By Michael Marshall
DottedZebra/Alamy
A dinosaur-era animal laid one of the largest eggs ever, measuring 29 by 20 centimetres. The egg is the second largest ever discovered, only beaten by that of the extinct Madagascan elephant bird.
We don’t know what animal laid the huge egg, but the prime candidate is a mosasaur: a giant marine reptile that looked a bit like a toothed whale.
The fossilised egg was discovered on Seymour Island, off the west coast of Antarctica, by a team of Chilean palaeontologists including David Rubilar-Rogers of the National Museum of Natural History in Santiago.
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They showed it to Julia Clarke at the University of Texas at Austin. “I had never seen anything like that before in my life,” says Clarke. It was found in a 68-million-year-old rock formation – from near the end of the dinosaur era. Despite its huge size, it had almost no shell and must have been very soft.
At first, the researchers were baffled. “We had no idea what could have laid it,” says co-author Lucas Legendre, also at the University of Texas at Austin.
No fossilised eggs have ever been found in Antarctica. “We’ve got dinosaurs from Antarctica, but they’re not big enough to lay this egg,” says Clarke.
Instead, the team thinks the egg could have been from a mosasaur. Although mosasaurs lived during the dinosaur era, they weren’t dinosaurs. They belong to a group of reptiles called squamates, which also includes lizards and snakes. The ancestors of squamates split from those of dinosaurs around 240 million years ago.
Other groups of marine reptiles, like the long-necked plesiosaurs, are believed to have given birth to live young. “But for mosasaurs, the evidence is much more limited,” says Clarke. One adult has been found with embryos inside, curled up as if inside eggs, but with no eggshell preserved.
Given that the egg was so soft, Clarke and Legendre suspect it hatched as soon as it was laid – a halfway house being egg-laying and giving birth to live young, known as ovoviviparity.
“In lizards and snakes, live birth with a vestigial eggshell has evolved over and over again,” says Clarke. It would not be surprising if some mosasaurs also evolved it, she says.
Journal reference: Nature, DOI: 10.1038/s41586-020-2377-7
