Antarctica would actually be an exciting place for paleontologists. Because the continent was not always an icy desert, but was once home to a variety of dinosaur species. The fossils discovered so far bear witness to this. Although the climatic conditions and the ice tanks are not very conducive to the preservation of fossils, researchers keep finding remains from the reptile era. A fossil stored in a museum in Chile for nearly ten years has now been identified by an international research group as a soft-shelled egg of a dinosaur. The findings of the research group increase our understanding of the reproduction of those extinct reptiles.
The fossilized egg is one of the largest eggs ever discovered and is 28 by 18 centimeters. According to the study’s lead author, postdoctoral researcher Dr Lucas Legendre of the University of Texas, the egg is very different from previously known dinosaur eggs. “It is most similar to the eggs of lizards and snakes, but it is from a truly giant relative of these animals,” he explains. Analysis of the fossil using microscopes and CT scanners, however, suggested that the egg’s shell had not been hard, but soft as snake eggs, for example. This, and the fact that the fossil has been found in layers of a fossilized former seabed, suggests that it could be an egg of a marine dinosaur.
The fossil, which was discovered in Antarctica by Chilean researchers in 2011, has long remained unidentified in Chile’s National Natural History Museum. David Rubilar-Rogers, one of the explorers, had to wait a long time and show it to various experts until a useful idea could be formulated. Professor Julia Clarke, head of the UT’s geoscience department in Austin, Texas, in 2018 was the first to think it could be an empty egg. Thus, the researchers began to examine the fossil more closely. No skeletal residues could be detected in the egg itself. But the layers, which only became visible by microscope and scanner, showed egg shell structures similar to today’s snakes and some lizard species. Their eggs do not have a thick shell with a lot of calcium, but a rather leathery shell, which indicates a rather primordial egg type. Using comparisons of egg and body size of reptiles living today, the researchers concluded that it must have been an egg of a giant dinosaur. Fossil finds around the egg reveals skeletal parts of numerous small mosasaurs and plesiosaurs and some adults. As a result, Legendre and his colleagues suspect that it could be the egg of such a mosasaur. This species of marine reptile lived between 80 and 66 million years ago and was at home in almost all seas. They are among the largest marine predators with a length of up to 18 metres.
The findings made by the research group paint a new and as yet unknown picture of the reproduction of dinosaurs. Until now, it has been assumed that all marine dinosaur species had live-birth for their offspring. Hard-shelled eggs do not work in this environment and soft-shelled eggs have been unknown to dinosaurs until now. But despite the findings, the researchers can only make assumptions about how the egg laying worked. One thesis sees the animals laying their eggs like sea turtles on beaches. But to do this, the gigantic Mosasaurs would have had to push their long tail and powerful fins to the beach, which in turn would be associated with difficult maneuvers. Because the animals were heavy and therefore had to stay in the water with their bodies. “We can’t exclude the idea that they shoved their tail end up on shore because nothing like this has ever been discovered,” says Julia Clarke.
Link to the original work:
Legendre et al (2020) Nature, https://doi.org/10.1038/s41586-020-2377-7
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