Flora of the Late Triassic

Abstract

The Triassic was a time of diversification of the global floras following the mass-extinction event at the close of the Permian, with floras of low-diversity and somewhat uniform aspect in the Early Triassic developing into complex vegetation by the Late Triassic. The Earth experienced generally hothouse conditions with low equator-to-pole temperature gradients through the Late Triassic. This was also the time of peak amalgamation of the continents to form Pangea. Consequently, many plant families and genera were widely distributed in the Late Triassic. Nevertheless, two major floristic provinces are recognizable during this interval—one in the Southern Hemisphere (Gondwana) and another in the Northern Hemisphere (Laurussia); these being largely separated by the Tethys Ocean and a palaeotropical arid belt. Regional variations in topography, climate and light regime imposed further constraints on the distribution of plant groups in the Late Triassic such that two floristic sub-provinces are recognizable within Gondwana, and nine within Laurussia based on the plant macrofossil and dispersed spore-pollen records. In a broad sense, the Late Triassic saw the diversification of several plant groups that would become important components of younger Mesozoic floras (e.g., Bennettitales, Czekanowskiales, Gnetales and several modern fern and conifer families). The representation of these groups varied not only geographically, but waxed and waned through time in response to climatic pulses, such as the Carnian Pluvial Event. Significant turnovers are apparent in both macrofossil- and palyno-floras across the Triassic–Jurassic boundary, especially in the North Atlantic and Gondwanan regions. The geographic and temporal variations in the floras have necessitated the establishment of numerous regional palynozonation schemes that are tentatively correlated in this study. Major plant macrofossil assemblages of the Late Triassic world are also placed in a stratigraphic context for the first time. The Late Triassic floras also record the re-diversification of insect faunas based on a broad array of damage types preserved on leaves and wood. By the Late Triassic, all modern terrestrial arthropod functional feeding groups were established, and several very specialized feeding traits and egg-laying strategies had developed. Although age constraints on various fossil assemblages need to be improved, this study provides the first global overview of the temporal and geographic distributions of Late Triassic floras, and establishes a basis for future targeted research on Triassic phytogeography and phytostratigraphy.