The Iguanodon bernissartensis was about 33 feet long and weighed about 3 and a half tons and are believed to have lived during the early Cretaceous period, about 146 million years ago. But, the coal is believed to have been formed during the Carboniferous period ending about 290 mya. The explanation given is a ravine was formed in the coal seam from erosion due to subterranean dissolution of anhydrites. A carnivore, or carnivores, attacked a herd of Iguanodons as they were eating near the ravine and the herd fled and fell into the ravine. That theory was probably based on another fossil found at Bernissart, fragments of a predatory dinosaur within the bed where Iguanodon's were found.
"Erosional processes had cut right through the ancient coal deposit. It was into this deep ravine that no fewer than thirty-one adult dinosaurs of the genus Iguanodon had fallen in quick succession, lodging at various levels on its lower slopes. With the passage of time the ravine underwent numerous floods and was gradually filled with Cretaceous muds that buried the dinosaur skeletons and in time completely filled the ravine."
Besides coal there is also chalk, limestone, clay, layers of silex nodules and conglomerates at Bernissart. Chalk forms under relatively deep marine conditions from the accumulation of microfossils called coccoliths which come from coccolithophores; single-celled algae, protests and phytoplankton and are exclusively marine organisms. The primary source of calcite in limestone is usually marine organisms. In the Bernissart mines there crans, sinkholes and pockets, often found inside coal layers composed of sedimentary rock but the Iguanodons were found in different layers of a cran. The crans of Bernissart yielded not only fossil of iguanodons but also some a dwarf crocodiles, coprolites, thousands of fish and many plant fossils. (2)
What eroded the solidified coal and why it happened only in certain areas isn't explained sufficiently. On the page on fascinatingearth.com it says the mine was mapped meticulously but according to a book called 'Engineering Geology for Infrastructure Planning in Europe' there was plenty of information available about the areas which had been mined but knowledge of the sinkhole and the region above it was very poor (3).
In a blog called "New light on Iguanodon" the author says there's no evidence that herds of animals plunged into a ravine. There is no name given for the author but it is taken from "Dinosaurs: A Very Short Introduction" by David Norman.
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.
According to The Proceedings of the Dorset Natural History and Antiquarian Field Club published in the late nineteenth century the depresstion had been a lake during the Wealden period and dinosaurs and other large animals would sometimes be engulfed in floods and drown (4). David Norman doesn't believe the Iguanodons were trapped alive in fissures but a series of catastrophic flash floods is responsible for the fossilized Iguanadons at Bernissart (5, 6). The mines there are said to be pock-marked with crans. Unless proven otherwise it's more reasonable to believe that solidified coal wasn't worn away in a large area and smaller areas but massive amounts of sediment were deposited on top of vegetation, the weight forced them downward to the bottom of the plant material while smaller sedimentary deposits sank to various depths forming crans, or pockets, in what became a coal mine.
1. Dinosaur Impressions by Philippe Taquet, Kevin Padian, pp. 23, 37; see also http://en.wikipedia.org/wiki/Iguanodon
3. Engineering Geology for Infrastructure Planning in Europe by Henri Robert George, 2004, p. 359
4. Proceedings of the Dorset Natural History and Antiquarian Field Club, 1896, p. 118
5. Terrestrial Ecosystems Through Time by Anna K. Behrensmeyer , p. 63
6. Ibid, p. 74