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Paleontologists have found the earliest known evidence that dinosaurs — unlike reptiles and today’s penguins and other birds — lived in flocks and socialized with one another according to age groups.
Working on a rich fossil bed at a site in Argentina’s Santa Cruz province at the southern tip of South America, scientists found more than 100 eggs and the skeletons of 80 individuals of varying ages, from embryos to adults.
All fossils, including the embryos inside the eggs, belong to the genus Mussaurus patagonicus. These dinosaurs were about 10 feet high and 26 feet long when fully grown, offset by a long tail with an equally long neck ending in a head that looked too small for the huge animal it was attached to. This is the only place where Mussaurus remains have been found.
Little is known about the behavior of dinosaurs, but these large numbers of fossils and their distribution at the site have given scientists new insights into their social lives. The study appeared Scientific Reports Thursday.
The bones and eggs are spread over about 250 acres, a small area to find numerous fossils of the same species. Most of the eggs were found in between eight and 30 claws in nests close together, suggesting that the animals used a common breeding ground. Inside the nests, the eggs are arranged in trenches that the animals apparently dug for this purpose.
The scientists found the eggs, newborns, fry and adults clustered close together; this suggests that the animals live in socially cohesive groups, rather than temporarily gathering together only to breed and lay eggs. The authors suggest that age groupings like these maintain social connections with one another throughout the animals’ lives.
Examples include 11 one-year-old children, and analysis of the bones indicates they are likely members of a single brood buried together. The researchers also found many adults close together in their natural resting poses, suggesting that the animals lived and died together.
Often fossils are found in large numbers in one place, not because the animals died together, but because a stream or river piled up bones of different ages and species and buried them under the silt. But these Mussaurus bones were found in sediments made from wind-blown dust, and the authors conclude that they probably died out at the same time in periodic droughts. There have been at least three mass deaths in the region.
“The sediments have evidence that animals still had soft tissue when they were buried,” said lead author Diego Pol, a researcher at the Museo Paleontologico Egidio Feruglio in Trelew, Argentina. “This indicates a simultaneous death.”
The Latin name for the species is clearly a misnomer. The site had been studied by other researchers in the late 1970s, but they only found a few teenagers small enough to hold in a palm. They named them Mussaurus, which means “rat lizard” in Latin. These new excavations, which began in 2012, revealed a much larger bone assemblage and an animal much larger than a mouse.
It has been known for some time that dinosaurs sometimes lived in packs, but the behavior was only found in dinosaurs that lived around 150 million years ago at the earliest. But the long-necked, plant-eating Mussaurus evolved 193 million years ago, meaning dinosaurs probably lived in herds earlier than previously thought.
X-ray analysis of bone growth patterns by the researchers shows that the animals do not reach adult size until they are at least 15 years old.
Dr. “All this time the young individuals were vulnerable and exposed to predation,” Pol said. “This adds to the interpretation that herd behavior is beneficial for the species in order to protect the young during their growth.”
Body shape – the long neck and tail that immediately come to mind when imagining a dinosaur – may also have been a factor in Mussaurus’ evolutionary success.
Dr. “Once it emerged, this body became the dominant form over millions of years,” Pol said. “So we’re very interested in the evolution of this form. Behavior was perhaps another element of this successful evolutionary description.”
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