PALAEOECOLOGICAL IMPLICATIONS OF THE DISTRIBUTION OF MESOSAURID REPTILES IN THE PERMIAN IRATI SEA (PARANA BASIN), SOUTH AMERICA

Abstract

A reconstruction of the upper Permiian Irati sea and conditions in the waterbody is attempted. The reconstruction is based on faunal and facies distributions and the biostratigraphy of the formation. It is deducted that the Irati, like the southern Africa Whitehill Formation is a chronostratigraphic unit and that the rnesosaurid reptiles colonised the sea halfway through its existence. The sea itself is seen as a relatively shallow embayment of a central sea arm that occupied a position between Africa and South America. In the Irati embayment (sea), black shales accumulated in the stratified waters of the relatively deeper central areas whilst limestones and dolomites formed in nearshore and generally shallow areas. The accumulation of limestones in the Säo Paulo-Mato Grosso-Goiás area is a result of the restricted nature of this sub basin. Transgressive and regressive events caused a horizontal displacement of the shallow and deeper environments and resulted in isochronous facies changes in different parts of the basin. This explains e.g. the vertical displacement of deeper water black shales by shallow water limestones or even silty shales in some outcrops. The lithological changes because it was tied up with palaeoenvironmenal changes were accompanied by faunal migrations which is reflected in the fossil record.