Excavation, preservation and assembly of the Iguanodon skeletons

A vast programme began shortly after their discovery aimed at excavating the bones, which were highly fragile owing to the pyrite they contained, and protecting them from the inevitable deterioration associated with exposure to ambient air and variations in relative humidity. The fossils had been buried for millions of years in their clay coating, which created anaerobic conditions leading to their remarkable preservation.  We now know that when pyrite comes into contact with ambient air, it changes and transforms into other minerals which cause the bones to shatter as they grow and swell. The excavation techniques used at the time are still used today. The bones were removed after having been encapsulated in carefully numbered blocks of plaster so that the entire skeleton could subsequently be reconstructed. After extricating the bones by removing the plaster and clay surrounding them, the bones were coated in strong glue to prevent them from crumbling. The pyrite contained in the bones’ crevices was systematically removed and replaced by carton-pierre (mixture of chalk, hide glue and paper). The skeletons were then assembled in a supposed life-like position and exhibited to the public.

Years of exposure to the open air caused significant damage and a new method to protect the bones was developed in the 1930s. All the specimens were dismantled and dipped into a mixture of alcohol and shellac (a varnish naturally secreted by cochineals that live as parasites on tropical tree species in India and Thailand). Two glass cages were built around the Iguanodons to maintain a constant temperature and level of relative humidity. In 2003, all the skeletons were dismantled for restoration. The pyrite residues were carefully removed, the bones were impregnated with synthetic resin, the breaks were stuck back together thanks to strong modern adhesives and the fissures were filled with a hardening paste. The skeletons were finally placed in new glass cages.

How did these more recent layers of clay come to be in the older coal fields?

Sinkholes are natural shafts that occur when soluble rocks in the carboniferous limestone located deep down are dissolved. These rocks were dissolved during the Cretaceous by underground water. The empty spaces created in the rock caused a collapse spreading from bottom to top. The upper, more recent strata (shale, coalfields), collapsed into the shaft in the form of an opening. The superficial soft rocks (sand and clay) were gradually drawn into the shaft leading to the creation of a marsh on the surface.

The formation of this sinkhole, in the Wealden period, lies at the origin of the Bernissart fossil deposit. At the beginning of the Cretaceous, many Iguanodons undoubtedly lived in the Mons basin but only one of the 117 shafts in the basin’s coalfield yielded Iguanodons. The site was probably located at one of the deepest levels in the region and coincided with the future position of the natural shaft. The Iguanodon corpses slid down here and piled up. Once the marsh had been formed, the dinosaurs probably got stuck in it and slowly disappeared with the clay as they sunk, thus escaping causes of destruction, such as erosion, which is what caused the remains of the vertebrae hidden in the sediments to disappear. After the shaft ceased to function, it was covered by other types of soil that weren’t sucked under.