PAUL D STEWART / SCIENCE PHOTO LIBRARY PAUL D STEWART / SCIENCE PHOTO LIBRARY
Iridescent shell of an extinct cephalopod genus, Baculites from the cretaceous age, Pierre Shale, of Montana USA. They became extinct about 60 million years ago at the end of the cretaceous. The shell nacre preservation of aragonite and conchiolin results in very thin layers of material and structural colours due to interference of light.
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