The palaeopsychrosphere in the Devonian
Résumé
The interpretation of Palaeozoic marine benthonic ostracods of the Thuringian “Ecotype” or “Mega-assemblage” as indicative of a palaeopsychrosphere has been controversial. We review the evidence and conclude that the characteristics and distribution of these ostracods are consistent with the existence of deep, cold, well-oxygenated water masses, formed by high-latitude sinking of surface waters, in the Devonian oceans, comparable with those of the modern ocean that constitute the psychrosphere (waters below the thermocline with temperature <10 oC). We present a new palaeoceanographic model for the Frasnian-Famennian (Late Devonian) Kellwasser events that resulted in the extinction of 75% of marine ostracod taxa, mostly neritic or pelagic forms, while the deep-water Thuringian Mega- assemblage was relatively unaffected. We offer an explanation for the unlikely preservation of examples of such a deep-water (bathyal to abyssal) ostracod fauna that involves upwelling of deep cold waters on continental margins.