World's biggest reptile egg laid by a prehistoric sea monster 66 million years ago and unearthed in Antarctica has a soft shell like a turtle's egg

  • The soft-shelled egg belonged to an ancient sea lizard known as a mosasau and may have been laid in water
  • The egg is second in size only to the extinct Madagascan elephant bird egg that laid eggs up to 13 inches long
  • Researchers say the egg was first unearthed on Seymour Island on the Antarctic Peninsular back in 2011
  • The egg remained unlabelled and unstudied for a decade at Chile's National Museum of Natural History until Texas University researchers visited and realised it looked like a 'collapsed soft lizard shell'
A giant reptile egg unearthed in Antarctica was laid by a 23ft long sea monster 66 million years ago, a new study shows.
Geoscientists from Texas University believe the 11-inch egg was produced by a creature the size of a large dinosaur, but the shell is completely unlike any known dinosaur egg. 
The supersized soft-shelled egg belonged to an ancient sea lizard known as a mosasau, and is second only in size to the extinct Madagascan elephant bird egg. 
The rocks where the egg was found also host skeletons of mosasaurs and other prehistoric marine creatures called plesiosaurs - both babies and adults. 
An artist’s interpretation of a baby mosasaur shortly after hatching. The mother mosasaur is laying an egg while a baby mosasaur swims towards the surface shortly after hatching from a different egg
An artist's interpretation of a baby mosasaur shortly after hatching. The mother mosasaur is laying an egg while a baby mosasaur swims towards the surface shortly after hatching from a different egg 
A side view of the fossil of the giant egg that was found in Antarctica, which at the time would have been a warmer area free of snow and ice
A side view of the fossil of the giant egg that was found in Antarctica, which at the time would have been a warmer area free of snow and ice
A diagram showing the fossil egg, its parts and relative size. The giant egg has a soft shell. This is shown in dark gray in the drawing, with arrows pointing to its folds and surrounding sediment shown as light gray. The cross section (lower left insert) shows that the egg consists mostly of a soft membrane surrounded by a very thin outer shell
A diagram showing the fossil egg, its parts and relative size. The giant egg has a soft shell. This is shown in dark gray in the drawing, with arrows pointing to its folds and surrounding sediment shown as light gray. The cross section (lower left insert) shows that the egg consists mostly of a soft membrane surrounded by a very thin outer shell
Lead author Dr Lucas Legendre, a geoscientist at Texas University, said the egg was most similar to the eggs of lizards and snakes. 
'The egg belonged to an individual that was at least 7 metres (23ft) long - a giant marine reptile,' he said.
Aside from its astounding dimensions, the discovery challenges the prevailing idea that such giant marine animals - from a time just before the dinosaur killing asteroid - did not lay eggs.
Corresponding author Professor Julia Clarke, also of UT, said the almost-complete, football-sized soft-shelled egg changes our understanding of the creatures of this period.
Its proportions and thin shell - which lacks a crystalline outer layer - suggest the predator was 'ovoviviparous', which means the egg develops inside the mother and hatches as soon as it is laid.
The mother who laid this egg, called Antarcticoolithus bradyi, effectively gave birth to live young, the team said. 
It was dug up at the Lopez de Bertodano Formation of Seymour Island, which is part of the Antarctic peninsula, where, in prehistoric times, the region was ice-free and much warmer, with forests covering much of the land.
It was dug up at the Lopez de Bertodano Formation of Seymour Island, which is part of the Antarctic peninsula, where, in prehistoric times, the region was ice free and much warmer with forests
It was dug up at the Lopez de Bertodano Formation of Seymour Island, which is part of the Antarctic peninsula, where, in prehistoric times, the region was ice free and much warmer with forests
'Many authors have hypothesised 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,' said Legendre.
The egg may have hatched in the open water - which is how some species of sea snakes give birth - or it could have been deposited on a beach, with hatchlings scuttling into the ocean like baby sea turtles.
The beach-laying approach would depend on some fancy manoeuvring by the mother. Giant marine reptiles were too heavy to support their body weight on land.
Laying the eggs would require the reptile to wriggle its tail on shore while staying mostly submerged - and supported - by water.
'We can't exclude the idea they shoved their tail end up on shore because nothing like this has ever been discovered,' said Clarke.
An artist’s interpretation of a baby mosasaur emerging from an egg just moments after it was laid. The scene is set in the shallow waters of Late Cretaceous Antarctica. In the background, mountains are covered in vegetation due to a warm climate. In the upper right, an alternative hypothesis for egg laying is depicted, with the mosasaur laying an egg on the beach
An artist's interpretation of a baby mosasaur emerging from an egg just moments after it was laid. The scene is set in the shallow waters of Late Cretaceous Antarctica. In the background, mountains are covered in vegetation due to a warm climate. In the upper right, an alternative hypothesis for egg laying is depicted, with the mosasaur laying an egg on the beach 
An artist’s interpretation of the hypothesized egg-layer, an extinct marine reptile called a mosasaur. An adult mosasaur is shown next to the egg and hatchling for size comparison
An artist's interpretation of the hypothesized egg-layer, an extinct marine reptile called a mosasaur. An adult mosasaur is shown next to the egg and hatchling for size comparison
A cross section of the giant egg’s shell shows a very thin, hard outer shell surrounding a thick, soft inner membrane
A cross section of the giant egg's shell shows a very thin, hard outer shell surrounding a thick, soft inner membrane
An artist’s interpretation of a baby mosasaur hatching from an egg. The illustration shows the egg laying, the baby emerging from the egg, and an image of the empty egg after fossilization
An artist's interpretation of a baby mosasaur hatching from an egg. The illustration shows the egg laying, the baby emerging from the egg, and an image of the empty egg after fossilization
An artist’s interpretation of a baby mosasaur hatching from an egg in the Antarctic sea. The mother is visible in the background. The egg is on the sea floor
An artist's interpretation of a baby mosasaur hatching from an egg in the Antarctic sea. The mother is visible in the background. The egg is on the sea floor
The egg remained unlabeled and unstudied for a decade at Chile's National Museum of Natural History until Texas University researchers visited and realised it looked like a 'collapsed soft lizard shell'
The egg remained unlabeled and unstudied for a decade at Chile's National Museum of Natural History until Texas University researchers visited and realised it looked like a 'collapsed soft lizard shell'
The fossilised egg - described as looking like a 'deflated football' - was unearthed by Chilean scientists in 2011 but remained unlabelled in the collections of Chile's National Museum of Natural History for nearly a decade.
It was nicknamed 'The Thing' - after the sci-fi movie due to its unusual nature and mysterious origins.
Co-author Dr David Rubilar-Rogers, one of the expedition members, showed it to every geologist who visited the museum - hoping somebody had an idea.
But he didn't find anyone until Professor Clarke visited the museum 2011 - he showed 'The Thing' to Clarke and after a few minutes she said 'I can see a deflated egg'.
Now an analysis has established the specimen as the first fossil egg found in Antarctica and pushes the limits on how big soft-shell eggs can grow. 
Using a suite of microscopes to study samples, Dr Legendre found several layers of membrane that confirmed the fossil was indeed an egg.
The structure is very similar to transparent, quick-hatching eggs laid by some snakes and lizards today, he said.  
The findings have been published in the journal Nature.   

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