Bernissart a ravine where Iguanodon perished

Some of the earliest work at Bernissart focused on the extraordinary circumstances of the original discovery. The dinosaurs had been unearthed in a coal mine at depths of between 356 and 322 metres below the surface (Figure 18). This was unexpected, as the coal seams being excavated were known to be Palaeozoic in age and dinosaurs are of course unknown in rocks of such antiquity. However, the Iguanodon skeletons were not found in the coal seams themselves, but in a pocket of shale of Cretaceous age that cut across the more ancient coal-bearing rocks. Mining geologists had a commercial interest in discovering the extent of these clays, and the degree to which they might affect coal extraction, so they began mapping the area.

Cross sections of the mine, created during these geological investigations, suggested that the horizontal layers of Palaeozoic rocks (with their valuable coal seams) were occasionally cut through very steeply by beds of Mesozoic shale (finely laminated clays). The cross sections gave the first impression of steep-sided ravines cut into the ancient rocks, and formed the basis for a graphic and rather M appealing notion that the Bernissart dinosaurs represented a herd | that had tumbled to their deaths (Figure 18). Dollo, himself no £ geologist, was more inclined to the idea that these dinosaurs had lived, and died, in a narrow gorge. However, the more dramatic story had the greater impact, and was further embellished by suggestions that they had been stampeded into the ravine by huge predatory dinosaurs (megalosaurs), or by some freak event such as a forest fire. This was not entirely wishful thinking: extremely rare fragments of a large predatory dinosaur were discovered within the Iguanodon-bearing beds; and charcoal-like lumps of coal were recovered from some of the rubble-like deposits found in the region between the coal-bearing rocks and the dinosaur-bearing shaly beds.

The discoveries at Bernissart presented a huge logistic challenge in the 1870s and early 1880s. Complete skeletons of dinosaurs measuring up to 11 metres in length had been discovered at the bottom of a deep mine; they were the focus of worldwide interest at the time, but how were they to be excavated and studied?

Bernissart Iguanodon
18. Geological section of the Bernissart mine

A cooperative venture was arranged between the Belgian government, funding the scientists and technicians of the Royal Natural History Museum in Brussels, and the miners and engineers at the colliery in Bernissart. Each skeleton was carefully exposed and its position in the mine recorded systematically on plan diagrams. Every skeleton was divided into manageable blocks approximately 1 metre square. Each block, protected by a jacket of plaster of Paris, was carefully numbered and recorded on plan drawings (Figure 19) before being lifted and transported to Brussels.

Back in Brussels, the blocks were reassembled from the records, rather like a gigantic jigsaw puzzle. The plaster was painstakingly removed to reveal the bones of each skeleton. At this point an artist, Gustave Lavalette, specially commissioned for the project, drew the skeleton in its death pose before any further preparation or extraction was undertaken (Figure 20). Some skeletons were completely extracted from the shale and mounted to create a

Dinos Bernissart
19. Plan diagram of an excavated Iguanodon skeleton from Bernissart

magnificent display that can be seen to this day at the (renamed) Royal Institute of Natural Sciences, in Pare Leopold, Brussels. Other skeletons were cleared of the shale matrix on one side only and arranged in their burial pose on wooden scaffolding supporting vast banks of plaster. This display mimics their entombed positions when they were first discovered in the mine at Bernissart.

The original plans of each excavation, and some crude geological sections and sketches of the discoveries, are preserved in the archives of the Royal Institute in Brussels. This information has been 'mined', this time for clues concerning the geological nature of the dinosaur burial site.

The geology of the coal-mining area of the Mons Basin, in which lies the village of Bernissart, had been the subject of study before dinosaurs were ever discovered. A major review in 1870 pointed out f g

20. Lavalette's drawing of the Iguanodon skeleton seen in Figure 19

that the coal-bearing strata of the Mons Basin were pock-marked by 'cran' (naturally formed subterranean pits). Each 'cran' was of limited extent and filled with shales. It was concluded that these had formed by the dissolution of Palaeozoic rocks deep underground. The roofs of such caverns collapse periodically under the sheer weight of the overlying rocks, so the spaces become filled with whatever lies above: in this case soft clays or shales. The collapse of such sediments had been recorded locally in the Mons area as rather alarming, earthquake-like shocks. By amazing coincidence, a minor 'earthquake' of this type took place while the dinosaurs were being excavated in August 1878 at Bernissart. Minor collapses in the galleries were noted, as well as flooding, but the miners and scientists were soon able to resume their work once the flood water had been pumped out.

Despite all the local geological knowledge, it is very curious that the M scientists from the Museum in Brussels incorrectly interpreted the | geological nature of the 'cran' at Bernissart. The mining engineers £ produced crude geological sections from the tunnels that yielded the dinosaurs. These showed that immediately beyond the coal-bearing seams there was a section of 10-11 metres of breccia (broken beds containing irregular blocks of limestone and coal mixed with silt and clay, the 'collapsed coal-bearing rocks' of Figure 18) before entering steeply dipping, but more regularly stratified, shales that yielded the fossils. Toward the middle of the 'cran' the clay beds were horizontally bedded, and as the tunnel approached the opposite side of the 'cran' the beds once again became steeply tilted in the opposite direction before passing again into a brecciated region and finally re-entering the coal-bearing deposits. The symmetry of the geology across the 'cran' is exactly what would be expected if overlying sediments had slumped into a large cavity.

The sediments in which the dinosaurs are embedded also directly contradict the ravine or river-valley interpretations. Finely stratified shales containing the fossils are normally deposited in low-energy, relatively shallow-water environments, probably equivalent to a large lake or lagoon. There is simply no evidence for catastrophic deaths caused by herds of animals plunging into a ravine. In fact, the dinosaur skeletons were found in separate layers of sediment (along with fish, crocodiles, turtles, thousands of leaf impressions, and even rare insect fragments), proving that they definitely did not all die at the same time and therefore could never have been part of a single herd of animals.

Study of the orientation of the fossil skeletons within the mine suggests that dinosaur carcasses were washed into the burial area on separate occasions and from different directions. It was as if the direction of flow of the river that carried their carcasses had changed from time to time, exactly as happens in large, slow-moving river systems today.

So, as early as the 1870s, it was clearly understood that there were neither 'ravines' nor 'river valleys' in which the dinosaurs at Bernissart might have perished. It is fascinating how the dramatic discovery of dinosaurs at Bernissart seems to have demanded an equally dramatic explanation for their deaths, and that such fantasies were uncritically adopted even though they flew in the face of the scientific evidence available at the time.

The image of Iguanodon as a gigantic kangaroo-style creature has become iconic because of the generous distribution of full-sized skeletal casts to many museums around the world. But does the evidence for this restoration survive further scrutiny?

Was this article helpful?

+1 0

Post a comment