Saniwa est un genre éteint de lézard varanidé de l’Eocène européen et nord-américain et taxon frère du groupecouronne Varanus. Jusqu’à maintenant, seule une espèce, Saniwa orsmaelensis était rapportée en Europe, dans l’Eocène basal de Dormaal, Belgique. Cette espèce, originellement nommée par Louis Dollo il y a presqu’un siècle, est le plus ancien varanidé d’Europe. Malheureusement, le matériel diagnostique était limité à quelques vertèbres, décrites assez brièvement et non figurées, si l’on excepte une vertèbre dorsale désignée comme lectotype. Nous décrivons et illustrons ici de nouveaux spécimens de Dormaal ainsi que du Quesnoy, Bassin de Paris, France, incluant des restes crâniens (maxillaire, dentaires et pariétal), permettant de confirmer la validité de ce taxon européen. Ces nouveaux spécimens permettent en effet de nouvelles comparaisons avec l’espèce-type Saniwa ensidens, de l’Eocène moyen des formations de Bridger et de Green River, Wyoming, Etats-Unis et permettent d’amender la diagnose de S. orsmaelensis. La présence de S. orsmaelensis est restreinte à l’Eocène inférieur du Nord-Ouest de l’Europe et son origine géographique n’est pas encore certaine car Saniwa apparait simultanément en Amérique du Nord en Europe. La présence relativement brève des lézards varanidés dans le Paléogène Européen pourrait résulter des rapides changements environnementaux aux alentours du Paleocene Eocene Thermal Maximum qui ont permis de nombreux échanges fauniques dans l’hémisphère nord. Cependant, le sens de ces migrations n’est pas encore connu. Par ailleurs, les considérations paléogéographiques liées à la distribution du genre Saniwa suggèrent une origine asiatique bien qu’une origine africaine ne puisse être complètement exclue. Ce résumé est une contribution au projet réseau Belspo Brain BR/121/A3/PalEurAfrica financé par le Bureau de la Politique Scientifique Belge.
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RBINS Staff Publications 2019
The Paleocene-Eocene thermal maximum (PETM) is characterized by a worldwide 5-8 °C warming of Earth’s surface as well as the deep oceans, major global faunal and floral turnovers and large changes in ocean chemistry. In order to establish clear biogeographic patterns of how shallow benthic foraminiferal communities responded to these climate changes, we compare shallow marine ecosystems in three separated regions and provide a synthesis of the short-term biotic responses. These regions are located in Tunisia (Northern and Gafsa Basin), Egypt (Nile Basin) and the North Atlantic Coastal Plain (Salisbury Embayment, United States). In Egypt, widespread anoxia during PETM peak warming led to the collapse of Paleocene deep shelf communities and a basin-wide downslope migration of pioneering shallow water taxa (A. aegyptiacus) during the initial recovery phase. In the shallower Tunisian settings, the PETM is marked by dysoxia, increased water depth and an elevated sedimentation rate. This resulted in the migration of deeper-dwelling species (lagenid and buliminid fauna) at the onset of the PETM, replacing the former shallow water community. At the onset of the PETM in the North Atlantic Coastal Plain, deposition occurred in a basin-wide mud belt, inhabited by opportunistic deep shelf taxa. Increased eutrophication, high sedimentation rates and widespread hypoxia are linked to the establishment of a river-dominated shelf during the PETM. As a result, in shallow and deep shelf settings, diverse Paleocene assemblages were replaced by characteristic river-outflow assemblages (P. prima, A. acutus, T. selmensis), either by upslope migration or increased abundances of these background taxa. Due to the magnitude and tempo of global warming, the PETM exerted worldwide environmental stress on benthic foraminiferal communities, triggering prominent transient changes in population structure and biodiversity, yet the evolutionary impact was minor compared to the deep-sea extinction event. This implies the existence of refugia on the shelves. In general, stable latest Paleocene benthic foraminiferal assemblages were abruptly replaced by more stress-tolerant faunas, reflecting stressed dysoxic to anoxic eutrophic environments, due to higher nutrient delivery (increased runoff and upwelling) and stratification. These hypoxic conditions occurred in the early stages of the PETM continually or with high frequencies and evolved towards periodic (seasonal?) oxygen depletion during the latter stages of peak warming or initial recovery. The final recovery phase reflects a reoxygenation of the sea floor and a distinctive buliminid bloom (B. callahani) occurred at both sides of the Atlantic Ocean. These eutrophic conditions remained stable and continued in the aftermath of the PETM, yet the oxygenation of bottom waters became restored. The PETM sequences thus document a progression of environmental regimes that is somewhat similar in all studied settings, indicating a widespread mutual response to the massive injection of carbon dioxide at the onset of the PETM.
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