Some 193 million years ago in what is now Argentina, a group of 11 dinosaurs all died within a few feet of each other. It’s not clear how. Perhaps they succumbed to drought, as the spring or summer sun baked the ephemeral lakeshore on which they had gathered. Or perhaps they died in the vicious dust storm that buried their carcasses in piles of silt.
What’s more certain is their age: It’s likely that none of these dinosaurs made it to their second birthday.
The assemblage, unveiled today in the journal Scientific Reports, is among many astounding new fossils of Mussaurus patagonicus, a distant ancestral cousin of the famous long-necked dinosaurs such as Brachiosaurus or Brontosaurus. The remains include more than a hundred new Mussaurus eggs and 69 new skeletal fossils, and some appear in clusters of dinosaurs that are the same size and roughly the same age.
The study team interprets this clustering as evidence that Mussaurus moved in age-segregated herds, with animals of similar sizes and ages moving together within the group. If so, the discovery gives paleontologists the oldest evidence ever found of this kind of herd behavior within dinosaurs.
“We know very little about dinosaur behavior, but most of what we know is based on very late dinosaurs from the Cretaceous,” says study leader and National Geographic Explorer Diego Pol of the Paleontological Museum Edigio Feruglio in Trelew, Argentina. “We have very, very little to no information about dinosaur behavior at the beginning of their history.”
Mussaurus is a type of dinosaur called a sauropodomorph, and it’s been known to science since the 1970s, based on 11 fossils found in Argentina's Laguna Colorada Formation. Other sites around the world have hinted at social behavior among Mussaurus’s sauropodomorph cousins. Sites in Germany preserve assemblages that contain multiple fossils of Plateosaurus, while sites in South Africa preserve nesting sites and eggs of the related dinosaur Massospondylus.
But Kimi Chapelle, a postdoctoral researcher at the American Museum of Natural History who wasn’t involved with the new study, emphasizes just how astounding the Mussaurus site’s latest batches of fossils truly are.
“You have the eggs, [and] the individuals—and it’s not just individuals of the same age, but individuals of different ages. That’s just crazy,” she says. “To have all of that within one square kilometer and a three-meter-thick [rock] section? Amazing.”
Pros and cons of herding
Many of today’s large-bodied herbivores move in herds, and the basic evolutionary calculus today is the same as it would have been for Mussaurus more than 190 million years ago.
Herding comes with pros and cons, notes Temple University paleontologist Timothy Myers, who wasn’t involved with the new study. On the one hand, moving in large groups offers more protection from predators, which lets each individual within that group spend less time keeping watch and more time eating. On the other hand, groups must share food, and there’s a higher risk of diseases and parasites.
For herds to work well, Myers adds, animals within the herd need to be synchronized, which can be tricky for animals that change size dramatically as they age. In Mussaurus’s case, hatchlings started out the size of a human palm, grew to 20 pounds and two feet tall at the hip by one year old, and weighed upwards of 3,300 pounds by adulthood—nearly twice the mass of an adult moose.
That’s where age segregation comes in. “The costs, basically, of having to synchronize your behavior go up as you get increasing differences in body size,” Myers says. “For things like sauropods and sauropodomorphs, it’s definitely more convenient for them to form these herds of immature individuals that are separate from adults.”
That said, inferring this kind of social behavior—or any kind of social behavior—from the fossil record is tricky. Trace fossils such as track sites can help, and for some dinosaurs far more recent than Mussaurus, such preserved footprints have shown signs of multi-generational herds.
Skeletal evidence can also hint at social behavior, if paleontologists find groups of skeletons that were buried all at once. Without that crucial evidence, any two skeletons within the assemblage could be from animals that lived and died years apart.
Pol and his colleagues have been excavating the Mussaurus site for the past two decades, dramatically improving their understanding of the dinosaur. For instance, researchers now have a good sense of how Mussaurus’s body changed as it grew up; they are confident that the dinosaur switched from walking on all fours as a juvenile to walking on its rear legs in adulthood. New research also has shown that Mussaurus laid soft, leathery eggs, which suggests that the dinosaur buried rather than brooded its eggs. (Find out more about how new technology is reimagining dinosaurs such as Mussaurus.)
One of the biggest surprises from the site came in 2003, when Pol found the block of stone that contained the cluster of 11 young individuals. “I remember lifting part of the concretion and I saw that there was a top of a skull right there, and the neck was going into the rock,” Pol says. “I knew this was something different.”
Other fossils across the site also show hints of dinosaurs grouping together. The first known Mussaurus fossils, of young hatchlings, were clustered close together. The new excavations also found two adults whose bodies were nearly intertwined.
But to see whether these animals really died in groups and not just in the same place at different times, study co-authors led by Roger Smith from South Africa’s University of the Witwatersrand, Johannesburg, had to carefully study the site’s sediments.
Smith’s research found that there are three distinct layers of Mussaurus fossils at the site, and that many of the site’s fossils and nesting sites share the same layer, meaning that they were each buried at around the same time. The sedimentary rock that surrounds the fossils likely formed from wind-borne dust, perhaps deposited during dust storms.
The team also used several techniques to confirm that the dinosaurs were all Mussaurus and to check the dinosaurs’ ages and sizes. In 2017 Pol flew 30 of the eggs to Grenoble, France, to examine them using one of the world’s brightest sources of x-rays, the European Synchrotron Radiation Facility. Several of them contained fossilized Mussaurus embryos.
Pol and his colleagues also sampled some of the dinosaurs’ bones to see their internal structures, which can reveal information about age and growth patterns. All the fossils in the 11-individual cluster are roughly the same size, likely weighing between 18 and 24 pounds when they died. The bone samples suggest that if young Mussaurus grew in seasonal spurts, the cluster’s juveniles were all probably less than a year old when they died.
The Mussaurus site shows juveniles forming age-segregated groups at least seasonally, but the behavior of adults is somewhat less certain.
On the one hand, the site doesn’t have clusters of many adult Mussaurus, so there’s no direct evidence of adult herds. Among living reptiles, it’s not unheard of for juveniles to congregate and then split up in adulthood. Then again, the clusters of juveniles lie within a Mussaurus nesting site, which could indicate that the juveniles were part of a bigger herd that went to the nesting site to breed.
Still, there’s no doubt that this dinosaur showed versions of social behavior—which makes its place on the dinosaur family tree extremely important.
On top of all the new fossils, the Pol team’s research refines age estimates for the fossils and the surrounding sediments. For years paleontologists thought that Mussaurus was more than 205 million years old, placing it within the late Triassic period. Based on the latest work, though, Mussaurus is roughly 193 million years old, which places the dinosaur in the early Jurassic period.
That distinction is crucial because it means Mussaurus appeared right after a mass extinction that disrupted many kinds of land animals—but left sauropodomorphs relatively unscathed. In the aftermath, sauropodomorphs diversified rapidly and got much bigger than they had been, which would have tipped the balance in favor of age-segregated herds like the ones now suggested for Mussaurus.
Pol and his colleagues argue that more basic aspects of social behavior may have arisen among Mussaurus’s ancestors in the late Triassic, before the extinction event. If so, early sauropodomorphs’ social knack may have helped the group weather the extinction and thrive in the aftermath.
“This is the time when they actually ‘conquered’ the world—when they became dominant, when they first succeeded in ecological terms, in evolutionary terms. What are the keys for dinosaurs’ success? ” Pol says. “Here, we’re saying: Hey, behavior may have been one of the causes.”