The fossils indicate a common nesting area and adults who fed and cared for the young as a flock, scientists say.
To borrow a line from the film “Jurassic Park: “Dinosaurs move in herds. And a new study shows that prehistoric creatures lived in herds much earlier than previously thought.
In an article published today (October 21, 2021) in Scientific reports, researchers from MIT, Argentina and South Africa detail their discovery of an unusually preserved primitive dinosaur group that shows signs of complex herding behavior 193 million years ago – 40 million years earlier than other records of dinosaur herds.
Since 2013, team members have excavated over 100 dinosaur eggs (about the size of chicken eggs) and partial skeletons of 80 juvenile and adult dinosaurs from a rich fossil deposit in southern Patagonia.
Using X-ray tomography imaging, they were able to examine the contents of the eggs without separating them and discovered embryos preserved inside, which they used to confirm that the fossils all belonged to Mussaurus patagonicus – a herbivorous dinosaur that lived in the early Jurassic Period and is classified as a sauropodomorph, a predecessor of the massive long-necked sauropods that later roamed the Earth.
Surprisingly, the researchers observed that the fossils were clustered by age: dinosaur eggs and hatchlings were found in one area, while juvenile skeletons were clustered in a nearby location. Meanwhile, adult dinosaur remains have been found singly or in pairs throughout the site.
Extract from CONICET Documental: “First evidence of herd life and age segregation among dinosaurs. “
This “age segregation”, according to the researchers, is a strong sign of a complex social structure, similar to a herd. The dinosaurs probably worked in community, laying their eggs in a common nesting place. Juveniles gathered in “schools” while adults roamed and looked for food for the herd.
“This may mean that the young people did not follow their parents in a small family structure,” explains Jahandar Ramezani, a member of the team and a researcher at the Department of Earth, Atmospheric and Planetary Sciences at MIT. “There is a larger community structure, where adults have shared and participated in the education of the whole community.
Ramezani dated ancient sediments among the fossils and determined that the herd of dinosaurs dated back to around 193 million years ago, during the early Jurassic period. The team’s findings represent the first evidence of social breeding in dinosaurs.
Living in a herd may have given Mussaure and other social sauropodomorphs an evolutionary advantage. These first dinosaurs originated from the late Trias, shortly before an extinction event wiped out many other animals. For some reason, sauropodomorphs stuck around and eventually dominated the terrestrial ecosystem in the early Jurassic.
“We have now observed and documented this early social behavior in dinosaurs,” says Ramezani. “This now raises the question of whether living in a herd may have had a major role in the early evolutionary success of dinosaurs. This gives us some clues as to how dinosaurs evolved.
Since 2013, the team’s paleontologists have been working in the Laguna Colorada Formation, a site in southern Patagonia known to contain fossils of the first sauropodomorphs. When scientists first discovered fossils in this formation in the 1970s, they named them Mussaure for “mouse lizard” because they assumed the skeletons were miniature dinosaurs.
It wasn’t until much later that scientists, including members of the Argentina team, discovered larger skeletons, indicating Mussaure the adults were much larger than their rodent namesakes. The name stuck, however, and the team continued to unearth a rich collection of Mussaure fossils of a small square kilometer of the formation.
The fossils they have identified so far have been found in three closely spaced sedimentary layers, indicating that the area may have been fertile ground where dinosaurs have returned regularly, perhaps to take advantage of favorable seasonal conditions. .
Among the fossils they uncovered, the team discovered a group of 11 juvenile skeletons articulated, intertwined and overlapping, as if they had suddenly been thrown together. In fact, judging by the remarkably unspoiled nature of the entire collection, the team believes that this particular herd of Mussaure died “synchronously”, perhaps quickly buried by sediment.
Based on evidence of ancient flora in nearby outcrops, the Laguna Colorada formation has long been considered relatively ancient on a dinosaur scale. The team wondered: Could these dinosaurs have been able to come together from the start?
“People already knew that at the end of the Jurassic and Cretaceous, the large herbivorous dinosaurs exhibited social behavior – they lived in herds and had nesting sites, ”says Ramezani. “But the question has always been, when was such herd behavior the earliest? “
A gregarious line
To find out, Diego Pol, a paleontologist at the Egidio Feruglio Paleontological Museum in Argentina who led the study, looked for volcanic ash samples among the fossils to send to Ramezani’s lab at MIT. Volcanic ash can contain zircon, mineral grains containing uranium and lead, the isotope ratios of which Ramezani can accurately measure. Based on the half-life of uranium, or the time it takes for half of the element to decay into lead, he can calculate the age of the zircon and the ash in which it was found. . Ramezani managed to identify zircons in two ash samples, all of which he dated to around 193 million years ago.
Since the volcanic ash was found in the same layers of sediment as the fossils, Ramezani’s analyzes strongly suggest that the dinosaurs were buried at the same time as the ash was deposited. A likely scenario could have involved drought and wind-blown dust that quickly starved and buried the herd, as ash from a distant eruption drifted and, luckily for science, deposited zircons in the sediment.
The team of scientists used high-energy X-rays at the European Synchrotron (ESRF) to penetrate dinosaur eggs without destroying them and to gain a full view inside them. Credit: Vincent Fernandez / Diego Pol / ESRF
Together, the team’s results show that Mussaure and perhaps other dinosaurs evolved to live in complex social herds 193 million years ago, around the dawn of the Jurassic period.
“Evidence suggests that Mussaure optimized foraging potentials in the early Jurassic via age-based social partition – hatchlings, juveniles and adults apparently searched and perished in age-based groups, ”says Raymond Rogers, professor of geology at Macalester College, who was not involved in the study. “This type of gregarious behavior is common today in large terrestrial herbivores. It is amazing to see clear evidence of the same phenomenon in this first species of dinosaur. “
Scientists suspect that two other types of primitive dinosaurs – Massospondyle South Africa and Lufengosaurus from China – also lived in herds around the same time, although the dating of these dinosaurs has been less accurate. While several distinct lineages of dinosaurs lived in herds, researchers believe social behavior may have evolved earlier, perhaps as far back as their common ancestor, during the late Triassic.
“Now we know that ranching took place 193 million years ago,” says Ramezani. “This is the first confirmed evidence of herd behavior in dinosaurs. But paleontological understanding says that if you find any social behavior in this type of dinosaur around this time, it must have appeared earlier. “
Reference: “Early evidence of herd-living and age segregation between dinosaurs” by Diego Pol, Adriana C. Mancuso, Roger MH Smith, Claudia A. Marsicano, Jahandar Ramezani, Ignacio A. Cerda, Alejandro Otero and Vincent Fernandez, October 21, 2021 , Scientific reports.
DOI: 10.1038 / s41598-021-99176-1
This research was funded, in part, by the National Science Foundation in the United States and the National Council for Scientific and Technical Research of Argentina.