Fossil Preparation III: Molding and Casting

Several years ago, I had the opportunity to visit the Paleontological Institute of the Russian Academy of Science in Moscow, Russia, and to tour its facilities. One of the things that excited me most, as I looked at the skeletons on display, was that I could see acquisition numbers on the bone. Acquisition numbers are the catalogue numbers given to fossils and other specimens by museums, so they can trace where the specimen was collected with their filing system. Seeing these numbers meant that I was looking at actual bones on display!

"So what?" you are thinking, "I saw fossil bones during my last visit to my local natural history museum." Not necessarily. Keep in mind that fossilized bones are very heavy; after all, they have become filled with minerals. If museums were to mount the actual bones, it would require specially designed frames to support the immense weight. At the Institute in Moscow, I could see the welded iron frames that had been constructed for the skeletal displays earlier in the century.

Also, due to the way preservation and fossilization work, bones are seldom perfectly preserved. Often they are crushed or distorted by the weight of millions of years of sediment bearing down on them. Skulls are especially vulnerable to damage because they are largely hollow. What this means for the museum visitor is that often the real bones are rather ugly to behold. Also, sometimes bones might be missing. Perhaps the skeleton was discovered as parts of it weathered out in a streambed, and smaller, lighter bones were carried away in the water. Maybe parts of the animal were carried away millions of years ago by scavengers. And, sadly, modern day fossil thieves may have taken bones as well.

In order to deal with these problems-the weight of the original bones, damaged or missing bones, and the need to preserve the original fossils and make them available for study-museums often use casts of the original fossils made from synthetic resins for their displays.

In my next article, I will discuss how molds and casts are made, and how these new "bones" are mounted for display.

Be sure to visit my web site for more links to Natural History Museums. Also visit Mark Tailleur's site at Suite 101 for his articles about Natural History Museums.

Photographs by Geoff Habiger.

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