New research from a large set of fossils from Patagonia shows that some of the earliest dinosaurs lived in herds and suggests that this behavior may have been one of the keys to the success of dinosaurs. The discovery of embryos of the same species within fossilized eggs contributed to the results. The findings are published in the journal Scientific Reports .
In the past, studies have shown that some dinosaurs that existed in the last stage of the age of dinosaurs (the Cretaceous Period) lived in herds. However, a big pending question was when and how this behavior appeared in its evolutionary history.
In the early 2000s, an international team of scientists found a 190-million-year-old dinosaur nesting site that also contained juvenile skeletons belonging to Mussaurus patagonicus , a primitive herbivorous sauropodomorph dinosaur (precursor to the large long-necked dinosaurs). in Patagonia (Argentina). ” Such a preserved site would surely provide us with a lot of information about how the first dinosaurs lived ,” explains Diego Pol, a paleontologist at CONICET who discovered the site.
One of the items that could shed light on how the first dinosaurs lived were the eggs found at the site and Pol wanted to know if they were laid by Mussaurus or not. “It is difficult to find fossil eggs, and even more so to find fossil eggs with embryos inside them, since very special conditions are needed for their fossilization”, explains Pol. The ESRF, the European Synchrotron, is the perfect tool to investigate this type of samples: ” We use high-energy X-rays to penetrate the sample without destroying it and have a complete view of its interior, ” said Vincent Fernandez, a paleontologist at the Natural History Museum in London and a former ESRF scientist.
The high-resolution CT scan revealed the existence of fossilized Mussaurus embryos within some of the eggs and showed that all of these fossils belonged to a communal breeding site for a single dinosaur species.
The researchers, in parallel, studied the site itself. The fossils were found on multiple rock horizons in the same locality, indicating that Mussaurus returned to the same location during successive seasons to form breeding colonies. Based on the sediments, the scientists were able to infer that the nesting site was located on the dry margins of a lake.
A key aspect of this locality is that the dinosaur skeletons were not randomly scattered throughout the fossil site, but instead were grouped according to their age. The baby dinosaur fossils were located near the nests. The one-year-olds were found to be closely associated with each other, including a group of 11 skeletons in a resting posture, suggesting that Mussaurus formed schools of young individuals. Adults and sub-adults were frequently associated in pairs or alone, but all within an area of one square kilometer. To determine the age of the juvenile fossils, the scientists carried out histological studies, that is, they cut a thin slice of bone and looked at the bone tissue under a microscope. ” The bones of these dinosaurs grew in annual cycles, much like the tree rings, so by counting the growth cycles we could infer the age of the dinosaur, ” adds Pol.
Being sociable, the key to success
All of the findings show a well-organized herd structure and is the first record of this type of complex social behavior in an early dinosaur (predating other records of dinosaurs with evolved social behavior by more than 40 million years). The scientists compared these results with other fossil egg sites in South Africa and China and suggested that social behavior dates back to the time of the origins of the dinosaurs. “These are not the oldest dinosaurs, but they are the oldest dinosaurs for which herd behavior has been proposed. Mussaurus belongs to the first successful family of herbivorous dinosaurs, so we posit that being social and protecting their young together as a herd may have been part of the reason why these long-necked dinosaurs were so common on all continents. ” Pol concludes.
