ANTARCTIC CRETACEOUS BIODIVERSITY CHANGE 147
Fig. 6. Palaeolatitudinal gradients through South America and the Antarctic Peninsula highlighting the
increase in diversity of mesic elements (hepatics) and the decrease in abundance of thermophilic species (e.g.
Classopollis) from low to high latitudes. Diversity data for hepatics derived from Pons (1988), Riccardi (1988)
and Falcon-Lang el al. (2001). Abundance data for Classopollis derived from Herngreen (1975), Dettmann &
Thomson (1987), Herngreen et al. (1996) and Pramparo & Volkheimer (1999).
from herbaceous plants to probable trees
(Cantrill & Nichols 1996). In the macrofloras
angiosperms rapidly increase in importance
rising to a peak in the Coniacian (72%); this is
followed by a decline into the Santonian. In the
microfloras angiosperms appear in the early
Albian (Dettmann & Thomson 1987) and show
a rapid diversification, reaching up to 75%
within-flora diversity by Turonian times (Fig.
4F). From this peak of diversity they initially
decline, before rapidly recovering in the
Campanian and Maastrichtian.
Summary
Angiosperm within-flora diversity increases at
the expense of bryophytes/hepatophytes, ben-
nettites and other gymnosperms and, initially,
ferns and lycophytes. This suggests that the
angiosperm invasion took place in the niches
occupied by these groups. Within the Antarctic
Cretaceous ecosystem liverworts were colonizers
of bare sediment and an understorey component
of the vegetation (Cantrill 1997), whilst ferns and
lycophytes were either understorey or thicket-
forming plants (Cantrill 1996). So it seems most
likely that the early angiosperms occupied
colonization and understorey niches. Further
support for this idea can be seen in the over-
storey elements: conifers, the main tree species,
maintained a relatively constant diversity
throughout the Cretaceous. Indeed, angiosperm
wood does not become common until Coniacian
times, and the early angiosperm floras of the
Antarctic Peninsula (Albian) are dominated by
herbaceous leafy forms, although some leaf
morphologies are suggestive of trees (Cantrill
& Nichols 1996). The lack of wood also
suggests that these early angiosperms occupied
under/middle-storey niches. We conclude then
that the angiosperms were invading the under-
storey and colonization niches within the eco-
system as those are the groups that were being
replaced. Angiosperms later expanded into the
overstorey, perhaps by filling gaps in the canopy
left by conifers. The recovery of fern diversity
(Fig. 4E) suggests that pteridophytes were able
to colonize new niches created by the
angiosperms.
Antarctic Peninsula: biotic gateway or
biological backwater?
In order to evaluate whether the Antarctic
Peninsula acted as a biotic gateway during the
mid-Cretaceous radiation of angiosperms, the
floral succession needs to be densely sampled
and well age-constrained. A pilot study
(Dettmann & Thomson 1987) identified
angiosperm pollen in early Albian samples from
