How Are Dinosaur Skeletons Held Together In Museums?

Most museum dinosaur skeletons are not single, complete fossils. They are lightweight fiberglass casts, often combined with a few real bones and pieced together from several individuals. The whole assembly is held up by a hidden steel framework, called an armature, that keeps every bone in its proper position.

If you have ever spent any time in a Natural History museum, then you’ve likely strolled past a looming dinosaur skeleton, frozen in time. Perhaps its fossilized jaw was open in a silent roar. Dinosaurs remain some of the most fascinating creatures to have ever lived on our planet, and people around the globe are perennially drawn to museums where they can learn about these ancient titans of Earth. It isn’t unusual for a natural history museum to have multiple dinosaur skeletons dominating an exhibit hall, posed perfectly for the greatest dramatic effect!

If these fossils are so precious, how do museum curators manage to stand them up and suspend them safely, after they’d been buried underground for millions of years?

Are The Skeletons In Museums Real?

Many people interchangeably use the words bones and fossils, but this is a simplification of the situation. Most fossils are actually made of rock, not the original bone; over the course of millions of years, mineral-rich groundwater seeps into the tiny pores of buried bone, where dissolved minerals such as silica and calcite precipitate out and, in many cases, gradually replace the original organic material, leaving a “fossil” behind. When paleontologists come across one of these fossils, they rarely find a complete skeleton, and it is almost never in perfect condition. Pieces will be missing, broken or chipped, in addition to being extremely heavy. In the case of massive dinosaurs, such as titanosaurs, the largest of which stretched to roughly 37 meters (120 feet) in length, their fossils (made of solid stone) can weigh tons!

Some dinosaur fossils are well over 230 million years old, dating back to the dawn of the dinosaurs in the Late Triassic, yet they remained remarkably well-preserved underground, waiting for paleontologists to stumble across them. Given how important these fossils are to our understanding of these larger-than-life species, some institutions consider it irresponsible to display them in a museum like a Lego sculpture! For that reason, almost all the dinosaur skeletons you see in museums are at least partially casts of the original fossils.

In the past, when dinosaur fossils were first being discovered and put on display, holes were drilled into the original fossils for easier mounting and reconstruction. The irreparable damage this caused was not considered at the time, but advancements in our knowledge and sense of responsibility have led to different techniques. Some skeletons do incorporate original fossils, but due to the lack of complete skeletons, the gaps need to be filled in with casts, the details of which are often derived from other skeletons or closely related species. This is why most of the mounted skeletons you see are composites: rather than coming from one perfectly preserved animal, a single mount is frequently assembled from the bones (and casts of bones) of several different individuals of the same species. The famous Triceratops at New York’s American Museum of Natural History, for instance, is built from the bones of four individuals found in two different states, Montana and Wyoming, with its forelimbs restored in plaster. The museum’s own 1933 description of the mount even concedes that the skull and skeleton probably represent more than one species! In many cases, the original fossils are also needed elsewhere, such as for research in a laboratory. Casts can also be shipped around the world so other museums can enjoy the wonder of these prehistoric creatures, whereas sending the original fossils would increase their risk of damage.

To protect the original fossils, fill in gaps, and make it easier to create museum exhibitions, lightweight casts are made, typically constructed from fiberglass. These are meticulously created, mimicking even the tiniest details of the original fossil. These casts can then be combined with original fossils, which may have some of their damage fixed with plaster, as well as sculptural elements (models of fossils that were too fragile or rare to make casts from). In fact, even the most celebrated real skeletons lean on casts. At Chicago’s Field Museum, the home of Sue, the most complete adult T. rex ever discovered, only 250 of “her” roughly 380 fossilized bones were actually found; the remainder are casts and sculpted models that fill in the gaps!

How Are Dinosaur Casts Made?

So how do you copy a bone that has spent millions of years underground without harming it? The answer is a careful, hands-on craft called molding and casting, and the goal, according to the American Museum of Natural History, is to reproduce the information in the original as completely as possible without damaging the specimen. That matters, because a cast may be all that some researchers ever get to see of a particular fossil.

The process usually goes something like this:

1. Protect the fossil. Before anything touches it, the fossil is treated, typically by coating fragile surfaces and filling cracks and holes.
2. Make a rubber mold. The bone is covered in a flexible rubber, generally silicone, polyurethane or latex rubber, which records its surface detail. Complicated bones get molds made in two or more parts that fit together like a clamshell.
3. Build a jacket. Rubber is floppy, so a rigid “mother mold” is built around it to hold its shape. These jackets are usually plaster, although large ones are sometimes made of fiberglass.
4. Pour the cast. Once the fossil is carefully freed from the mold, the empty mold is filled with plaster or with epoxy, polyurethane or polyester resin. Epoxy is often chosen for small fossils that need extremely high resolution, polyurethane for lightweight medium-to-large casts, and polyester for larger casts because it is cheaper. Plaster is easy to work with, but it captures less detail and dries an opaque white.

Unlike plaster, resin can be colored, so a well-made cast can be tinted to match the look of the original fossil, which is why it can be so hard to spot the difference from the other side of a museum rope!

These days, the process increasingly starts on a computer. When the Field Museum built Máximo, its cast of the giant titanosaur Patagotitan mayorum that stretches 37 meters (122 feet) across the museum’s main hall, every known Patagotitan fossil was 3D scanned and printed, and the missing bones were sculpted. Those printed bones were then used to make molds, and the finished skeleton is made of resin and fiberglass around a metal structure. Not a single bone in it is real, and that’s exactly the point: no priceless original has to spend its days hanging in the air over a crowded hall.

Skeletal Mounts

Now that you understand the mixed-bag style of fossil exhibits in museums, we can dig into how those skeletons manage to stay standing! As mentioned, a century ago, original fossils were simply drilled into, connected with screws and put together like a puzzle.

Things have certainly changed in the past hundred years, but if you go to some of the older museums around the world, you can still see these old methods of displaying dinosaur bones, many of which are the original fossils, instead of casts. However, regardless of the “authenticity” of the bones, the process of mounting them is quite similar. Once the skeleton is arranged, the museum curator will work with specialized metalworking artisans called mount makers to develop the final exhibit.

After deciding what position the dinosaur skeleton will be in, curators and mount makers can collaborate on a scaled-down model of the design. When this model is blown up to full size, the mount makers will construct an iron framework, called an armature, that can support the weight of the cast, and hold all the bones in place, accurately depicting the skeletal structure of the dinosaur. Basically, an iron skeleton needs to be created to hold up the fossilized skeleton, and keep it from moving around. These metal armatures must be sturdy and durable, particularly if they are holding up original or irreplaceable fossils. For example, if an earthquake hits, the skeletal mounts must stay intact and maintain the integrity of these precious fossilized finds.

After the fossils and casts are assembled on the steel framework, the entire construction is usually painted, which can help to make the armature less visible or obvious, completing the breathtaking illusion of a towering dinosaur skeleton in the middle of an exhibition hall!

How Do Modern Mounts Protect The Real Fossils?

The old habit of drilling holes into fossils to fit ropes and bars did lasting damage to many early museum specimens. Today, the guiding rule is that a mount should protect the bones and keep them available for science, not just hold them up for the crowds.

A great example is the Nation’s T. rex at the Smithsonian’s National Museum of Natural History in Washington, D.C. It was discovered in 1988 by rancher Kathy Wankel while hiking with her family near Montana’s Fort Peck Reservoir, and it has since been loaned to the Smithsonian for 50 years by the U.S. Army Corps of Engineers. About 80 to 85 percent of the animal (by size) was recovered, but several fossils, including the head and most of the ribs, were too fragile to put on display. As a result, just over half of the bones on show are genuine, and carefully made replicas fill in the rest. For bones that have no true counterpart, the team modeled stand-ins on the mirror-image bone from the other side of the body or on other T. rex skeletons.

The mount was built by Research Casting International, an Ontario firm that assembles dinosaurs for museums around the world. Crucially, each of the metal pieces that secures the fossils is removable, so scientists can still get at individual bones for future research. The finished skeleton, standing about 4.6 meters (15 feet) tall and 12 meters (40 feet) long and posed about to decapitate a fallen Triceratops, went on display when the museum’s new fossil hall opened on June 8, 2019.

The Field Museum takes a similar approach with Sue. The skull on her mounted skeleton is actually a cast; the real one, which weighs about 272 kilograms (600 pounds), sits in a freestanding case so visiting scientists can easily study it. Mounts can also be revised as the science moves on. When Sue returned to display in a new gallery in December 2018, the museum added her belly ribs (gastralia), which had been left off her original mount because scientists didn’t know exactly how they fit onto the skeleton.

So how can you tell which bones in front of you are real? Your best bet is simply to read the signs. As the Field Museum points out, museum labels and signs identify which specimens are fossils and which are casts.

A Final Word

If your childhood dream of being a paleontologist didn’t quite work out, you can still get that rush of excitement by visiting an old natural history museum, such as the one in Washington, Paris, New York or London. While many of the bones on display will only be casts, there will almost certainly be some original fossils attached to delicate armatures, allowing you to stand in the presence of prehistory’s greatest beasts!