The "dinosaur bones" that you see on display at the Museum aren't really bones at all. Through the process of fossilization, ancient animal bones are turned into rock.

What is a fossil?

A fossil is any evidence of prehistoric life (plant or animal) that is at least 10,000 years old. The most common fossils are bones and teeth, but fossils of footprints and skin impressions exist as well. Fossils are excavated from many environments, including ancient riverbeds and lakes, caves, volcanic ash falls, and tar pits. Fossils are classified as either body fossils or trace fossils. Body fossils were parts of the organism, such as bones or teeth. Trace fossils are all other types of fossils, including foot impressions, burrows, and dung.

How does something become a fossil?

Most ancient animals never became fossils. Their carcasses were likely consumed by other organisms, or worn away by wind or water. But sometimes the conditions were right and their remains were preserved. The most common process of fossilization happens when an animal is buried by sediment, such as sand or silt, shortly after it dies. Its bones are protected from rotting by layers of sediment. As its body decomposes all the fleshy parts wear away and only the hard parts, like bones, teeth, and horns, are left behind. Over millions of years, water in the nearby rocks surrounds these hard parts, and minerals in the water replace them, bit by bit. When the minerals have completely replaced the organic tissue, what's left is a solid rock copy of the original specimen.

Where are fossils found?

Fossils are found almost exclusively in sedimentary rocks—rocks that form when sand, silt, mud, and organic material settle out of water or air to form layers that are then compacted into rock. So in looking for non-avian dinosaur fossils in particular, paleontologists look for outcrops of sedimentary rocks that formed during the Mesozoic Era (251–65.6 million years ago), the geologic time period when non-avian dinosaurs lived. Scientists typically search in regions where little vegetation covers the surface of the ground, so that any fossil fragments weathering out of the sedimentary rock layers can be more easily seen. These regions of barren ridges and ravines are often referred to as ‘badlands.’

In order to find appropriate Mesozoic, sedimentary rock layers, paleontologists often use geologic maps, which show the kinds of rock layers of different geologic ages that are exposed on Earth’s surface in different regions. Once appropriate rock layers are found, the search for dinosaur fossils can begin with a reasonable hope of finding the kinds of dinosaurs one is searching for. And other kinds of fossils are often serendipitously discovered during the search.

How are dinosaur fossils discovered and collected?

To find fossils, paleontologists conduct expeditions to regions around the world where fossils are likely to be found. To be successful, this fieldwork requires considerable funding and careful planning. Each trip is designed to try and find fossils that will shed new light on particular research questions. Often, scientists choose destinations for their field work in regions where fossils have already been found, but if not, geologic maps and satellite photos are used to identify areas where rocks of the right age and ancient environment are exposed on the surface.

To find fossils, paleontologists first carry out an operation called prospecting, which involves slowly hiking across ridges and through ravines, while keeping one's eyes focused on the ground in hopes of finding fragments of fossils weathering out on the surface. Commonly, one covers 5 to 10 miles in a day while prospecting. Once a fossil fragment is found, the collector brushes away the loose dirt on the surface to see whether more of the specimen is buried in the ground. If so, quarrying is initiated to collect the fossil. First, awls, rock hammers, chisels, and other tools are used to remove the rock covering the bones to see how much of the skeleton is present. As fossil bone is exposed, special glue is applied to the cracks and fractures to hold the fossil together.

Next, a trench is dug around the fossil so that it essentially sits on a low pedestal, still encased in its surrounding rock, or matrix. A covering of damp toilet paper is placed over the fossilized bones before a layer of plaster bandages is wrapped around the matrix to create a hard cast, just like a doctor does around a broken bone. Once the cast hardens, excavating the fossil is completed by snapping the matrix from the underlying rock, and the fossil in its cast is packed for shipment back to the Museum.

How are dinosaur fossils prepared in the laboratory?

Fossil preparators are highly skilled technicians who extract fossils from the surround matrix, use adhesives and consolidants to stabilize the fossils and prepare molds and casts of the specimens.

When fossils arrive from the field, they are encased in plaster jackets, as well as the rock, or matrix, which surrounds the fossils. Fossil preparation involves cutting open the plaster jacket and removing this matrix surrounding the fossil. The matrix may be soft and crumbly, when the sand or mud is poorly cemented together, or it can be extremely hard, when the sediments are well cemented. Accordingly, a wide variety of tools are required to remove the matrix and stabilize the fossil. Commonly, dental tools are used to carefully pick away sediment near the bone, along with custom-made needles composed of carbide steel. Formerly, chisels and hammers were used to remove blocks of matrix farther away from the bone, but recently, smaller mechanical tools have taken their place. These include small grinding wheels, miniature jackhammers called air scribes, and tiny sand-blasters powered by compressed air.

Preparators often use these tools while examining the fossil through a precision microscope under high-quality lighting to make sure delicate features on the fossils are not damaged. They carefully select the materials used to strengthen or repair specimens. Adhesives, glues, and fillers must stand the test of time and not become brittle or discolored, just like the materials used to conserve works of art. The types of materials used are recorded in order to aid future preparators should additional preparation or repair be required.