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The importance of plankton and nekton distributions in Ordovician palaeogeographical reconstructions

¹ Institut für Paläontologie, Universität Erlangen. Loewenichstraße 28, D-91054 Erlangen, Germany; and Laboratoire de Paléontologie et de Paléogéo-graphie du Paléozoïque, UMR 8014 CNRS, SN5, Université de Lille 1, F-59655 Villeneuve d’Ascq cedex, France, E-mail: Thomas.Servais{at}univ-lille1.fr. (Correspondant)
² Laboratoire de Paléontologie et de Paléogéographie du Paléozoïque, UMR 8014 CNRS, SN5, Université de Lille 1, F-59655 Villeneuve d’Ascq cedex, France. E-mail : Alain.Blieck{at}univ-lille1.fr ; Martial.Caridroit{at}univ-lille1.fr.
³ Nanjing Institute of Geology and Palaeontology, Academia Sinica, 39 East Beijing Road, 210008 Nanjing. China. E-mail: xuchen{at}jlonline.com
⁴ Géosciences Rennes, UMR 6118 CNRS, Université de Rennes 1, Campus de Beaulieu, F-35042 Rennes cedex, France.E-mail : Florentin.Paris{at}univ-rennes1.fr
⁵ Departamento de Paleontología Invertebrados, Museo de la Plata, Paseo del Bosque, La Plata 1900, Argentina. E-mail: tortello{at}infovia.com.ar

Trilobites and brachiopods are the two main fossil groups that allowed construction of the first palaeogeographical maps for the early Palaeozoic. Together with the bivalves and ostracodes, the benthic elements of these fossil groups have proved to be of great palaeobiogeographical importance. For this reason, these groups are usually considered to be ‘better’ fossils for inferring Ordovician palaeogeography. The present study indicates that planktic and nektic fossil groups should not be neglected in such palaeobiogeographical studies. The plotting on a palaeogeographical reconstruction for the Arenig (Lower Ordovician, – 480 Ma) of some planktic (acritarchs, chitinozoans) and nektic (vertebrates, pelagic trilobites) fossil groups indicates that their distribution appears in part surprisingly similar to that of the benthic trilobite faunas that are considered to display the greatest provincialism. For example, the distribution of the ‘peri-Gondwanan’ acritarch province including Arbusculidium filamentosum, Coryphidium and Striatotheca, and the distribution of the Eremochitina brevis chitinozoan assemblage are almost identical to the palaeogeographical distribution of the Calymenacean-Dalmanitacean trilobite fauna. A review of the different planktic and nektic fossil groups also indicates that it is very important to carefully select ‘good’ palaeogeographical indicators, in most cases from a large number of taxa. It appears that almost all fossil groups include some ‘good’ palaeobiogeographical ‘markers’. Therefore it is important to search for ‘better’ taxa within each fossil group, instead of looking only for the ‘better’ fossil groups as a whole.

Key Words: Acritarchs • Chitinozoans • Graptolites • Trilobites • Radiolarians • Vertebrates • Ordovician • Palaeobiogeography