Protoceratops and Mussaurus eggs would have looked similar to these snapping turtle eggshells, shown after hatching. "It is most similar to the eggs of lizards and snakes, but it is from a truly giant relative of these animals", Dr. Legendre said. The egg was hatched and contained no skeleton of any reptile inside, making it hard for Legendre to identify the type of egg and what reptile could have laid it.
One of the authors of the study, University of Calgary paleontologist Darla Zelenitsky, said many fossilized eggshells have been found for certain bird-like, meat-eating dinosaurs as well as some long-necked and duck-billed plant-eaters.
The egg is the largest soft-shell egg ever discovered and the first to have been found in Antarctica. Soft-shelled eggs are not readily preserved as fossils. It also suggests that calcified eggs evolved independently at least three times in the dinosaur family tree.
"At the same time, we've found thousands of skeletal remains of ceratopsian dinosaurs, but nearly none of their eggs". The academic of the Faculty of Sciences of the University of Chile Alexander Vargas explained that the egg had a "very thin" soft shell and that the mother, a marine reptile that was probably a mosasaur, measured "between 7 and 17 meters".
According to a statement from UT Austin, there are two theories as to how the ancient marine reptiles could have laid their eggs.
Adding to that evidence, the rock formation where the Antarcticoolithus bradyi egg was discovered also hosts skeletons from baby mosasaurs and plesiosaurs, along with adult specimens.
"We essentially "fingerprinted" a large number of modern and fossil samples to build a dataset of the overall molecular picture of the eggshell through time".
"It's an exceptional claim, so we need exceptional data", said study author and Yale graduate student Jasmina Wiemann.
"We've made a lot of progress toward understanding that soft tissues preserve actually quite commonly, but they don't always look the way we would expect them to look".
The clutch of fossilized Protoceratops eggs and embryos examined in this study was discovered in the Gobi Desert of Mongolia at Ukhaa Tolgod. "Otherwise, you wouldn't pick it up in the field", she said.
The early dinosaur eggs, the researchers found, had a chemical residue that was non-mineralized-meaning they were more like today's leathery turtle eggs, protecting their embryos with a soft outer covering.
A mysterious 68-million-year-old fossil found on Seymour Island off Antarctica's coast that looked like a deflated football has turned out to be a unique find - the second-largest egg on record and one that may have belonged to a huge marine reptile that lived alongside the dinosaurs.
Anything earlier, Fabbri said, has been a "black hole".