A deflated football-shaped fossil found in Antarctica in 2011 has been identified as the largest soft-shelled egg ever found (Credit: Legendre et al. 2020)

Scientists from the University of Texas at Austin have finally solved the mystery of a massive fossil that has been sitting unlabeled and unidentified at a Chilean museum for almost a decade. The relic, which resembles a deflated football, is the largest-known soft-shelled egg from a marine reptile that inhabited Earth over 66 million years ago — about the time of the mass animal extinction event. Measuring more than 11 by 7 inches it is also the second-largest egg belonging to any known animal, behind only the now-extinct elephant bird.

The rare fossil nicknamed, "The Thing" after the popular 1980s science fiction movie, was discovered inside a rock formation in Seymour Island off the coast of Antarctica in 2011 by a team of researchers that included David Rubilar-Rogers. Over the years, the paleontologist at Chile's National Museum of Natural History showed the peculiar-looking fossil to every geologist that came to the museum. However, no one was able to identify "The Thing's" origin until Julia Clarke, a professor at UT Austin's Jackson School of Geosciences, visited in 2018.

"I showed it to her, and, after a few minutes, Julia told me it could be a deflated egg!" Rubilar-Rogers said.

The soft shell fossil egg shown in dark gray in the drawing (upper right), with arrows pointing to its folds and surrounding sediment, shown as light gray. The cross-section (lower left) shows the soft membranes surrounded by a very thin outer shell
(Credit: utexas.edu/Lucas Legendre and Grace Musser)

To confirm Clarke's suspicion, Lucas Legendre, a postdoctoral student at UT Austin who led the study, examined the fossil under a microscope. Sure enough, he found several layers of membrane, confirming that the fossil was indeed a soft-shelled egg — similar to the transparent, quick-hatching eggs laid by some modern-day snakes and lizards. "The shell is very thin. It can rip almost immediately upon exiting the mother, and the juvenile emerges from that egg," says Clarke.

The fossil's identification led to an even bigger mystery — what animal could have laid an egg this size? With no skeletal remains left inside the casing to help, the researchers had to find an innovative solution to the question. After comparing the body sizes of 259 modern reptiles to the sizes of their eggs, Legendre and his team deduced that the egg could have only have belonged to a creature over 20 feet long from snout to the end of its body — not including the tail. "It is from an animal the size of a large dinosaur, but it is completely unlike a dinosaur egg," Legendre said. "It is most similar to the eggs of lizards and snakes, but it is from a truly giant relative of these animals."

An artist's rendition of the interpretation of a mosasaur, an extinct marine reptile that scientists think may have laid the massive soft-shelled egg (Credit: utexas.edu/Francisco Hueichaleo, 2020)

After considering several potential parents, the researchers concluded that the egg was laid by an ancient marine reptile like a mosasaur. The fact that the rock formation where the egg had been found had fossil evidence of baby mosasaurs and the offspring of other marine animals further strengthened their theory. "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," Legendre said.

The researchers speculate the giant reptiles laid the quick-hatching eggs either directly in the water, like sea snakes, or deposited them on the beach, similar to sea turtles. Since it would have been difficult for the massive marine animals to move on land, the scientists think they probably used their tails to place the eggs on the beach, while leaving the majority of their bodies under the water. "We can't exclude the idea that they shoved their tail end up on shore because nothing like this has ever been discovered," Clarke said.

An artist's rendition of a baby mosasaur emerging from an egg in the Antarctic sea (Credit: utexas.edu/ John Maisano/Jackson School of Geosciences)

The researchers, who published their findings in the journal Nature on June 17, 2020, hope to return to Antarctica to search for additional soft-shell egg fossils and collect more data. "There are more and more studies showing that egg structure in reptiles was way more variable than previously thought," Legendre said. "We are currently expanding our dataset to better understand the evolution of those eggs in reptiles as a whole."

Resources: utexas.edu, NPR.org