Preparing a Trilobite

by AMNH on

From the Collections posts

The Museum has about 32,000 lots of trilobite specimens, with each lot containing anywhere from one to thousands of examples of these ancient arthropods. Assistant Curator Melanie Hopkins recently took us through a preparation process that helps her identify a given specimen’s species.

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The Museum is home to thousands of examples of trilobite fossils. 
©AMNHGSC 69255a, Earth Sciences Sector, Natural Resources Canada

Given the subtle differences between one species and another—and there are over 20,000 recognized species of trilobite, spanning 10 orders—correctly identifying a given specimen can be a trying task. The fact that many trilobites, like this one, are very small, doesn’t help!

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The details on this fossil, which is just over a centimeter long, are hard to see.
©AMNH GSC 69255a, Earth Sciences Sector, Natural Resources Canada

To make it easier to identify key trilobite characteristics like ridges, spines, and eye stalks, Museum staffers first prepare a batch of ammonium chloride by grinding it into a powder. This chemical, used in fertilizer, cough syrup, and some brands of licorice is a bright white color. That powder is placed in a glass drying tube and heated until it sublimates—transitioning immediately from a solid to a gas. 

Person at a laboratory table with a small object in one hand, treating it with a gaseous substance coming from a glass tube attached to a small hose with a bulb at the end.
Clouds of gaseous ammonium chloride can whiten fossils, making small details easier to see. 

Above, Dr. Hopkins uses a plastic bulb attached to the glass one to produce small clouds of ammonium chloride gas, which coat the fossil in a thin, bright white film. (The powder will later be gently washed off the fossil and has no permanent effect, making it a good option for introducing contrast in fossils.)

The final result is a startlingly white trilobite fossil, with easily discernable bumps and ridges that paleontologists like Hopkins use to identify the species. 

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Coated in ammonium chloride, the fossil's details now stand out. (Click for a closer look).
©AMNH GSC 69255a, Earth Sciences Sector, Natural Resources Canada