Scientists find 66-million-year-old sea lizard egg in Antarctica, the biggest ever recorded

A 66-million-year-old fossil found in Antarctica almost ten years ago was recently found to be one of the biggest eggs ever recorded, scientists say.

In 2011, a group of Chilean researchers were exploring the southern frozen continent looking for fossils of marine reptiles, when they came across a weird looking fossil. It was almost a foot wide and looked like a deflated football.

Not knowing what it was, the researchers brought it back to Chile’s National Museum of Natural History, where it remained unstudied for years. But in 2018, Julia Clarke, a professor visiting from the Jackson School of Geosciences at the University of Texas at Austin, saw the specimen and immediately thought it could potentially be a giant fossilized egg. With permission, she took a few fragments back with her to UT Austin.

On June 17, researchers from UT Austin and the Chilean National Museum of Natural History released a study that indicates that the mysterious fossil is actually an 11-inch soft-shell egg — the largest ever recorded — that was likely laid by an ancient marine reptile.

According to Lucas Legendre, lead author of the new study and a postdoctoral researcher at UT Austin, fossils of soft-shell eggs are extremely rare to find. As opposed to the more traditionally-known hard-shell egg, which has a solid mineralized exterior, soft-shell eggs mostly consist of a soft membrane, which breaks very quickly after being laid.

“It is, by far, the largest soft-shell egg ever discovered,” Legendre said. “The next one in line would be those of large pythons, or komodo dragons, which are about half the size of this one. It is the second-largest egg ever discovered in general.”

The first challenge for scientists was to try and determine the origin of the animal that laid the egg. Legendre says he and his team compared the chemical composition of the egg to hundreds of other lizards and reptiles — but they discounted dinosaurs since they primarily are known to have laid hard-shelled eggs.

By comparing their findings to the type of creatures living in Antarctica 66 million years ago, Legendre said it seems likely the egg is that of a mosasaur, a giant marine reptile that would typically grow up to 10 metres in length.

Up until this point, since no specimens were ever found, scientists often hypothesized that mosasaurs and other marine reptiles were viviparous — which meant they carried their embryos internally. Legendre says the common belief was that mosasaurs birthed their young, who then immediately came up to the surface for air.

But with the discovery of the new, record-breaking egg, Legendre says it could mean that these marine reptiles would first lay a soft-shell egg, which would hatch within minutes, to allow their children to come up for air.

“So what we think happened, is that, you have to picture a 10-metre-long mosasaur in these shallow waters trying to expel an egg out of its womb,” Legendre said. “And immediately after that, maybe even seconds, because it’s so thin, the baby would have been able to come out and go to the surface to breathe instantly.”

Since soft-shell eggs so rarely become fossilized, the researchers believe that after it hatched, the eggshell was covered in sediment and bacteria, which allowed it to be preserved so well.

Legendre said that scientists can now use the egg to learn more about the development of soft-shell eggs over the course of millions of years.