Palaeobiogeographic patterns of the brachiopod assemblages of the Iberian Subplate during the Late Toarcian–Early Aalenian (Jurassic)

Abstract

Late Toarcian–Aalenian brachiopod assemblages recorded in different basins located in and around Iberia present a pattern of spatial and temporal distribution which enables two palaeobiogeographic biochoremas to be differentiated: the Iberian-Atlantic District (IAD), which includes the Lusitanian Basin, Asturias and the Basque-Cantabrian Basin, on one hand, and the Iberian-Mediterranean District (IMD); the latter basically comprises the Iberian Range and the Tortosa Platform of the Catalonian Basin. Changes in the composition of the assemblages throughout the stratigraphical interval studied have enabled three main stages characterizing each palaeobiochorema to be recognized. The first two assemblages reflect gradual individualization and isolation, with frequent endemisms. These two stages correspond to the IAD1, IAD2, IMD1 and to the IMD2 assemblages respectively. As from the Pseudoradiosa Chronozone, and during the Aalensis and Opalinum chronozones, an increase in communication between the two districts can be observed, as well as between these and other remote areas. This stage is represented by the IAD3 and the IMD3 assemblages.Some of the brachiopods colonizing the districts are widely dispersed due to their capacity for expansion: they have been found in other Tethysian basins. Other species are exclusive to the district basins and appear to have originated in the areas close to them. For the latter species the existence of two different “evolutionary factories” is proposed: one that supplies species to the IAD, mainly from generic stocks as Neozeilleria, Soaresirhynchia or Praemonticlarella; and another for the species reaching the basins of the IMD, clearly shared by other European and North African basins.The Atlantic “evolutionary factory” remains active until the Aalenian and part of the Early Bajocian, while the Mediterranean is diluted since the Aalenian.