Why – and how – do scientists reconstruct fossils in the first place?

Take a careful look this black and white illustration of the skeleton. Did you notice some of the bones are missing? Now look at the photograph of the skeleton. It looks complete. Which is right? Both of them are.

WHY DO SCIENTISTS RECONSTRUCT FOSSILS?

Paleontologists (people that dig up the remains of ancient animals, like dinosaurs) almost never find complete fossils. Fossils found in the field are usually chipped, broken, and largely incomplete. However, if you walked into a natural history museum right now, you would see dozens of complete dinosaur skeletons on display. How is this possible? As it turns out, each of those skeletons has been subjected to some degree of fossil reconstruction. That means that a scientist working in a lab examined the fossils that were found in the field, put all the broken pieces together, filled in all the cracks and holes, and then also created dummies for the bones that she didn’t have at all.

 How do scientists reconstruct fossils?

Take a look at the picture to the right. What do you think this is a picture of? Most of you probably see a tiger, and you would be correct! Even though this picture is far from complete, you still had enough information to determine what it is an image of. When scientists first find a bone in the field, they must go through a similar process. The fossil might be broken or incomplete, but the parts that are there provide enough information to figure out where that bone was in the body, and possibly even what animal it is from! The first step in fossil reconstruction is determining everything you can about the fossil material you have in your hands. Once you do that, you are in good shape to make inferences (educated guesses) about the material you do not have.

 How can scientists be sure their reconstructions are accurate?

Scientists can never be sure that a reconstruction is completely accurate, but there are methods they can use to insure it is as accurate as possible. Let’s take a look at the muzzle of our tiger puzzle. One side of it is missing, but the other half looks pretty good. We know from evidence that an animal’s entire body is symmetrical (the same on both sides). We can use the left/right symmetry of animals to our advantage. If we wanted to reconstruct this picture, we would use the left side of the face as a model for drawing the right side. Of course, it is possible that the tiger in this picture was deformed, or was in a fight and lost its right lip. But these conditions are rare so it is highly unlikely that this tiger suffered from them—and as scientists, we want to base our reconstructions on what the animal is most likely to look like. This left/right trick can be used for arms and legs as well. If you find the left leg of a dinosaur and not the right leg, just make a mirror-image copy of the left leg and you’re all set.
One of the best ways to reconstruct something that is broken is to find a complete example to guide you. Imagine trying to put together a puzzle without the picture on the box! For our tiger puzzle, we might look for pictures of other tigers sitting in similar positions to help us imagine what our reconstruction should look like. So how does this apply to dinosaur fossils?

Let’s say that you find the femur (upper leg bone) of Nigersaurus, but it’s been broken and you only have the lower half. Based on the material that you have, it would be difficult to make an educated guess on what the knee-end of the bone looks like. However, if you compared your bone to other Nigersaurus femurs that have already been discovered, you would be able to make a pretty accurate estimate of what yours looked like. What if no-one ever found a Nigersaurus femur before you? How would you know how to reconstruct the one you found? You would look at the femur of Nigersaurus’ closest relative (or at least the closest relative we have a femur for) and base your reconstructions on that. It will not be exact or perfect, but this method ensures that it will be more accurate than anything you could simply imagine.