Entering the record books as the second largest animal egg ever found on Earth, paleontologists from the National Museum of Natural History of Chile and the University of Texas at Austin have identified a giant fossilized egg that belongs to a reptile Extinct marine that once swam in Antarctic waters about 66-68 million years ago.
The large Cretaceous fossil, first discovered in Antarctica in 2011 by Chilean scientist David Rubilar-Rogers and his colleagues, was originally nicknamed “The Thing” because of its strange appearance of deflated football. Now this rare relic has been officially named Antarcticoolithus bradyi and represents the first fossilized egg unearthed in Antarctica.
Paleontologist Julia Clarke of the University of Texas at Austin initially observed the strange fossil in 2018 while visiting Rubilar-Rogers at the National Museum of History in Chile, where she quickly deduced that the object was an ancient egg measuring 11.4 by 7.9. inches and weighed 14.33 pounds.
“It was weird enough that they decided to pick it up, even though it wasn’t clear what it was about. It was definitely not bone, but it was surprisingly unusual,” Clarke explained. “Many turtles, snakes, and lizards lay eggs with soft, flexible shells. The thing is the largest soft-shelled egg in history, by far.”
Excavated in offshore marine deposits in Antarctica, this unique specimen far exceeds the size of the eggs belonging to all known non-avian dinosaurs in volume and is significantly different in structure, lacking a distinct prism and pore layer. Only the egg of Madagascar’s extinct elephant bird is marginally larger, but it’s also five times thicker.
These findings, published this week in the online scientific journal Nature, stretch the limits of how great paleontologists theorized that these soft-shell eggs could grow. The oblong fossil is also of great importance because scientists have concluded that it was put in by an extinct giant marine reptile known as a mosasaur, which flies in front of evidence that these sea creatures did not lay eggs.
“It is an animal the size of a large dinosaur, but it is completely different than a dinosaur egg,” said lead author Lucas Legendre, a postdoctoral researcher at the Jackson School of Geosciences at UT Austin. “It is very similar to the eggs of lizards and snakes, but it is from a truly giant relative of these animals.”
All of the mosasaurs from the Cretaceous period had similar body styles, with highly muscular cylindrical bodies, long jaws, and a powerful tail for quick propulsion. Its nimble limbs had evolved over time into efficient oars used to maneuver along primitive waterways in search of prey.
Legendre used a variety of microscopes to study samples from “The Thing”, where he identified multiple layers of membrane that confirmed that this unusual fossil was absolutely an egg. Their structure was found to be more similar to the transparent, fast-hatching eggs produced by many modern snakes and lizards. But because its content contains no real skeletal evidence, Legendre and his team had to build a theory about which prehistoric reptile deposited it.
The researchers formulated a data set comparing the body size of 259 living reptiles with their eggs, and found that this ancient marine reptile would have easily measured more than 20 feet long from the tip to the end of the body. To further cement its position, the rock formations where the celebrity egg was discovered is also a known repository of both adult and infant mosasaur and plesiosaurus skeletons.
“Many authors have hypothesized that this was a type of nursery with protected shallow water, a cove environment where young people would have had a quiet environment to grow,” said Legendre.
Exactly how this pregnant mosasaur gave birth and laid the egg is still being debated. Scientists have proposed that it may involve a process by which the egg hatches in open water, according to the methods used by modern sea snakes. Another speculates that the birth event had the massive reptile throwing eggs on a sheltered beach, with the little monsters crawling out to sea like baby sea turtles.
However, the researchers quickly point out that this would be a difficult endeavor since Mommy Mosasaur would have been too heavy to navigate its immense weight on dry land.
“We can’t exclude the idea that they stuck their tails on shore because nothing like this has ever been discovered,” Clarke said.