Found by coal miners in 1878, the Early Cretaceous Iguanodon Sinkhole – or natural pit - at Bernissart (Belgium) is a unique fossil deposit owing to the quantity and preservation quality of Iguanodon (Dinosauria: Ornithischia) skeletons along with other taxa. Although this fossil locality has already been the subject of numerous scientific papers and monographies since its discovery 136 years ago, the processes leading to the local accumulation of so many complete skeletons remain completely unexplained. The aim of the present project is to refine an integrated model for the Bernissart Iguanodon locality that will be used as a framework for evaluating different taphonomic scenarios.

Several information sources will be re-examined at the occasion of this project: the Bernissart Iguanodons themselves, associated fossils, original maps and drawings made at the occasion of the excavations, and core data from the Ber 3 borehole that was drilled within the Iguanodon Sinkhole in 2002. Advanced non-invasive technologies will be used in order to preserve the integrity of these precious collection: computed 3D mapping of excavation area, core logger at large scale and microXRF at higher resolution for key intervals on the core drilled through the Bernissart Wealden deposits, analyses of multiple tracers (isotopes, trace elements, molecular biomarkers, etc.) from the core sediments, palaeohistological (“fossilopsies”), molecular palaeontology, and biogeochemical studies of the Iguanodon bones.

A series of taphonomic elements ('indices') will be collected in order to reconstruct the circumstances of the iguanodon demise. The analysis of the “crime scene” will clarify the distribution of the skeletons within the sinkhole. Forensic analyses of the Bernissart Iguanodons will help in reconstructing the structure of their population, will reveal eventual traces of predation and scavenging, and will bring new information about their diet, and paleoecology. Collateral victims, particularly fresh-water fishes, will also be studied for reconstructing the evolution of the ecological conditions reigning in the Bernissart lake/swamp. “Witness” testimonies will be cross-checked in order to clarify the relationships between the Bernissart Iguanodons and their environment: isotopic work (∂¹³C ∂¹⁵N) on organic matter and palaeobotanical changes across the bone layers; core logger at large scale & microXRF at higher resolution for key intervals …

In the light of these taphonomic elements, taphonomic models that potentially explain such a local concentration of skeletons in a limited area, will be evaluated: (1) "natural death" scenario, resulting from normal biological activity under normal circumstances, but requiring efficient and exceptional processes of concentration and conservation; (2) "accidental" scenario resulting from the quick burying of an Iguanodon population resulting from catastrophic sedimentary events; (3) "mass or serial murder" scenario, caused either by herding or abnormal behaviour under perilous situation. In this case, the collected taphonomic elements will be used to reconstruct the causes of death (or recovering the "murder weapons": exhaustion/starvation, miring, drowning, poisoning/suffocation, mass predation...) and then to propose a profile of the mass or serial killer (predators, wildfires, floods, droughts, H₂S from deep brines, cyanobacteria,...).

Since 1878, geologists and palaeontologists have tried to explain the processes leading to the formation of the unique accumulation of more than 30 sub-complete and articulated dinosaur skeletons within the Early Cretaceous Iguanodon Sinkhole at Bernissart. About 117 similar sinkholes are known within Carboniferous basement of the Mons Basin, however, only one has yielded dinosaur remains (Godefroit et al., 2012). On the basis of geological sections of the site, Dupont (1878, 1897) hypothesized that the Bernissart locality represented a narrow gorge (‘crevasse’) in which iguanodontids lived, died, and were periodically buried during flooding episodes. Soon, Cornet and Schmitz (1898) assessed that the accumulation of numerous iguanodontids skeleton at Bernissart was clearly a slow attritional process, resulting from sliding / stacking of carcasses of dead animals in a subsiding lake. Dollo (1923) also proposed several possible scenarios for the Iguanodon accumulation at Bernissart: Bernissart was some kind of dinosaur graveyard by Early Cretaceous time, or flashfloods selectively killed the older and less agile animals. He also suggested that some iguanodontids specimens from Bernissart showed evidence of a violent death, maybe through combats.

Nevertheless, all these hypotheses have never been tested so far and therefore remain 'nice stories'. The aim of the present project is to produce an integrated formation-model for the iguanodon-rich deposit of Bernissart. A series of taphonomic elements ('indices'), collected from different sources (fossils, iconographic documents, drilling and core data), will be investigated to reconstruct the circumstances of the iguanodon demise:

• The analysis of the 'crime scene'. The distribution of the skeletons within the sinkhole is particularly important. Are the skeletons randomly distributed and oriented within the sinkhole or are they organized into discrete bone-beds with or without preferential orientations?
• Forensic analyses of the victims. Structure of the Bernissart Iguanodon population through palaeohistological studies; traces of predation, scavenging, trampling; analyses of multiple tracers (stable isotopes, trace elements, etc.) for reconstructing their diet, and palaeoecology...
• Looking for collateral victims. The Iguanodons were not the only fossils found at Bernissart. The proposed taphonomic scenario explaining the local accumulation of iguanodontid skeletons must also take into consideration collateral victims. Particularly interesting is the abundant fish fauna, which will potentially provide key information about the evolution of the ecological conditions reigning in the Bernissart lake/swamp. So far no or little attention has been devoted to the other fossil present in the deposit. Investigating the cause of massive fish die-off could help understanding why many iguanodons accumulated at Bernissart.
• Cross-checking the witness testimonies. The relationships between the Bernissart Iguanodons and their environment must also be clarified. Large-scale palaeoecological reconstructions exist for whole Mons Basin (e.g. Spagna et al., 2012). We shall focus more on the local circumstances based on drilling and core data: isotopic work (∂¹³C ∂¹⁵N) on organic matter and palaeobotanical changes across the bone layers; core logger at large scale & microXRF at higher resolution for key intervals, …

Based on these taphonomic elements, three possible taphonomic models that potentially explain such a local concentration of skeletons in a restricted area will be evaluated:

• Passive attrition scenario ("natural death" scenario), resulting from normal biological activity under normal circumstances, but requiring efficient and exceptional processes of concentration and conservation. In this case, it will be necessary to understand the mechanisms leading to the local concentration of the skeletons, and consequently reconstruct the palaeoenvironments at very high-resolution.
• Obrution scenario ("accidental" scenario): quick burying of a population of Iguanodon resulting from catastrophic sedimentary events. The latter remain to be identified through further sedimentological investigations.
• Catastrophic scenario ("murder" scenario): mass death caused either by herding or abnormal behaviour under perilous situation. In this case, the collected taphonomic elements will be used to reconstruct the causes of death (or recovering the "murder weapons": exhaustion/starvation, miring, drowning, poisoning/suffocation, mass predation,...) and then to propose a profile of the mass or serial killer (predators, wildfires, floods, droughts, H₂S from deep brines, cyanobacteria,...).

Related publications

Baele, J. - M. , Godefroit, P. , Spagna, P. & Dupuis, C. . Bernissart Dinosaurs and Early Cretaceous Ecosystems (Godefroit, P. ) 35-42 (Indiana University Press, 2012).