Multivariate analysis of Holocene marine isopod faunas from the Arctic, Antarctic, northwestern Atlantic, southeastern Pacific, and northwestern Pacific regions confirms that on a global scale, generic-level faunal resemblance is highest between cold-water biofacies regardless of water depth or latitude. In general, warm-water biofacies are most similar to each other and least similar to cold-water biofacies. These faunal trends are apparently related to the division of the present world oceans into two general thermal realms: the psychrosphere, consisting of cold high-latitude shallow waters and deep waters at all latitudes; and the thermosphere, consisting of mainly lower latitude warm waters and temperature-variable waters above the permanent thermocline. Analogous trends in faunal resemblance and distributional characteristics have been documented for some Late Cambrian trilobite faunas, and they probably also existed during other parts of the Cambrian and during at least some parts of the Ordovician. This suggests that a two-layered thermally stratified ocean may have existed during some parts of the early Paleozoic.
Regional trends in taxonomic resemblance during present thermal stratification of the world oceans suggest by analogy that early Paleozoic paleozoogeographic data are inconclusive for reconstruction of Paleozoic predrift continental positions. Concomitantly, taxonomic resemblance between early Paleozoic vagile benthic faunas cannot be used with complete fidelity to critically “test” continental reconstructions that have been based primarily on geological or geophysical evidence.