An unexpected discovery in Antarctica is challenging long-standing scientific assumptions. A massive egg fossil, recently identified as soft-shelled and possibly belonging to a mosasaur, may reveal that these powerful marine reptiles reproduced in ways never before imagined.
A Long-overlooked Specimen Reveals New Secrets
The egg, first unearthed in 2011, remained overlooked for years while sitting in storage at Chile’s National Museum of Natural History. Researchers were initially unsure what to make of this ancient trace, the structure was unmistakable. The study, published in Nature, estimates the egg to be approximately 66 million years old, placing it shortly before the mass extinction that marked the end of the Cretaceous period.
The fossil, which measures over 11 inches (27.9 cm) long and 7 inches (17.8 cm) wide, resembling a partially deflated football. Unlike the familiar rigid-shelled dinosaur eggs, this specimen showed none of the defining hard shell structure scientists usually look for. It wasn’t until a team from the University of Texas closely examined the specimen that its true nature became apparent. Researchers identified multiple membrane layers inside the egg.
Second-largest Egg Ever Discovered
This paleontological find holds two remarkable distinctions. It is not only the second-largest egg ever recorded from any animal, living or extinct, but also the largest known soft-shelled egg. According to Lucas Legendre, lead author of the study, the egg is “from an animal the size of a large dinosaur, but it is completely unlike a dinosaur egg.” He described it as most similar to the eggs of snakes and lizards, though the scale is vastly greater.
To determine the possible size of the reptile that laid it, the research team studied the relationship between egg and body size across 259 modern reptile species. Their analysis suggests the animal measured at least 23 feet (7 meters) long, not counting the tail. Given its size and age, the most likely candidate is a mosasaur — a marine reptile that dominated prehistoric seas but was unrelated to dinosaurs.
Did Scientists Get Mosasaur Birth All Wrong?
Before this discovery, most paleontologists believed that mosasaurs gave birth to live young, much like modern sea snakes or some species of sharks. Because mosasaurs were fully aquatic, it seemed unlikely they would return to land to lay eggs. This assumption painted them as highly evolved for ocean life, capable of viviparity — a reproductive mode where embryos develop inside the parent.
Now, that theory is being revisited. The egg’s soft-shelled structure raises the possibility that mosasaurs laid eggs directly into the water, allowing them to hatch almost immediately. Another possibility is that these marine reptiles sought out shallow, protected coastal coves to deposit their eggs. Supporting this idea, fossilized baby mosasaurs have been found nearby, suggesting the existence of a prehistoric marine nursery.
A New Chapter In Reptilian Evolution
This Antarctic discovery may prompt a broader reassessment of how prehistoric marine reptiles reproduced. The presence of such a large, soft-shelled egg suggests this form of reproduction may have been more widespread than previously assumed. It also hints that the transition from egg-laying to live birth could have occurred in stages, varying significantly across different species and environments.
If further evidence supports the egg’s mosasaur origin, scientists may need to redraw key parts of the evolutionary tree regarding aquatic reptile reproduction. This rare fossil stands as a striking reminder of how much remains undiscovered in the fossil record — and how a single overlooked artifact can change our understanding of life in Earth’s ancient oceans.
