Many Cretaceous dinosaurs lived in groups and formed nest colonies, for example, or roamed the area as a herd. But when the first dinosaurs began to form social communities has so far been unclear. Now paleontologists in Patagonia have discovered the fossils of the oldest known herd of dinosaurs – they are already 193 million years old. Among the finds are nests with eggs and embryos, a group of adolescent young animals and pairs of adult representatives of the herbivorous dinosaur Mussaurus patagonicus. The age-specific grouping of these around 80 animals speak for a complex, lifelong social association, say the researchers.
The large, herbivorous long-necked dinosaurs of the late Jurassic and Cretaceous periods have long been known to be common in herds. Evidence for this is provided by fossils of entire groups of such sauropods who died together in a primeval catastrophe – for example a flood or a mudslide. Further finds show that some predecessors of the sauropods, the sauropodomorpha living in the early Jurassic, at least already brooded together: Similar to some modern birds, they established nest colonies around 190 million years ago. However, it remained unclear whether these dinosaurs only came together to raise boys or whether they also spent the rest of their lives in social associations.
Eggs, kittens and adults in one place
A fossil site in Patagonia now provides evidence for the first time that at least some Sauropodomorpha formed real herds. As early as the 1970s, paleontologists on the southern tip of Argentina discovered the first fossils of the herbivorous Mussaurus patagonicus in the successive rock layers of the Laguna Formation at this site. In new excavations from 2013, Diego Pol from the Egidio Feruglio Paleontological Museum in Argentina and his team have found further representatives of this species from all ages. In addition to nests with a total of around 100 eggs and embryos that can be identified in X-ray analyzes, young animals of various sizes as well as adult specimens were represented. Dating with the help of zirconium crystals in the find layer showed that these dinosaurs had died 193 million years ago.
“Such a well-preserved site can provide a lot of information about how these early dinosaurs lived,” explains Pol. In fact, it turned out that the fossil Mussaurus relics were not jumbled up randomly, as is the case at some other fossil sites. Instead, there was a clear spatial grouping of the various age groups. At one point on this former lake shore were the nests with the eggs. About 50 meters away, the paleontologists came across a group of eleven young animals. “These young animals were apparently members of a year and died together shortly before they reached their first year of life,” reports the team. Farther out in the outskirts of the site were the fossils of adult dinosaurs of this species, which had been traveling individually or in pairs.
Long-necked dinosaur’s secret of success?
In the opinion of Pol and his team, this grouping is a strong indication that these dinosaurs not only brooded together, but also formed a stable social group of all ages. “The boys did not form a nuclear family with their parents. Instead, there was a larger community where the adults shared the chores and all the boys raised together, ”explains co-author Jahandar Ramezani of the Massachusetts Institute of Technology (MIT) in Cambridge. The findings of the nests in three successive layers indicate that the Mossaurus herd returned to this place several times to raise a new generation of young animals. Meanwhile, the adolescent Mussaurus cubs gathered in their own small groups, which, however, also remained in the herd.
(Video: CONICET)
“Our finds are the oldest evidence of a structured herd formation in dinosaurs differentiated according to age groups,” state Pol and his colleagues. This evidence is more than 40 million years older than comparable evidence from the late Jurassic and Cretaceous periods. As early as 193 million years ago, the forerunners of the herbivorous sauropods formed complex social communities. “This raises the question of whether this life in the herd might also have played a decisive role in the success of these early dinosaurs,” adds Ramezani. Because the Sauropodomorpha had already spread strongly in the late Triassic and gradually displaced almost all other herbivorous dinosaurs from their habitats.
“At that time they were the most numerous terrestrial vertebrates in many terrestrial ecosystems,” explain the paleontologists. Unlike other large herbivores of this time, the sauropodomorpha even survived the mass extinction at the end of the Triassic 200 million years ago unscathed and thus formed the basis for the development of the long-necked dinosaur. According to Pol and his colleagues, the highly developed social skills of Mossaurus, and likely other dinosaurs in this group, may have been one of the reasons for this success. “Mossaurus was part of the first successful family of herbivorous dinosaurs. We therefore postulate that the protection of the young in the herd and the social nature of these long-necked herbivores were one of the reasons why they were so common on all continents, ”says Pol.
Source: Diego Pol (Museo Paleontológico Egidio Feruglio, Argentina) et al., Scientific Reports, doi: 10.1038 / s41598-021-99176-1