Antarcticoolithus bradyi: Paleontologists Find Giant Soft-Shelled Egg of Cretaceous-Period Marine Reptile in Antarctica

Saturday, June 20, 2020

An artist’s rendering of a pair of mosasaurs and their egg. Image credit: Francisco Hueichaleo.

A giant fossilized egg of an extinct marine reptile has been found in the 68-million-year-old nearshore marine deposits in Antarctica.

Named Antarcticoolithus bradyi, the new fossil is the first fossilized egg found in Antarctica.

The specimen exceeds eggs of all known non-avian dinosaurs in volume and differs from them in structure.

Measuring 29 by 20 cm (11.4 by 7.9 inches) and weighing 6.5 kg, it is the largest soft-shell egg ever discovered and the second-largest egg of any known animal. Although the elephant bird egg is slightly larger, its eggshell is roughly five times thicker.

University of Texas at Austin paleontologist Lucas Legendre and his colleagues from the United States and Chile think that Antarcticoolithus bradyi was laid by a giant marine reptile, such as a mosasaur — a discovery that challenges the prevailing thought that such creatures did not lay eggs.

“The fossil egg 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,” Dr. Legendre said.

A diagram showing the Antarcticoolithus bradyi egg, its parts and size relative to an adult human. Image credit: Legendre et al, doi: 10.1038/s41586-020-2377-7.

The researchers found several layers of membrane that confirmed that Antarcticoolithus bradyi was indeed an egg.

“The structure is very similar to transparent, quick-hatching, eggs laid by some snakes and lizards today,” Dr. Legendre said.

However, because the fossil egg is hatched and contains no skeletal material, the scientists had to use other means to zero in on the type of reptile that laid it.

An artist’s interpretation of a baby mosasaur emerging from an egg. Image credit: Francisco Hueichaleo.

They compiled a data set to compare the body size of 259 living reptiles to the size of their eggs.

They found that the reptile that laid the egg would have been 7 m (23 feet) long from the tip of its snout to the end of its body, not counting a tail.

Adding to that evidence, the rock formation where the Antarcticoolithus bradyi egg was discovered also hosts skeletons from baby mosasaurs and plesiosaurs, along with adult specimens.

“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,” Dr. Legendre said.

The study was published in the journal Nature.


L.J. Legendre et al. A giant soft-shelled egg from the Late Cretaceous of Antarctica. Nature, published online June 17, 2020; doi: 10.1038/s41586-020-2377-7