The breakup history of Gondwana and its impact on pre-Cenozoic floristic provincialism

The concept of 'Gondwana', an ancient Southern Hemisphere supercontinent, i s firmly established in geological and biogeographical models of Earth hist ory. The term Gondwana (Gondwanaland of some authors) derives from the reco gnition by workers at the Indian Geological Survey in the mid- to late 19th century of a distinctive sedimentary sequence preserved in east central In dia. This succession, now known to range in age from Permian to Cretaceous, is lithologically and palaeontologically similar to coeval non-marine sedi mentary successions developed in most of the Southern Hemisphere continents suggesting former continuity of these landmasses. Palaeomagnetic data and tectonic reconstructions suggest that the main assembly of Gondwana took pl ace around the beginning of the Palaeozoic in near-equatorial latitudes and that the supercontinent as a whole shifted into high southern latitudes, a llowing widespread glaciation by the end of the Carboniferous. From Carboni ferous to Cretaceous times the southern continents had broadly similar flor as but some species-level provincialism is apparent at all times. The break -up of Gondwana initiated during the Jurassic (at about 180 million years a go) and this process is continuing. The earliest rifting (crustal attenuati on) within the supercontinent initiated in the west (between South America and Africa) and in general terms the rifting pattern propagated eastward wi th major phases of continental fragmentation in the Early Cretaceous and La te Cretaceous to Paleogene. Gondwanan floras show radical turnovers near th e end of the Carboniferous, end of the Permian and the end of the Triassic that appear to be unrelated to isolation or fragmentation of the superconti nent. Throughout the late Palaeozoic and Mesozoic the high-latitude souther n floras maintained a distinctly different composition to the palaeoequator ial and boreal regions even though they remained in physical connection wit h Laurasia for much of this time. Gondwanan floras of the Jurassic and Earl y Cretaceous (times immediately preceding and during break-up) were dominat ed by araucarian and podocarp conifers and a range of enigmatic seed-fern g roups. Angiosperms became established in the region as early as the Aptian (before the final break-up events) and steadily diversified during the Cret aceous, apparently at the expense of many seed-fern groups. Hypotheses invo king vicariance or long distance dispersal to account for the biogeographic patterns evident in the floras of Southern Hemisphere continents all rely on a firm understanding of the timing and sequence of Gondwanan continental breakup. This paper aims to summarise the current understanding of the geo chronological framework of Gondwanan breakup against which these biogeograp hic models may be tested. Most phytogeographic studies deal with the extant , angiosperm-dominated floras of these landmasses. This paper also presents an overview of pre-Cenozoic, gymnosperm-dominated, floristic provincialism in Gondwana. It documents the broad succession of pre-angiosperm floras, h ighlights the distinctive elements of the Early Cretaceous Gondwanan floras immediately preceding the appearance of angiosperms and suggests that lati tudinal controls strongly influenced the composition of Gondwanan floras th rough time even in the absence of marine barriers between Gondwana and the northern continents.