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79 8 On the Age of the Bernissart Iguanodons Johan Yans*, Jean Dejax, and Johann Schnyder We summarize the studies dealing with the dating of the Bernissart iguanodons . Both palynology (especially pollens of angiosperm affinities, such as the biorecord Superret-croton and probably the paleotaxon Superretsubcrot ) and chemostratigraphy (carbon isotope composition of dispersed organic matter and fossil wood) have recently been applied to refine the age of the Iguanodon-bearing Wealden facies trapped in the Iguanodon Sinkhole at Bernissart (Sainte-Barbe Clays Formation). These studies suggest that this formation is late Late Barremian to earliest Aptian in age. Although numerous studies are dedicated to the faunal and floral content of the Iguanodon Sinkhole at Bernissart, the age of the Sainte-Barbe Clays Formation (Wealden facies) remained poorly constrained until recently. In the most recent synthesis on the Cretaceous of Belgium, Robaszynski et al. (2001) concluded that this formation was Late Jurassic to Early Cretaceous in age (161.2 to 99.6 Ma, according to Gradstein et al., 2004). Here we summarize the previous attempts to date the Bernissart iguanodons and discuss new datings on the basis of both palynology of the angiosperm pollen content and carbon isotope chemostratigraphy on organic matter. Palynology The present study is based on, first, samples collected at −322 m in the Iguanodon Sinkhole at Bernissart during the 1878–1881 excavations and stored in the collections of the Royal Belgian Institute of Natural Sciences of Brussels (Dejax et al., 2007a), and second, on the preliminary results of samples from the BER 3 borehole, drilled in 2002–2003 (Yans et al., 2005). Standard procedures, as described in Dejax et al. (2007a), were used to eliminate the carbonate and the insoluble fluorides in each sample and to concentrate the organic matter. The final residue, obtained through sieving on a 5-µm micromesh nylon sieve and centrifuging, was strewn on a coverslip by means of hydroxyethyl cellulose (Cellosize), dried, and mounted upside down with Canada balsam on a microscope slide. The observations and determinations noted herein are based on light microscopic examination, mainly using an interferential–differential contrast objective. The preparations were stored in the paleontological collections of the Royal Belgian Institute of Natural Sciences in Brussels. Introduction Material and Methods Yans et al. 80 Chemostratigraphy The carbon isotope ratios of bulk organic matter and isolated wood fragments were measured on 141 samples from the BER 3 borehole, collected between −315 and −265 m. Samples were prepared according to standard procedures, as described in Yans et al. (2010). Isotopic measurements were carried out on bulk samples with a Finnigan MAT Deltaplus mass spectrometer connected to a Thermo-Finnigan Flash EA1112 series microanalyzer at the Free University of Brussels by means of standard techniques. Up to three distinct measurements have been made for each sample. Reproducibility of standards is within ~0.2 ‰. Numerous various ages, based on different approaches, have been proposed for the dating of the Bernissart iguanodons (Yans et al., 2005). In 1900, Vandenbroeck compared the paleofloras from Bernissart and southeastern England and suggested a Late Jurassic age for the Bernissart locality. In 1954, Marlière proposed an age ranging from the Late Jurassic to the “Neocomian ,” also on the basis of the paleoflora. However, the same author subsequently revised this hypothesis and argued for an age more recent than the Valanginian–Hauterivian (Marlière, 1970). In 1975, Taquet compared the morphologies of different taxa within the Iguanodontidae and concluded that the Bernissart iguanodons are Barremian in age. On the basis of a comparative study of the lithology and the paleontological content of Bernissart and England, Allen (1955) suggested a Late Aptian–Early Albian age. Later, Allen and Wimbledon (1991) proposed a Hauterivian–Barremian age, and Pelzer and Wilde (1987) and Vakhrameev (1991) a “Neocomian” age. In a recent synthesis (Norman and Weishampel, 1990), the species Iguanodon bernissartensis (including I. orientalis, according to the recommendations of Norman, 1996) was suggested to range from the Valanginian to the Albian and Mantellisaurus atherfieldensis, from the Berriasian, to the Aptian. Taking into account all these previous studies, Robaszynski et al. (2001) concluded that the Iguanodon-bearing Wealden facies (Sainte-Barbe Clays Formation) were Late Jurassic to Early Cretaceous in age. Several previous studies have dealt with the palynology of the Wealden facies from the northern part of Europe, including the pioneering palynological works of Delcourt and Sprumont (1955; see references in Dejax et al., 2007a). The Wealden facies, encountered at −322 m depth in the Iguanodon Sinkhole at Bernissart (the level...

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