Fossil of a brooding dinosaur

Artist’s impression of a breeding Oviraptorosaur. (Image: Zhao Chuang)

Similar to today’s birds, some dinosaur species have already incubated their eggs, confirms the investigation of a spectacular fossil ensemble: It is the partial skeleton of a dinosaur from the group of oviraptors, including eggs, in which the remains of young animals were discovered for the first time. In addition to this connection, the breeding behavior is also evident from isotope analyzes of the eggs: They were therefore exposed to temperatures of up to 38 degrees Celsius. The different stages of development of the young also suggest that they hatched in stages, report the paleontologists.

Even at the beginning of their history of discovery, eggs were typical additional finds in connection with the fossils of the oviraptorosaurs. This has also shaped their name: Oviraptor means “egg thief”, because at the beginning paleontologists suspected that they fed on the clutches of other species. But then it became apparent that they were their own eggs and more and more features of these small to medium-sized bipeds of the Cretaceous period became clear. Accordingly, they were probably feathered and had bird-like beaks, with which some species may crack mussels.

Fossils of oviraptors had already been discovered directly on eggs, which suggested a breeding behavior – but since no remains of young animals were found in the eggs, there was a possibility that the animal had perished while laying eggs or that it was only guarding the nest, similarly as it is known from crocodiles. But the results of the investigation of the current find now remove these ambiguities, report the paleontologists working with Shundong Bi from Yunnan University.

For the first time also unhatched young animals

The fossil nest with the eggs and the bones of the adult animal above. (Image: Zhao Chuang)

The fossil ensemble comes from a site in southern China and was dated to an age of around 70 million years. As the studies showed, it consists of the incomplete skeleton of a two-legged running dinosaur, which the researchers identify as a representative of the oviraptors based on its characteristics. As they report, it appears as if the animal was sitting on its nest in a bird-like, brooding position. This clutch consisted of at least 24 eggs. The paleontologists finally made the decisive discovery in seven of them: they contain the remains of unhatched young animals. “This is the first time that a non-avian dinosaur has been found in a single spectacular specimen – and that sits on a nest of eggs that also contain embryos,” sums up Bi.

The late stage of development of the young and the close proximity of the adult suggest that it was a parent that died while incubating its nest. In addition to these indications, the results of an oxygen isotope analysis also speak for the bird-like breeding behavior. As the researchers explain, the ratio of the oxygen isotopes in the fossil embryos shows that the eggs were exposed to typical incubation conditions of 30 to 38 degrees and not the ambient temperature.

“Petrified Behavior”

This clearly shows a breeding behavior that is also known from today’s birds such as the ostrich. “This type of discovery – ultimately fossilized behavior – is very special in dinosaurs,” explains Matthew Lamanna of the Carnegie Museum of Natural History in Pittsburgh. “With the new specimen, the babies were almost ready to hatch, which suggests that this Oviraptor has been tending its nest for a long time.”

Interestingly, the paleontologists also found that the juveniles in the eggs were at surprisingly different stages of development. This suggests that they hatched at different times. Something similar is known of some of today’s bird species – but it is considered a rather unusual adaptation to certain living conditions. Apparently, so-called asynchronous hatching developed independently of one another in oviraptorosaurs and modern birds, say the scientists.

“It’s amazing how much biological information is contained in just one fossil,” says co-author Xing Xu of the Chinese Academy of Sciences in Beijing. He also says this with regard to another finding: the researchers discovered a collection of stones in the abdominal region of the adult Oviraptor. These are probably so-called gastroliths that were intentionally swallowed by the animals to aid digestion. This is the first time that such stomach stones have been found in an oviraptorosaur and could provide clues about the diet of these animals. “We may be able to learn from this specimen for many years to come,” says Xu in conclusion.

Source: Scinece China Press, technical article: Science Bulletin, doi: 10.1016 / j.scib.2020.12.018

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