Curious Collections: True Blue Fossils

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Vertebrae fragments of Champsosaurus sp. Photo: ©AMNH

Nestled deep within the Museum’s vertebrate paleontology collection are several gloriously blue bones.

They are vertebrae of the long-extinct Champsosaurus, a crocodile-like creature that lived between about 60 and 45 million years ago, straddling the non-avian dinosaur extinction. They were found in 1882 in the San Juan Basin, New Mexico.

“The overwhelming majority of vertebrate fossils fall into the earth tones, browns, black, and white,” says Carl Mehling, collections manager for fossil amphibians, reptiles, and birds at the Museum.

Specimens that deviate from this drab palette are rare, which makes finding them all the more exciting. Mehling, who discovered the fossils in the vast collections, couldn’t believe the color was natural. “I thought, ‘Get out of here!’” says Mehling. “I thought somebody had put a coating on it that just happened to be blue.”

The muted sapphire tone was no coating but rather a part of the bones, which had undergone a process called mineralization after burial. Dissolved minerals in groundwater deposited minerals in the bones, changing their chemical composition. What minerals could possibly turn a bone blue? Without extensive lab work, it’s hard to be sure.

“Typically, it only takes a very little amount of a mineral to change the color [of an object],” says George Harlow, curator of minerals and gems in the Department of Earth and Planetary Sciences.

But, so long as the structure of the bones is intact, what mineral caused this metamorphosis would be just a secondary concern to those who study fossils.

This story originally appeared in the Summer issue of Rotunda, the Member magazine.