A Strange Fossilized Egg Has Surfaced from the Frozen Depths of Antarctica

3 min readHere’s what you’ll learn when you read this story:A fossilized egg that hatched in what is now Antarctica during the Late Cretaceous apparently had a soft shell. This means it was probably laid by a monstrous ancestor of lepidosaurs (lizards and snakes) called mosasaurs.The object has now become the largest soft-shelled egg—and second largest egg—ever discovered.68 million years ago, at the edge of the Late Cretaceous, an enormous marine reptile laid an egg near the shore of the landmass that is now Antarctica. When that egg hatched and the offspring swam away into shallow waters, it left behind a leathery soft shell that would eventually fossilize in the sand—and break records millions of years later.This fossil egg is second in size only to that of the extinct elephant bird, at 11.4 by 7.9 inches (29 by 20 cm), and is both the largest soft-shell egg ever found and the largest known egg of any non-avian dinosaur. It stands out even more because it is more similar in structure to the eggs of snakes and lizards (or lepidosaurs) than dinosaurs. What species hatched from the egg is unclear because there are no skeletal remains, but paleontologist Lucas Legendre (from the University of Texas at Austin) thinks the size of the egg means it may have come from a mosasaur, which is an ancestor of modern lepidosaurs.“The identity of the animal that laid the egg is unknown, but these preserved morphologies are consistent with the skeletal remains of mosasaurs (large marine lepidosaurs) found nearby,” Legendre said in a study recently published in Nature. “They are not consistent with described morphologies of dinosaur eggs of a similar size class.”Because Legendre and his research team had no way of assigning it a species, they created a separate taxon for the eggshell alone—Antarcticoolithus bradyi. Previously, experts had speculated that mosasaurs were viviparous, meaning they either birth live young or lay “vestigial” eggs holding the embryo in a late stage of development, with thin shells that are not too mineralized so more water and gas can enter and leave in utero. This egg seems to lend credence to that theory.For existing marine reptiles that lay soft-shelled eggs, larger bodies are usually associated with smaller egg mass and larger clutch size. But the exceptional size of the Antarcticoolithus egg may not be explained by the body size of its parent alone—some exceptions evolved depending on the surroundings of a species. Sea snakes, for instance, tend to have smaller clutch sizes because of the streamlined bodies they developed to navigate their surroundings, and the sleek forms of mosasaurs might have had something to do with larger eggs in a (presumably) smaller clutch.Modern-day viviparous lepidosaurs usually need to conserve energy during reproduction because their metabolisms and body temperatures are relatively low. But something as large as a mosasaur probably could have expended more energy for larger eggs. Antarcticoolithus is also at the upper limit for volume and shell thickness in lepidosaur eggs, and such a merging of gigantism with streamlined body plans and viviparity may have only occurred in mosasaurs (if they actually were viviparous).We certainly haven’t found any evidence of other viviparous lepidosaurs, but that could just as easily be the result of a simple lack of discovered fossils or a lack of successfully fossilized eggs. Soft eggshells are more prone to degrading, after all, but they are more likely to preserve where oxidation can occur. Many fossilize through redox reactions, in which one atom or molecule is oxidized while another is simultaneously reduced as a result of electrons moving between them.Apatite and pyrite found in mineral deposits where the egg surfaced, however, \ revealed there was hardly any oxidation involved in the fossilization of this particular egg. Both apatite and pyrite will degrade if they oxidate when exposed to air or water, meaning Antarcticoolithus must have been buried under the ocean floor and successfully fossilized in the anoxic conditions.“The limited oxidation indicated by the presence of apatite, followed by the net-reducing conditions indicated by the pyrite framboids in Antarcticoolithus,” the researchers said. “[It] may be key to this preservational mode.”Elizabeth Rayne is a creature who writes. Her work has appeared in Popular Mechanics, Ars Technica, SYFY WIRE, Space.com, Live Science, Den of Geek, Forbidden Futures and Collective Tales. She lurks right outside New York City with her parrot, Lestat. When not writing, she can be found drawing, playing the piano or shapeshifting.