133845.pdf

Cretaceous patterns of floristic change in the Antarctic Peninsula

DAVID J. CANTRILL^1 & IMOGEN POOLE^2

l

British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley

Road, Cambridge CB3 OET, UK (e-mail: [email protected])

2

Wood Anatomy Section, National Herbarium of the Netherlands, University of Utrecht

branch, PO Box 80102, 3585 CS Utrecht, The Netherlands

Abstract: Cretaceous radiation of angiosperms from low to high palaeolatitudes, coupled

with the break-up of Gondwana, played a major role in establishing and maintaining bio-

geographic patterns across the southern hemisphere. Uncertainties in details of plate

reconstructions provide conflicting hypotheses about area relationships of Gondwana frag-

ments. This has led to a number of competing proposals concerning angiosperm migration

across Gondwana. Central to this debate is the role of the Antarctic Peninsula, a region

that is often envisaged as providing the main connection between east and west Gondwana.

The initial radiation of angiosperms into the Antarctic Peninsula region, however, post-

dates appearances elsewhere in east Gondwana (e.g. Australia), strongly suggesting that

the Antarctic Peninsula was not the main gateway, at least in the early stages of Gondwana

radiation. A steep climatic gradient in this part of the world probably acted as an effective

barrier to angiosperm radiation. The peak of floristic replacement coincides with the peak

of Cretaceous warmth (Turonian) which in turn suggests that climatic warming acted as a

forcing mechanism by pushing latitudinal belts of vegetation southwards. Once into the

southern high latitudes angiosperms diversified, and as climates cooled during the Late

Cretaceous a number of important groups seem to have their origins here. Recent investi-

gations of Antarctic macro- and microfloras indicate progressive floristic replacement

through the Cretaceous. Bryophytes, hepatophytes, bennettites and other seed plants all

show a rapid decline in diversity. In contrast, ferns initially decline then recover, while

conifers remain relatively stable. The ecological preferences of the replaced groups imply

that angiosperms initially occupied areas of disturbance and were understorey colonizers,

only later replacing fern thickets and becoming important in the overstorey. This pattern

is consistent with those observed elsewhere through the Cretaceous.

Two major events influenced Cretaceous to Antarctica) (Fig. 1). Consequently, the break-up

present-day biotic history of the southern hemi- of Gondwana underpins many biogeographic

sphere: the radiation of angiosperms and the and diversity patterns seen in the southern

concomitant break-up of Gondwana. During hemisphere today. Antarctica is unique amongst

mid-Cretaceous times (130-90 Ma BP) terrestrial fragments of Gondwana in that it was attached

vegetation underwent rapid modernization, at some time to all other major components of

with an explosive radiation of crown group the supercontinent (Fig. 1). For this reason

angiosperms (Lidgard & Crane 1990). In the understanding the geological evolution of

northern hemisphere it is well established that Antarctica is essential for evaluating terrestrial

angiosperms arose at low palaeolatitudes and connections across Gondwana through time,

spread polewards (Crane & Lidgard 1989; Crane Although the timing and pattern of break-up is

et al 1995). A paucity of knowledge about relatively well understood for the large conti-

southern hemisphere floras, and in particular nental fragments, this is less the case for smaller

those of Antarctica, means that the pattern in microplates (Storey et al 1988). The present-day

the southern hemisphere is less well constrained Antarctic region can be divided into two areas;

(Drinnan & Crane 1990). However, a general East Antarctica, a craton; and West Antarctica,

pattern of earlier inception in low latitudes is composed of a number of microcontinental

evident from the limited data available (Drinnan fragments (e.g. Antarctic Peninsula, Thurston

& Crane 1990). Island-Eights Land, Marie Byrd Land, Haag

During Cretaceous times disintegration of Nunalaks, Ellsworth-Whitmore Mountains)

Gondwana was important in maintaining, or (Fig. 2). These microcontinents have a long and

creating, biotic gateways between what are now complicated history (Storey et al 1988) and

widely separated landmasses (Australia, Africa, represent one of the major uncertainties

India, New Zealand, South America and in Gondwana reconstructions. Nevertheless,

From: CRAME, J. A. & OWEN, A. W. (eds) 2002. Palaeobiogeography and Biodiversity Change: the Ordovician
and Mesozoic-Cenozoic Radiations. Geological Society, Londor, Special Publications, 194,141-152.
0305-8719/02/$15.00 © The Geological Society of London 2002.