SEYMOUR ISLAND, ANTARCTICA — Nine years ago, scientists discovered a 68-million-year-old fossilised egg on Seymour Island in Antarctica. New research published in the journal Nature suggests the egg may have belonged to an ancient marine reptile called a mosasaur.

The fossilised egg has a soft shell and measures 29 centimeters in length and 20 centimeters in width. It is the second-largest egg recorded to date. That is roughly five times the size of a medium-sized chicken egg.

Mosasaurs were marine reptiles that lived during the dinosaur era. Evidence suggests that mosasaurus gave birth to live young. However, the egg could have hatched right after being laid. This mode of reproduction is called ovoviviparity.

Mosasaurs belong to an order of reptiles called squamates or scaled reptiles whose ancestors split from the ancestors of dinosaurs, birds and crocodiles roughly 240 million years ago. Lizards and snakes also belong to this group.