In some cases, scientists haven’t yet found a particular bone from a certain species. 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. This isn’t as accurate since it doesn’t come directly from the fossil bone. If there’s a missing bone in a skeleton, sometimes that shape will be carved like a sculpture. Other methods include 3D prints made from CT scanning, surface scanning, and photogrammetry, which are also very reliable. A fossil can also be a preserved imprint, like a footprint or a leaf.Ĭasts 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. 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). Over time, the sediment hardens into rock encasing the bones, often distorting them. 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. But fossils are rare since the conditions have to be right for them to form. Understanding fossils and castsįossils form over tens of thousands-up to hundreds of millions-of years. 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. 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. This is a question we often hear from visitors as they roam the Field Museum, especially about dinosaur bones.
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