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155 14 Geological Model and Cyclic Mass Mortality Scenarios for the Lower Cretaceous Bernissart Iguanodon Bonebeds Jean-Marc Baele*, Pascal Godefroit, Paul Spagna, and Christian Dupuis The Iguanodon Sinkhole at Bernissart (Belgium) is an exceptional fossil deposit as a result of the quantity and preservation quality of Cretaceous basal Iguanodontia found by coal mine workers in 1878. Efforts to unravel the processes that caused the accumulation and preservation of many dinosaurs , along with other taxa, are here based on a new geological model that relies on several discrete, continuous bonebeds. Several taphonomic scenarios are proposed and discussed within the specific geological and environmental specificities of the so-called Lower Cretaceous Bernissart paleolake. On the basis of sedimentological and taphonomic evidence, attrition and obrution processes appear less likely than mass death by drowning and/or intoxication. Contamination of the aquatic environment by sulfate-rich brines related to deep solution–collapse processes could support the hypothesis of intoxication by H2 S or biological toxins as a direct or indirect lethal agent in a context of seasonally shrinking water. The discovery, from 1878 until 1881, of about 40 skeletons of iguanodontid dinosaurs in Lower Cretaceous (“Wealden”) deposits at Bernissart (Belgium ) is exceptional in the history of paleontology, and immediately it intrigued geologists. Indeed, 117 similar sinkholes are known within Mississippian deposits of the Mons Basin, and only one has yielded dinosaur remains. Outside the Iguanodon Sinkhole, only two dinosaur bones have been identified to date in Wealden outcrops from the Mons Basin (see Chapter 13 in this book). Since 1878, geologists have tried to explain the processes leading to the formation of this unique accumulation. On the basis of geological sections of the site, Dupont (1878, 1897) hypothesized that the Bernissart environment back to Lower Cretaceous times was a narrow gorge (crevasse) in which iguanodontids lived, died, and were periodically buried during flooding episodes.Soon, Cornet and Schmitz (1898) and Cornet (1927) proposed an alternative explanation. They believed that the accumulation of numerous iguanodontids skeleton at Bernissart was clearly a slow, attritional process, resulting from the sliding or stacking of carcasses of dead animals in a subsiding lake. Because the fossiliferous layers were trapped within a sinkhole, they could escape the erosion that removed the coeval surrounding layers in other places in the Mons Basin (Bultynck, 1989). Introduction Baele et al. 156 Louis Dollo, the original describer of all the terrestrial vertebrates from Bernissart in articles published between 1882 and 1923, proposed several conflicting hypotheses to explain the accumulation of iguanodontid skeletons at Bernissart (see reviews in Casier, 1960, and Bultynck, 1989). After noting that most of the dinosaurs were old-age individuals, Dollo proposed that Bernissart was some kind of dinosaur graveyard by Early Cretaceous times, or that flash floods selectively killed older and less agile animals. He also suggested that some iguanodontid specimens from Bernissart showed evidence of a violent death, maybe through combat. Casier (1960) provided two further explanations for the mass burial at Bernissart. On the basis of the supposition that iguanodontids usually retreated into water to escape from predators or other startling events, he first supposed that some dinosaurs might have inadvertently slipped into the steep-sided marshy depression at Bernissart. In a second hypothesis (the “Hippopotamus” hypothesis), Casier assumed that iguanodontids were amphibious and therefore dependent on a permanent body of water; a period of low rainfall may have led to these animals becoming mired in the muddy ooze around shrinking water holes. More recently, Norman (1987) compared Bernissart with another Early Cretaceous iguanodontid bonebed in Nehden, Germany, and refuted a mass kill scenarios on the basis of a more detailed taphonomic analysis. Bultynck (1989) also spoke for an attritional scenario, agreeing with the sinkhole environment hypothesis previously developed by Cornet and Schmitz (1898) and Cornet (1927). Here we present a refined geological model for the Bernissart Iguanodon deposit. This model is then used as a framework for evaluating different taphonomic scenarios. We placed emphasis on the role of site-specific geological factors such as subsidence due to solution collapse deep underground and possible upwelling of sulfate-rich brines. Institutional abbreviation. RBINS, Royal Belgian Institute of Natural Sciences, Brussels, Belgium. Historical Discovery and Fossil Assemblage The iguanodon bonebeds were discovered in April 1878 by coal miners excavating a driftstone gallery at level −322 m approximately 250 m south– southeast from the Sainte-Barbe shaft (see Chapter 1 in this book for details on the discovery). Earlier in March, the gallery went through 10 m of faulted and...

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