How Are Dinosaur Fossils Excavated? From a Bone in a Hillside to a Mounted Skeleton

Nobody digs at random. Bones survive only in sedimentary rock of the right age laid down in the right conditions, so the search begins with a geological map and a formation known to be the correct age and to have produced material before, which is why the same basins in Montana, Alberta, Mongolia, Patagonia and the Gobi appear repeatedly. Within a promising formation, prospecting is done on foot, walking slopes and gullies and looking for fragments weathering out, since erosion does the exposing and the field team follows it uphill to the source. Satellite imagery and drones now narrow the ground, and in some areas ground-penetrating radar helps, but the eye of someone who knows what a weathered bone chip looks like among a million rock chips remains the main instrument. A find is photographed, recorded with satellite positioning, and left in place while a decision is made about whether there is enough there to justify a season.

Once a specimen is judged worth taking, the work becomes an archaeological excavation with heavier tools. The overburden, the rock lying above the bone layer, is stripped with picks, shovels and sometimes a mechanical digger, stopping well short of the bones. From there the sequence is deliberate:

• •A grid is laid over the site and every element mapped in three dimensions before anything moves, since the arrangement of the bones carries information about how the animal died and was buried
• •The matrix, the rock around the bone, is removed with awls, chisels and brushes, leaving each bone standing on a pedestal of rock
• •Cracks and exposed surfaces are stabilised as they appear with a dilute consolidant, because a bone that has waited 70 million years can crumble in an afternoon of sun
• •Everything is photographed continuously and increasingly scanned photogrammetrically, so the quarry can be reconstructed digitally after it is destroyed
• •Small material is bagged with its position, and sediment is often screened for teeth, and for the bones of mammals, lizards and fish that establish the ecosystem

A large bone is never lifted bare. Once it stands on its pedestal, it is covered with a separating layer of damp paper or foil and then wrapped in strips of burlap soaked in plaster of Paris, with wooden splints or metal pipe added for rigidity in the way a broken limb is set, a technique invented in the 1870s and essentially unchanged. The jacket is allowed to harden, the pedestal is undercut until the block breaks free, the whole thing is rolled over, and the underside is jacketed too. Blocks routinely weigh hundreds of kilograms and sometimes several tonnes, which is the point at which a remote site needs a winch, a road cut or a helicopter, and where a large specimen can cost more to move than to find. Each jacket is labelled with the site, the specimen number and an arrow showing which way was up.

The field season is weeks; preparation is years. In the laboratory the jacket is opened and the rock removed grain by grain under a microscope with pneumatic air scribes, dental picks and fine abrasive blasting, at a rate that can be measured in square centimetres a day when the rock is hard and the bone fragile. Acid preparation is used where the surrounding rock is limestone and the bone is not, dissolving the matrix away over weeks while the exposed bone is protected with resin. Breaks are glued, missing areas are filled with a material deliberately distinguishable from the original, and everything done is recorded, because a future researcher needs to know what is bone and what is restoration. Computed tomography now supplements the physical work, letting a braincase be read without opening it and, in some cases, letting a specimen be studied and printed while the original stays in its rock. The published description that follows is what turns a pile of bones into knowledge, and a famous specimen may wait a decade for it.

Ownership decides more about a fossil's fate than geology does. In Mongolia, China, Brazil, Argentina and most of Europe, fossils are the property of the state wherever they are found, and exporting them is a crime, which is why several high-profile specimens have been repatriated after being sold abroad. In the United States, fossils on federal land require a permit and belong to the public, while fossils on private land belong to the landowner and can be sold, which has produced both a commercial industry that finds and prepares specimens no museum had the budget to collect and an auction market in which single skeletons have sold for tens of millions of dollars. Scientists object to the latter because a specimen in a private collection is not available for study or for checking a published claim, and journals generally refuse to describe material that is not held in a permanent public repository. Specimens collected without records are worse than useless, since a bone with no location and no stratigraphy has lost almost everything that made it evidence.

Finding a specimen starts with a geological map and ends with someone walking a slope looking for fragments weathering out of rock of the right age. The site is mapped in three dimensions before anything is lifted, the bones are pedestalled, stabilised and encased in burlap and plaster, and blocks weighing tonnes are winched or flown out. Preparation in the laboratory takes years rather than weeks, under a microscope with air scribes and acid, and what a fossil is worth scientifically depends on records of where it came from and on its ending up in a public collection.