Microfloristic regions of the Middle–Upper Triassic: New insights into the phytogeographical affinities of southwestern Gondwana

Abstract

Two phytogeographic units were defined for the Middle–Upper Triassic of Gondwana, the warm humid Onslow and the cool-temperate Ipswich microfloras; nevertheless, many uncertainties remain regarding the geographical scope of such assemblages, as well as to the degree of similarity between phytogeographical regions and the existence of different microfloristic domains within the Laurasian territory. To tackle these questions, a hierarchical cluster analysis complemented with ordination methods and bipartite network analysis was performed over a list of 70 palynomorph species and selected localities of the Anisian–Ladinian, Carnian and Norian time-intervals, with special emphasis on southwestern Gondwana units. Obtained results showed a clear dichotomy between tropical and extratropical associations, within which different microfloristic regions also developed from gradual changes in the frequency distribution of palynotaxa. These findings are significant because they confirm for first time through quantitative methods a phytogeographic separation between ‘Onslow’ and ‘Euro-Mediterranean’ assemblages, considered for a long time to be part of the same microflora. Furthermore, and contrary to expectations, two distinct phytogeographic units were recognized for the Laurasian territory, where the first is related to the hot and dry margins of western Tethys, and the other to the temperate and humid zones of Boreal Pangea. Likewise, a separate and well defined assemblage was recognized for southwestern Gondwanan, during the late Anisian–Carnian of central-western Argentina, named here as the Cuyo microfloristic region. This unit showed a close resemblance to microfloras of northern Laurasia due to the shared presence of several species of miospores that are largely absent from the eastern part of Pangea. We hypothesize that the migration of such species occurred through a trans-equatorial temperate and humid landway, following the rift basins and topographic highs developed along the suture lines of the Variscan orogenic belt and western South America at the beginning of Pangea separation.