The concept of ‘Gondwana’, an ancient Southern Hemisphere
supercontinent, is firmly established in geological and biogeographical models
of Earth history. The term Gondwana (Gondwanaland of some authors) derives
from the recognition by workers at the Indian Geological Survey in the mid- to
late 19th century of a distinctive sedimentary sequence preserved in east
central India. This succession, now known to range in age from Permian to
Cretaceous, is lithologically and palaeontologically similar to coeval
non-marine sedimentary 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 place around the beginning of the Palaeozoic in
near-equatorial latitudes and that the supercontinent as a whole shifted into
high southern latitudes, allowing widespread glaciation by the end of the
Carboniferous. From Carboniferous to Cretaceous times the southern continents
had broadly similar floras but some species-level provincialism is apparent at
all times. The break-up of Gondwana initiated during the Jurassic (at about
180 million years ago) and this process is continuing. The earliest rifting
(crustal attenuation) within the supercontinent initiated in the west (between
South America and Africa) and in general terms the rifting pattern propagated
eastward with major phases of continental fragmentation in the Early
Cretaceous and Late Cretaceous to Paleogene. Gondwanan floras show radical
turnovers near the 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
supercontinent. Throughout the late Palaeozoic and Mesozoic the high-latitude
southern floras maintained a distinctly different composition to the
palaeoequatorial and boreal regions even though they remained in physical
connection with Laurasia for much of this time. Gondwanan floras of the
Jurassic and Early Cretaceous (times immediately preceding and during
break-up) were dominated by araucarian and podocarp conifers and a range of
enigmatic seed-fern groups. Angiosperms became established in the region as
early as the Aptian (before the final break-up events) and steadily
diversified during the Cretaceous, apparently at the expense of many seed-fern
groups. Hypotheses invoking 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 geochronological framework of Gondwanan breakup against
which these biogeographic 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, highlights the distinctive elements of the Early
Cretaceous Gondwanan floras immediately preceding the appearance of
angiosperms and suggests that latitudinal controls strongly influenced the
composition of Gondwanan floras through time even in the absence of marine
barriers between Gondwana and the northern continents.