The continuity through the past 300 million years of key tropical sediment types, namely coals, evaporites, reefs and carbonates, is examined. Physical controls for their geographical distributions are related to the Hadley cell circulation, and its effects on rainfall and ocean circulation. Climate modelling studies are reviewed in this context, as are biogeographical studies of key fossil groups. Low-latitude peats and coals represent everwet climates related to the Intertropical Convergence Zone near the Equator, as well as coastal diurnal rainfall systems elsewhere in the tropics and subtropics. The incidence of tropical coals and rainforests through time is variable, being least common during the interval of Pangean monsoonal climates. Evaporites represent the descending limbs of the Hadley cells and are centred at 10°to 40°north and south in latitudes that today show an excess of evaporation over precipitation. These deposits coincide with the deserts as well as seasonally rainy climates, and their latitudinal ranges seem to have been relatively constant through time. Reefs also can be related to the Hadley circulation. They thrive within the regions of clear water associated with broad areas of downwelling which are displaced toward the western portions of tropical oceans. These dynamic features are ultimately driven by the subtropical high-pressure cells which are the surface signature of the subsiding branches of the Hadley circulation. Carbonates occupy the same areas, but extend into higher latitudes in regions where terrestrial surface gradients are low and clastic runoff from the land is minimal. We argue that the palaeo-latitudinal record of all these climate-sensitive sediment types is broadly similar to their environments and latitudes of formation today, implying that dynamic effects of atmospheric and oceanic circulation control their distribution, rather than temperature gradients that would expand or contract through time.
The climatic influences on vegetation in the present world can serve as an excellent model for the Permian, there being approximately the same level of floral differentiation and the same relatively cold polar regions. Accordingly, a system of present day climatically defined biomes, developed by Walter, is adapted herein for palaeophytogeographic purposes. There are ten biomes altogether which range from the tropical 'everwet' (biome 1) to the polar 'glacial' (biome 10).Biome 1 is represented by the tropical rainforests of the Cathaysian province sensu stricto which was populated by the arborescent lycopods and sphenophytes, and the Gigantopterids which are interpreted as lianas (vines). This is flanked by the lower diversity 'summerwet' biome 2, the Atlantic Province, a zone which today is characterized by savanna vegetation, and in the Permian was represented by the pteridosperm Callipteris and the primitive conifers. Biome 3 was effectively 'abiotic' and is expressed geologically by evaporite belts.The 'winterwet' or Mediterranean climate of today (biome 4) was not well developed in the Permian, or perhaps just not well preserved, but I attribute some low diversity floras of Kazakhstan to this biome. Biome 5, the 'warm temperate' biome is well developed in both hemispheres, and, like biome 1, is characterized by high diversity floras and abundant swamp deposits. Not surprisingly, the biome 5 floras have been mistaken for biome 1 with which they are transitional in the modern world and with which they share similar climatic conditions. In the Permian Angaran Realm, the Pechora Province represents this biome, while the Austroafroamerican Province is the Gondwanan equivalent. Often, biome 5 floras have been simply termed 'mixed'. Proceeding poleward across the 'hard frost line', the cool temperate floras of biome 6 were populated by diverse herbaceous sphenophytes and deciduous trees (the cordaitids of Angara and the similarly diverse glossopterids of Gondwana). Areas in the temperate zone that were remote from moisture sources are assigned to biome 7 and are only known in the southern hemisphere by some aeolian deposits in Argentina. The 'cold temperate' biome 8 is a low diversity equivalent of biome 6, and has been given the name Gangamopteris Flora in the southern hemisphere. The 'tundra' environment, biome 9, and the truly glacial deposits, biome 10, are known only from the southern hemisphere.
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Five benthic communities occupied the shelf regions of the British Isles, Norway, and North America in Upper Llandovery times. The communities are listed below in order of increasing distance from shore.(1) T h e Lingula Community is the least diverse; it has both infaunal elements, including a protobranch, and two lingulids, and epifaunal elements, including a rhynchonellid, a pterioid, and a cornulitid. A restricted and protected near-shore environment, such as a bay or estuary, is-postulated.(2) T h e Eocoeliu Community shares elements in common with the former community, but is more diverse and is dominated by epifaunal forms; the many small pedunculate brachiopods probably lived attached to the large leptostrophiid brachiopod.(3) T h e Pentamerus Community is dominated by this genus which lived free and upright on the bottom; smaller pedunculate brachiopods probably attached to this large neighbor.(4) The Costistricklandia Community was similar in structure to the former community with the many small pedunculate brachiopods being attached to the large Costistricklandia.(5) T h e Clorindu Community is the most diverse, with a great variety of small brachiopods which were probably able to attach to small objects in this quiet off-shore environment, or to some moderately sized brachiopods, such as Clorinda and Cyrtiu, which apparently lived free on the bottom.The brachiopod dominated communities of the Silurian clearly inhabited the 'level bottom', an area now occupied mainly by infaunal forms. T h e main attachment surfaces for the epifaunal elements of the Silurian communities were disarticulated, convex-upward shells.
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