In 2011, Chilean scientists discovered a football-sized fossil off the coast of Seymour Island, near the northern tip of the Antarctic Peninsula. Though they didn’t know what it was at the time—and simply called it “The Thing”—new research shows that not only is it the first fossil egg ever found in Antarctica, it’s also the largest soft-shelled egg ever found anywhere.

In a study published today in the science journal Nature, researchers from the University of Texas at Austin and the University of Chile dated the nearshore rock formation where the fossil egg was found to be from the Late Cretaceous period—about 68 million years ago—and measured the fossil itself to be roughly 11.4 inches by 7.9 inches (29 centimeters by 20 centimeters). This empty, partially collapsed egg is smaller only than that of the elephant bird, an extinct, flightless species from Madagascar whose eggs averaged about 12 inches by 8 inches.

A side view of the fossil egg.Legendre et al. (2020)

But beyond their size, the eggs don’t have much in common; an elephant bird egg is about five times thicker than this fossil egg, and its hard shell has distinct pores and a prismatic layer that the fossil egg lacks. In other words, an elephant bird egg resembles a giant chicken egg. (And giant is no exaggeration—an elephant bird egg could hold the contents of about 150 chicken eggs.)

An elephant bird egg next to a chicken egg (and a man's head), to put it in perspective.Oli Scarff/Getty Images

With its soft shell and oblong shape, the new fossil egg, from the new taxon Antarcticoolithus bradyi, is more similar to a lizard or snake egg, which suggests it could’ve been laid by a large reptile. To test that theory, the researchers compared it to the egg traits of 259 species of lepidosaurs—a subclass of reptile that includes snakes and lizards—and surmised that the egg-layer may have been a marine reptile that measured roughly 23 feet (7 meters) or longer.

The researchers believe this mystery mother might have been a mosasaur, a type of large marine lepidosaur whose remains have also been discovered in the area. During the Late Cretaceous period, mosasaurs were among the most fearsome predators in the ocean. They had strong flippers and sharp teeth, and some species grew as long as 50 feet (though that’s still a good 10 feet shorter than the fictional mosasaur depicted in 2015’s Jurassic World). Fossilized contents of their stomachs show they feasted on a variety of wildlife, including fish, seabirds, turtles, plesiosaurs, and more—one mosasaur had even eaten a few other mosasaurs. And although mosasaurs did live in Antarctica, the continent during the Late Cretaceous period looked nothing like its current frigid landscape.

“Antarctica was rich in life,” Dr. Julia Clarke, a professor in UT Austin’s Department of Geological Sciences and co-author of the study, tells Mental Floss. “Temperate forests diverse in plant species covered exposed land. Giant marine reptiles and much smaller coiled ammonites and relatives of living birds hunted in the seas, while on land, mid-sized non-avian dinosaurs ambled.”

The egg looks a lot smaller when you compare it to a full-grown mosasaur.Francisco Hueichaleo, 2020

Since scientists have uncovered the remains of mosasaurs and plesiosaurs of all ages in the rock formation where the fossil egg was found, some think it may have been a popular place for creatures to hatch and raise their young.

“Many authors have hypothesized that this was sort of a nursery site with shallow protected water, a cove environment where the young ones would have had a quiet setting to grow up,” Lucas Legendre, a postdoctoral researcher and the lead author of the study, said in a press release.

If the fossil egg really did belong to a mosasaur, it could alter our understanding of how mosasaurs gave birth. In South Dakota during the 1990s, scientists unearthed the skeleton of a lizard-like mosasaur called a Plioplatecarpus with five unborn offspring preserved in its abdomen. Because they weren’t in eggs, it was generally thought that mosasaurs gave birth to live young. The existence of Antarcticoolithus bradyi, however, suggests the possibility that some mosasaurs laid soft-shelled eggs that hatched immediately after.

According to Clarke, the discovery of the fossil egg is especially exciting because it demonstrates “how much we have yet to learn about the evolution of eggs, from the first egg-layers that moved away from water to the immense diversity of eggs and reproductive strategies we see today.”