Physiography, palaeotectonics and palaeoenvironments as controls of changes in ammonite and brachiopod communities (an example from the early and middle jurassic of western Algeria)
Abstract Comparative studies of the palaeogeographical distributions of brachiopods and ammonites in western Algeria and its constituent basins give evidence of the control played by physiological differentiation. This factor, dependent upon tectonics, conditioned the bathymetry as well as the sedimentary context and the supply of nutrients. After a marked provincialism, caused by the large extent of carbonate platforms which prevailed during the Early and Middle Liassic, exchanges became easier. But from the Domerian to the Bathonian, the splitting of the margins into a mosaic of small subbasins bounded by steep seamounts and/or islands precented a wide distribution by currents. This led to diversification of the faunal assemblages inside the North African basins. The brachiopod communities consisted of abundant specimens of varied species on the shelves but were rare or absent in (more or less wide) epioceanic areas. Among ammonites the dominant morphological type varied from one palaeogeographical setting to another. Platycones were dominant on the shallow outer shelves, oxycones were abundant on the slopes, and the proportion of sphaerocones and serpenticones was higher in largely open basins and on outer shelves offering good connections with more open seas.