Case cracked: Mystery Antarctica fossil is massive prehistoric egg

The exceptionally preserved Protoceratops specimen includes six embryos that preserve nearly complete skeletons. Image credit M. Ellison  American Museum of Natural History

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The Mongolian eggs, which were oval and up to 15 centimetres long, belonged to a horned dinosaur estimated to be between 71 million and 74 million years old.

Protoceratops and Mussaurus eggs would have looked similar to these snapping turtle eggshells, shown after hatching. "The egg belonged to an individual that was at least 23 feet long - a giant marine reptile", Legendre said. The Protoceratops eggs are the first ever discovered for a horned dinosaur, while the Mussaurus eggs represent some of the earliest for a long-necked dinosaur. "From our study, we can also now say that the earliest archosaurs - the group that includes dinosaurs, crocodiles, and pterosaurs - had soft eggs". That's backed up by the discovery of fossilized bones from baby and adult mosasaurs and plesiosaurs, found in the same Antarctic rock formation as the egg.

"We suspect these large reptiles had the same reproductive strategy as viviparous lizards and snakes, which lay eggs with a very thin shell that hatch immediately after being laid", Legendre said. The exceptionally preserved Protoceratops specimen includes a clutch of at least 12 eggs and embryos, six of which preserve almost complete skeletons.

The fossilized egg of Mussaurus. Image credit Diego Pol
The fossilized egg of Mussaurus. Image credit Diego Pol

But now, the UT Austin team has finally given the thing a look over, and found that it's most likely the remains of a soft-shelled egg, after the hatchling had broken free.

By comparing the size of hundreds of reptiles alive today and their eggs, researchers say the animal that laid the egg would have been at least seven metres long.

They found chemical signatures showing the dinosaurs would have been surrounded by soft, leathery eggshells. The same was true for the Mussaurus specimen.

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"These dinosaurs buried their eggs in clutches, like modern animals that lay soft eggs, such as many lizards, snakes, and turtles..."

Hard-shelled eggs have been found dating back to the mid-Jurassic period 164 million to 170 million years ago, but the majority of specimens are from the Cretaceous period, which ended 66 million years ago. "We now have a new method that can be applied to all other sorts of questions, as well as unambiguous evidence that complements the morphological and histological case for soft-shelled eggs in these animals".

With data on the chemical composition and mechanical properties of eggshells from 112 other extinct and living relatives, the researchers constructed a "super tree" to track the evolution of the eggshell structure and properties through time.

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"From an evolutionary perspective, this makes much more sense than previous hypotheses, since we've known for a while that the ancestral egg of all amniotes was soft", Yale graduate student and one of the study's co-authors, Matteo Fabbri, added.

A paper published Wednesday in the journal Nature suggests eggs missing from the fossil record may have been soft, like a turtle's, instead of hard, like those of a bird. Mark Norell of the American Museum of Natural History (AMNH) is first author of the study.

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