A fossilized ancient egg discovered in Antarctica nearly a decade ago has finally been identified, shedding new light on prehistoric history. Scientists from the University of Texas at Austin have determined that it is a giant, soft-shelled egg, most likely laid by a mosasaur—an extinct marine reptile that ruled the seas during the late Cretaceous period.
A Long-overlooked Discovery
The fossil, found in 2011, remained unexamined for years, stored away in the Chile’s National Museum of Natural History. Measuring over 11 inches (27.9 cm) long and 7 inches (17.8 cm) wide, it resembles a deflated football. The researchers’ initial confusion stemmed from the fact that it did not resemble the typical rigid-shelled eggs seen in many dinosaur fossils.
Upon closer inspection, the University of Texas at Austin research team identified multiple layers of membranes inside the specimen, a characteristic of soft-shelled eggs.
These findings, published in the journal Nature, indicate that the egg dates back approximately 66 million years, placing it just before the mass extinction event that wiped out the dinosaurs and many other prehistoric species.
A Record-breaking An Ancient Egg
This discovery is particularly remarkable because the egg is the second-largest known egg from any animal, living or extinct. It also holds the title for the largest soft-shelled egg ever recorded.
“It is from an animal the size of a large dinosaur, but it is completely unlike a dinosaur egg. It is most similar to the eggs of lizards and snakes, but it is from a truly giant relative of these animals,” explained Lucas Legendre, lead author of the study.
However, the sheer size of the fossilized egg suggests it belonged to an enormous marine reptile, far larger than any existing egg-laying species today.
The team analyzed the relationship between egg size and body size across 259 living reptiles to estimate the size of the creature that laid it. Their calculations suggest the reptile measured at least 23 feet (7 meters) long, excluding its tail.
Mosasaur Reproduction Mystery
Until now, the general belief among paleontologists was that mosasaurs gave birth to live young, similar to modern-day sea snakes and some sharks. Unlike marine dinosaurs, mosasaurs were reptiles that thrived in the open sea, making it unlikely for them to nest on land like birds or turtles. This led scientists to assume that these creatures evolved viviparity, or live birth, to adapt to a fully aquatic lifestyle.
The discovery of this soft-shelled egg challenges the idea that mosasaurs gave birth to live young. Scientists suggest they may have laid eggs in water, like sea snakes, allowing them to hatch immediately, or used shallow, protected coves as nesting sites. Fossilized baby mosasaurs found nearby support the idea of a prehistoric marine nursery.
The Bigger Picture: Rewriting History
If future discoveries confirm that mosasaurs laid eggs, it could force scientists to rethink their reproductive evolution. This ancient egg suggests that soft-shelled eggs were more common among prehistoric marine reptiles than previously thought.
It also hints that the transition from egg-laying to live birth may have been more complex, occurring in different stages across various species.
