Published:June 11, 2019
“Is that real?”
This is a question we often hear from visitors as they roam the Field Museum, especially about dinosaur bones. And it’s a valid one: alongside fossil skeletons, we sometimes display casts, which are made from extremely accurate molds that are shaped directly from the fossils. While we try to show you the real thing whenever possible, there are some important considerations behind why we put both dinosaur fossils and casts on display.
Fossils form over tens of thousands—up to hundreds of millions—of years. But fossils are rare since the conditions have to be right for them to form. First, sediment like mud or sand covers an animal’s body, and the soft tissues rot away leaving behind the hard tissue—teeth and bones. Over time, the sediment hardens into rock encasing the bones, often distorting them. Minerals from the surrounding groundwater and sediment very gradually replace some of the bones’ original minerals (this is why fossils are a variety of different colors: they take on the color of the minerals in the earth around them). A fossil can also be a preserved imprint, like a footprint or a leaf.
Casts are made using precise molds of fossil bones and are one of the most accurate and common forms of 3D duplication you’ll see on display at the Field and other museums. Other methods include 3D prints made from CT scanning, surface scanning, and photogrammetry, which are also very reliable. If there’s a missing bone in a skeleton, sometimes that shape will be carved like a sculpture. This isn’t as accurate since it doesn’t come directly from the fossil bone. It’s based on an examination of existing bones, or on references or photos of bones from other specimens that are the same or related species.
In some cases, scientists haven’t yet found a particular bone from a certain species. They very rarely unearth an entirely intact dinosaur skeleton. Scavengers can disturb decomposing bodies, and erosion after fossilization may destroy some or most of the bones before they are discovered.
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