a communal nesting area. Juveniles gathered in schools, while adults wandered and foraged for the herd.
Ramezani timed old sediments found amid the remains and discovered that the dinosaur herd lived approximately 193 million years ago, during the early Jurassic era. The findings indicate the first evidence of social herding among dinosaurs.
Living in herds may have provided an evolutionary benefit to Mussaurus and other sociable sauropodomorphs. These early dinosaurs appeared in the late Triassic period, just before a mass extinction event that killed off many other creatures. Sauropodomorphs survived and eventually dominated the terrestrial environment in the early Jurassic.
The team’s paleontologists have been working in the Laguna Colorada Formation, a location in southern Patagonia notable for containing early sauropodomorph fossils, since 2013. When scientists found fossils inside this formation in the 1970s, they called them Mussaurus, which means “mouse lizard,” since they thought the bones belonged to tiny dinosaurs.
Only long later did scientists, including members of the Argentinian team, find larger bones, showing Mussaurus adults were much larger than their rodent namesakes. However, the name stuck, and the team has proceeded to discover a large collection of Mussaurus fossils from a tiny, square kilometer of the formation.
The fossils discovered so far have been found in three sedimentary layers that are close together, indicating that the location may have been a shared breeding place to which the dinosaurs returned frequently, maybe to take advantage of favorable seasonal circumstances.
Among the fossils, a paleontologist sought for volcanic ash samples to send to Ramezani’s lab at MIT. Volcanic ash can include zircon, mineral grains containing uranium, and lead, the isotopic ratios of which Ramezani can precisely quantify. He can determine the age of the zircon and the ash in which it was discovered using the half-life of uranium, or the time it takes for half of the element to disintegrate into the lead. Ramezani successfully found zircons in two ash samples, all of which he dated to about 193 million years old.
Scientists believe that two additional early dinosaurs, Massospondylus from South Africa and Lufengosaurus from China, lived in herds around the same period, however, dating for these dinosaurs has been less accurate. If many lineages of dinosaurs lived in herds, the researchers believe social behavior originated earlier, maybe as far back as their common ancestor in the late Triassic.