Which Dinosaur Bones Are “Real”?

“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.

Understanding fossils and casts

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.

The most complete T. rex fossil skeleton

This is what makes SUE the T. rex so rare, and so valuable to science. SUE is the most complete adult Tyrannosaurus rex skeleton discovered to date, with 250 of the approximately 380 total bones in a T. rex. This one-of-a-kind specimen is even more complete by bone volume, at 90 percent—meaning that many of the missing bones are smaller ones.

So, SUE is also the most complete dataset for studying Tyrannosaurus rex. There’s information that we can only get from the fossil bones themselves, which informs our understanding of T. rex growth, age and life span, diseases and injuries, and how T. rex moved and used those tiny arms.

A case for casts

You can still learn quite a bit about an animal—especially a rare specimen like SUE—from a cast. We have a complete research cast of SUE in our collections for visiting scientists to examine. Depending on the focus of the research, there are times when it’s easier to study a hollow plastic cast than the real bones, especially when they’re as big and as heavy as SUE’s.

Plus, mounted casts of SUE’s skeleton have visited museums across the US and in dozens of countries. This is a great way for people around the world to get a better understanding of the size and scale of the most complete T. rex, even if they can’t make it to Chicago to see SUE the fossil.

Along with SUE, which is on display here in the Griffin Halls of Evolving Planet, our other famous dinosaur personality is Máximo the Titanosaur. You’ll find Máximo, a mounted Patagotitan mayorum skeleton made from casts and sculpted bones, in our main Stanley Field Hall. Paleontologists identified this species of titanosaur from around 130 fossil bones that belonged to more than six individuals of Patagotitan. In other words, no one's found a single complete fossil skeleton of Patagotitan mayorum (yet). But scientists can estimate how big Patagotitan was and what its missing bones looked like based what they know about other long-necked titanosaurs.

To make Máximo, all the existing Patagotitan fossils were 3D scanned and printed, and the missing bones were sculpted. Then, the 3D printed bones were used to make molds. Those molds shaped the cast bones for the skeleton, which is resin and fiberglass around a metal structure.

Having a cast of the titanosaur lets our visitors see the true scale of the world’s biggest dinosaur, get up close, and even touch the cast. Casts help spark the imagination, educate, and inspire future discoveries.

Making science visible

Our philosophy at the Field is to show you the real deal—put fossils on display—whenever we can. But we also recognize the value in using scientifically accurate casts to help fill out the story of evolution.

When you walk through the dinosaur hall inside our Griffin Halls of Evolving Planet exhibition—or anywhere in the museum where there are skeletons!—keep an eye out for signs and labels that let you know what’s a fossil and what’s a cast. Later in 2019, we’ll be updating the signs for some of your favorite dinos with even more information, including Apatosaurus and Daspletosaurus. It shouldn’t be a guessing game for you, and it’s also a fascinating part of the story of how we study and display extinct animals and understand what they looked like.