What Is a Fossil Jacket? Wrapping a Bone in Plaster Before Moving It

The procedure has changed remarkably little. The exposed surface of the specimen is cleaned and stabilised with consolidant, then covered with a separating layer, traditionally wet paper or tissue and now frequently aluminium foil, so that the plaster does not bond directly to the bone. The surrounding rock is then trenched away, undercutting the specimen so that it sits on a pedestal attached to the ground only at its base. Strips of burlap soaked in plaster are laid over the exposed top and sides in overlapping layers, with wooden or metal splints incorporated for large blocks. Once the plaster has set, the pedestal is cut through, the whole block is rolled over, and the underside is jacketed in the same way. The result is a sealed container holding the specimen in the position it was found, which is what allows it to travel. Large jackets have required cranes, helicopters and in one case a tracked vehicle to move.

The jacket travels with information that cannot be recovered afterwards and which determines what the specimen is worth:

• •Precise location, now recorded by satellite positioning, since a fossil without a location cannot be tied to a rock layer and therefore has no age
• •Stratigraphic position, meaning exactly which bed it came from within a sequence, which is what dates it
• •Orientation, marked directly on the jacket, so that the original arrangement of bones can be reconstructed after preparation
• •Photographs and maps of the site before and during excavation, which are the only record of how elements lay relative to each other
• •Sediment samples, which record the environment and can contain pollen, microfossils and chemical evidence
• •Associated material including other specimens, plants and trace fossils, which supply the ecological context

Collecting is faster than preparing, and the gap has accumulated into a substantial backlog. Institutions hold jackets collected decades ago that have never been opened, and the reasons are resource rather than neglect, since preparation requires trained staff and thousands of hours per large specimen while collecting requires a field season. That backlog is not entirely a problem, since an unopened jacket is a sealed and stable container and the specimen inside is arguably safer than a prepared one on a shelf, and opening it later means opening it with better techniques. Several notable specimens have been described from material collected generations earlier, and computed tomography now permits a jacket to be scanned before opening, which reveals what is inside and allows institutions to prioritise. The practice of leaving some material deliberately unprepared for future methods has become explicit, which converts the backlog from an embarrassment into a policy.

The technique emerged during the intense and acrimonious American fossil collecting of the 1870s, when Edward Drinker Cope and Othniel Charles Marsh were competing to describe new species and their field crews were shipping material east in quantity. Earlier attempts to move fossils had produced a great deal of damage, and the plaster jacket, credited to workers in Cope's employ, solved the problem well enough to become standard immediately and to remain so. That period, generally called the bone wars, produced an enormous number of described species alongside rushed work, mislabelled material, sabotage of sites and specimens damaged or destroyed to prevent the rival obtaining them, and untangling its taxonomic legacy occupied the following century. The jacket is the most durable positive legacy of it, and the method is now used for anything fragile that must be lifted, including archaeological burials, mosaics and blocks of sediment removed intact for laboratory excavation.

The surrounding rock supports a fossil, so removing it without a rigid shell destroys the specimen, which plaster and burlap over a separating layer prevents. The block is undercut to a pedestal, jacketed on top, rolled and jacketed underneath. Location and stratigraphic position recorded in the field cannot be recovered later and are what give the fossil an age. Many jackets remain unopened, which is increasingly deliberate.