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Evolution of Late Cretaceous Radiolaria and the Caribbean Large Igneous Province

A revised Late Cretaceous radiolarian biochronology is essential for more accurate and non-controversial dating of many radiolarian-rich siliceous sediments associated with effusive phases of the Caribbean Large Igneous province (CLIP, Fig. 1), as well as with, partly coeval, arc-derived tuffaceous and volcano-clastic sediments. These sediments usually lack planktonic foraminifera. Hence, the reconstruction of the CLIP evolution is until now largely based on 39Ar/40Ar-ages, often contradicted by fossil ages, when available. Multicyrtid Nassellaria (Fig. 2) are commonly used in the biostratigraphy of the Cretaceous. However, many species have holotypes defined by drawings from thin sections in classical work published at the turn of the 19th to the 20th century, or by transmitted light images in 1950-1985. The comparison of the now most widely used SEM images with this earlier work is often ambiguous. As a consequence, the “semantics” of many species has broadened by inclusion of morphotypes that more or less loosely compare with the original descriptions. Yet, the broader the species concept, the longer tends to be the species range. To obtain a better resolution for a global, low-latitude, Late Cretaceous radiolarian biochronology, we are using the following strategies: 1. Obtain topotypic material from DSDP-ODP and land sites, where Late Cretaceous radiolarian taxa were described. 2. Compare SEM and transmitted light illustrations of topotypic material with original descriptions. 3. Use morphometrics to try to separate closely related forms in well-preserved material. 4. Construct evolutionary lineages for selected taxa. So far we have been working on the following taxa (Fig.2): “Dictyomitra” formosa group, incuding “D”. torquata, “D.” duodecimcostata, “D”. koslovae and other similar taxa. Dictyomitra multicostata, D. densicostata, and other similar taxa. “Pseudo-dictyomitra” pseudomacrocephala group and related “Pseudodictyomitra” spp. We define several new genera and species. Several local radiolarian biostratigraphies for the Late Cretaceous have been proposed in the last 40 years e.g. Pesagno (1976), Taketani (1986) and the low-latitude synthesis by Sanfilippo and Riedel (1985), which was based on DSDP Sites and land samples. These zonations have been used for a long time, although zonal marker taxa used are now known to have longer ranges. In addition, many species co-occurrences observed in samples from Central America, DSDP Sites and Southern Europe are incompatible with the ranges expressed in the fore-mentioned zonations. Several ranges are also shorter than those expressed in earlier publications. Until now, we have used the stacked chronostratigraphic ranges of several authors with the hope to get close to the “full” global range of each taxon. To assess the age of a sample we have used the concurrent “full” range of the species recorded in each sample (e.g. (Baumgartner 1984, Bandini et al. 2006, 2008, Denyer & Baumgartner 2006, etc.). This method is, however, very unsatisfactory, because of the important differences in the quality of calibration to other fossil groups and ultimately to the time scale. To overcome this situation, we propose to create a new, Late Cretaceous radiolarian biochronology based on Unitary Associations, a concept that has proven to be specially adapted to both temporally and spatially incomplete radiolarian fossil record (Baumgartner, 1984 ; Baumgartner et al., 1995 ; Carter et al., 1998 ; O’Dogherty, 1994). At present, we work with a database consisting of >12, 000, mostly unpublished, SEM images from some hundred Upper Cretaceous samples from low latitude DSDP/ODP Sites and land sections in Central and South America, the Caribbean and the Mediterranean area.

Late Cretaceous, Nassellaria, Caribbean Large Igneous Province

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