This Ancient Crocodilian Egg Looks Like a Geode

Smithsonian National Museum of Natural History
Smithsonian National Museum of Natural History / Smithsonian National Museum of Natural History

At first glance, this round object looks like an everyday geode that you might see being sold in a gift shop. In reality, the object is an ancient crocodilian egg that never got the chance to hatch. Inside its hard shell, shimmering white crystals have formed and lined the inside of the egg.

The 45 million-year-old egg underwent a process called biocrystallization, in which organic macromolecules of living organisms start to form crystals, usually as a result of stress or other outside factors. It happens to humans too: Anyone who's ever had a kidney stone has experienced lithiasis, which is a form of biocrystallization.

This particular crocodilian egg is from the Eocene epoch. It, as well as other similar specimens, were discovered in Wyoming's Bridger Formation in the 1930s. Fossils and sediments found in the area suggest that the environment was quite a bit different back when the egg was laid. What is now the site of ski towns and ranches was a balmy paradise filled with crocodilians; the region even had palm trees. Eggs and other remains from the time became encased in basin sediment (formed from eroding mountains and volcanic ash) and were occasionally preserved as fossils.

There is no evidence of embryonic remains, but scientists believe it's a crocodilian egg thanks to microscopic analysis. The shape and size of the shell are similar to those of Crocodylus johnstoni and Crocodylus porosus (fresh and saltwater crocodiles, respectively). The beautiful crystallized egg is currently housed at the Smithsonian National Museum of Natural History. 

[h/t Smithsonian National Museum of Natural History on Facebook]

Know of something you think we should cover? Email us at