Scientists have discovered that long ago, some dinosaurs may have produced soft-shelled eggs. They consider the possibility that some descents transitioned into laying hard shelled-eggs afterwards.
Two soft-shelled egg fossils were found from different species in Argentina and Mongolia. Reuters reports that researchers claim the fossils belonged to a species called Mussaurus from Patagonia, and a Protoceratops from the Gobi Desert.
They estimated the fossils to date back some 75-200 million years ago. The scientists say that the discovery of the fossils indicates that many lineages might have laid these kinds of eggs 230 million years ago. They add that soft-shelled eggs are not readily conserved as fossils.
Calcified eggshells are said to protect growing embryos against the stresses of the outside environment. Furthermore, the structures also contribute to reproductive success. The findings of the new study were published in the scientific journal Nature on June 17, 2020.
Not All Dinosaur Egg Shells are Hard
It has always been common knowledge that dinosaurs laid hard-shelled eggs. Comparable to modern birds, feathered dinosaurs, and crocodilians, the image of baby dinosaurs making their way out of cracked hard shells has always been the belief of many.
Although soft-shelled eggs exist in many animals such as snakes, lizards, and turtles, the possibility of dinosaurs laying these kinds of eggs was not considered until now. Moreover, only a few dinosaur egg fossils have ever been discovered, mostly belonging to meat-eating dinosaurs.
One of the soft-shelled eggs found in Argentina came from the long-necked plant-eater, Mussaurus. It stood approximately six meters long and belonged to the sauropod lineage. Its five-inch egg was spherical in shape compared to a traditional oval egg shape.
The other egg fossil found in Mongolia belonged to another herbivore called the Protoceratops. The species is a member of the ceratopsian lineage of beaked dinosaurs, which sometimes possessed horns. Its egg measured four inches and appeared more oblong.
A New Perspective in Dinosaur Reproductive Biology
The discovery of the soft-shelled egg fossils suggests that the most primitive dinosaurs were in a way reptilian in their reproductive practices.
The authors of the study claim that histology reports authenticate the organic composition of the newly-discovered soft-shelled dinosaur eggs. They reveal that the fossils somehow resembled turtles' soft eggshells.
Through their findings, the researchers conclude that the first dinosaur egg was soft-shelled. Furthermore, they say that the calcified, hard-shelled dinosaur egg evolved at least three times during the Mesozoic era. This would justify the bias towards collected dinosaur eggshells in the fossil record.
According to Mark Norell, the lead author, the team's findings offer a new perspective in dinosaurs' reproductive biology. Additionally, the paleontologist from the American Museum of Natural History in New York says that dinosaurs laid soft-shelled eggs that were most likely buried under the sand or in vegetation.
He explains that it could be the reason behind calcified fossil eggs being known from only a handful of dinosaur groups. It could also be why the fossils appear long after the inception of the dinosaur group.
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