'The Thing' Unveiled: Enormous Soft-Shelled Antarctic Egg Rewrites Marine Reptile Reproductive History

An 11-inch soft-shelled egg found in Antarctica proves that giant marine reptiles like mosasaurs likely laid eggs in coastal nurseries 68 million years ago.

By: AXL Media

Published: Apr 28, 2026, 6:51 AM EDT

Source: Information for this report was sourced from Earth.com

A Deflated Fossil Mystery in the Antarctic Sediment

For nearly a decade, a mysterious fossil nicknamed "The Thing" sat in the collections of Chile’s National Museum of Natural History, its identity obscured by its unusual, bag-like appearance. Originally unearthed in 1911 on Seymour Island, the object lacked the thick, chalky shell typically associated with dinosaur remains. It was only through a meticulous microscopic analysis led by Lucas Legendre of the University of Texas at Austin that the truth was revealed. The delicate, multi-layered wall—barely a fraction of a millimeter thick—confirmed that the specimen was a soft-shelled egg, a biological rarity in the fossil record that offers a rare glimpse into the reproductive life of the Late Cretaceous.

The Giant Behind the Shell

Measuring 11 inches long and 8 inches wide, Antarcticoolithus bradyi represents the second-largest egg of any known animal, surpassed only by the extinct elephant bird. To identify the likely parent, researchers conducted an extensive comparative analysis of 259 modern reptile species, linking egg size to body length. The results pointed to a mother exceeding 23 feet in length. Given the proximity of the find to skeletal remains of Kaikaifilu hervei, a 33-foot mosasaur that reigned as Antarctica's top marine predator 68 million years ago, scientists consider this giant sea lizard the primary candidate for the egg-layer.

Challenging the Live-Birth Narrative

Before this discovery, paleontologists widely assumed that large marine reptiles like mosasaurs had evolved away from egg-laying to give birth to live offspring in the open ocean. This was based on the lack of fossilized eggs and the anatomical difficulty of such massive creatures hauling themselves onto beaches to nest. The Antarctic egg suggests a sophisticated middle ground: a "mixed" reproductive strategy. In this scenario, the mother would carry the egg nearly to term before releasing it into the water, where it would hatch almost instantly. This allowed the offspring to enter the marine environment as mobile, independent hunters without the need for terrestrial nesting.