One of the study’s authors, paleontologist Darla Zelenitsky of the University of Calgary, said that many fossilized egg shells have been found for some bird-like meat-eating dinosaurs as well as some fish eaters. long neck and duckbill plants.
The eggs of other dinosaurs – especially more primitive species – were notably absent.
“Over the years, we had certainly thought that the dinosaurs may have laid soft-shelled eggs. Others have gone so far as to nurture the idea of live birth, “said Zelenitsky, a specialist in dinosaur eggs and nesting sites.
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“But there was no fossil evidence, plus the closest living relatives of dinosaurs – crocodiles and birds – all lay hard-shell eggs. It goes without saying that dinosaurs also only lay hard-hatched eggs. “
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The international research team examined egg laying discovered in Mongolia and Argentina with an unusual thin halo surrounding the fossilized embryos.
The Mongolian eggs, which were oval and up to 15 centimeters long, belonged to a horned dinosaur estimated between 71 million and 74 million years ago. This species, Protoceratops, is an ancestor of the dinosaurs discovered in Alberta which would have also laid soft eggs.
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Zelenitsky said this could be the reason why, after 30 years of finding eggs in Alberta, no hard-shelled eggs belonging to large horned dinosaurs were found there.
“They may be hidden in the fossil record like less discreet, thin-shelled soft eggs.”
The 12-centimeter-wide spherical eggs found in Argentina belonged to a small, long-necked dinosaur called Mussaurus and are 200 million years old.
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Study co-author Jasmina Wiemann, a doctoral candidate at Yale University, used cutting-edge technology to examine the molecular composition of fossils without damaging them.
She was able to determine the halos observed in the fossils were not thinner hard shells than usual, but a material that was probably “squishy and leathery in nature.”
“For a long time, we thought that soft tissue generally couldn’t fossilize,” she said.
“We have made a lot of progress in understanding that soft tissue actually keeps quite often, but it doesn’t always look like what we would expect.”
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Wiemann said scientists also have a better understanding of where the best-preserved soft-shell egg fossils can be found, such as in river sediments, and how to prepare fragile specimens so that they are not damaged.
“You must be determined to expect something like this. Otherwise, you would not pick it up on the ground, ”she said.
“I wouldn’t be surprised if we found many more soft tissue dinosaur eggs in the future, potentially in our collections, already there waiting. ”
Matteo Fabbri, another Yale researcher who contributed to the article, said it appears that dinosaurs started with soft-shelled eggs and hard shells evolved independently in certain types of dinosaurs later.
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Hard-shelled eggs date back to the mid-Jurassic from 164 million to 170 million years ago, but the majority of the specimens are Cretaceous, which ended 66 million years ago.
Everything that happened earlier, Fabbri said, has been a “black hole”.
“Now we know that the first dinosaurs were much more reptilian in terms of reproductive biology,” he said.
“This is a complete change of perspective.”
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