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SOME OF THE DEEP-SEA FAUNA IS ANCIENT

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Decreasing deep-sea floor temperatures during the mid Cainozoic, and a presumed widespread disoxia in the deep sea prior to this era has lead many authors to suggest that the deep-sea fauna has accumulated during the last 30-40 mybp only. This hypothesis argues for extinction and replacement of earlier faunas. Some taxa, such as the Ostracoda, show extensive taxonomic replacement during the Miocene that is correlated with declining sea floor temperatures. A recent evaluation of the deep Atlantic distribution of major isopod clades, however, demonstrated that two different historical patterns are present. One pattern (''Flabellifera'') conforms to a relatively recent Cainozoic and ongoing colonization of the deep sea, with relative impoverishment of species with depth. The other pattern (Asellota) is one that is rich in deep-sea species, and has a high level of endemic morphological diversity, suggesting a long period of evolution in isolation. Glaciation during the late Palaeozoic and an early phylogenetic origination of the Asellota support the hypothesis that these isopods colonized the deep sea prior to the disoxia events during the Mesozoic and the early Cainozoic. The Mesozoic deep sea is unlikely to have become completely anoxic globally owing to vertical halothermal circulation at low latitudes, allowing the possibility of oxygenated refuges in deep water. Elements of the Palaeozoic fauna, therefore, may have persisted through the Mesozoic without representation in marine shallow waters. Within the isopods, these Palaeozoic relicts have taken two adaptive directions. In freshwater, the Phreatoicidea show morphological stasis, with modern taxa resembling fossils from Carboniferous and Triassic eras. The Asellota, on the other hand, have evolved into a many different morphological types and a huge number of species globally.

La diminution de la temperature dans l'ocean profond pendant le Cenozoique ainsi qu'un presume appauvrissement en oxygene apparu avant cette ere a conduit beaucoup d'auteurs a suggerer que la faune profonde ne s'est accumulee que pendant les derniers 30-40 mybp seulement. Cette hypothese plaide pour l'extinction et le remplacement des faunes anterieures. Quelques taxa, tels les Ostracoda, montrent un remplacement taxonomique important pendant le Miocene qui est correle avec les temperatures en baisse de l'ocean profond. Une evaluation recente de la distribution atlantique profonde des clades principaux d'Isopodes montre cependant que deux configurations historiques differentes sont presentes. Une configuration (''Flabellifera'') se conforme a une colonisation Cenozoique recente et continue de la mer profonde, avec un appauvrissement relatif des especes avec la profondeurs. L'autre configuration (Asellota) traduit une grande richesse des especes profondes, et un niveau eleve de diversite morphologique endemique, suggerant une longue periode d'evolution en isolement. Les Glaciations a la fin du Paleozoique et une origine phylogenetique plus ancienne des Asellota supportent l'hypothese que ces Isopodes ont colonises la mer profonde avant les evenements d'appauvrissement en oxygene pendant le Mesozoique et le Cenozoique ancien. La mer profonde du Mesozoique est peu susceptible d'etre devenue completement anoxique en raison, globalement, de la circulation verticale halothermique sous les basses latitudes, ce qui avait permis la possibilite de refuges oxygenes dans l'eau profonde. Les elements de la faune du Paleozoique ont pu donc persister lors du Mesozoique sans representants dans les eaux peu profondes marines. Chez les Isopodes, ces relictes du Pal eozoique ont pris deux directions adaptatives. Dans les eaux douces, les Phreatoicidea montrent une stase morphologique avec des taxa modernes ressemblant a des fossiles du Carbonifere et/ou du Trias. Les Asellota d'autre part, ont evolue en beaucoup de types morphologiques differents et, globalement, en un

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/content/journals/10.1163/156854099503915
1999-11-01
2015-08-05

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