144 DAVID J. CANTRILL & IMOGEN POOLE
Antarctica, New Zealand) Gondwana is via the
Antarctic Peninsula, a region that has been
postulated as providing the main connecting link
during the radiation of angiosperms (Fig. 3).
Alternative scenarios have also been postulated;
palaeogeographic reconstructions of SE Asia,
Australia and east Antarctica during the
Cretaceous indicate land connections between
these areas (Metcalfe 1990), and point to a
possible invasion from Asia to the north (Burger
1981; Truswell et al. 1987; Hill & Scriven 1995,
fig. 5) (Fig. 3). Other workers have postulated
that angiosperms arrived in Gondwana via India
(Burger 1990), while more recently Dettmann
(1989, 1994) suggested that angiosperms
migrated along rift valleys, as these provided
highly disturbed environments that favoured
early angiosperms (Fig. 3).
To date, relatively little is known of
Cretaceous angiosperm floras from Antarctica
(Truswell 1991), yet they are critical for testing
the validity of hypothesized radiation routes. If
the Antarctic Peninsula acted as the main
gateway, then Cretaceous floras from this region
should have earlier records of angiosperms than
elsewhere in eastern Gondwana (i.e. Australia,
New Zealand, India). Alternatively, if angio-
sperms radiated into Gondwana via another
route then they should appear later in Antarctic
floras than elsewhere in Gondwana.
Floristic change in the Antarctic Peninsula
Over the last ten years our knowledge of the
floristic composition of the Antarctic Peninsula
macro- and microfloras has increased dramati-
cally. Well documented macrofloras are now
known from Aptian (Cantrill 1997, 2000; Cesari
et al. 1998, 1999; Torres et al. 1997), Albian
(Cantrill 1995, 1996; Cantrill & Nichols 1996;
Falcon-Lang & Cantrill 2000; Cantrill & Falcon-
Lang 2001), Coniacian (Hayes 1999), Santonian
(Hayes 1999), Campanian and Maastrichtian
(Poole & Francis 1999, 2000; Poole et al.
2000a,b,c; Poole & Gottwald 2001) strata.
However, the Maastrichtian record of leaf floras
is sparse and could not be used in this study. In
contrast, microfloras have been recorded from
all of the major stages of the Cretaceous (exclud-
ing the Hauterivian and Barremian) (Askin
1990, 1994; Duane 1996; Riding et al. 1998;
Cantrill 2000), and so provide a record for the
latest Cretaceous (Maastrichtian).
Patterns of floristic replacement through the
Cretaceous were examined using within-flora
diversity plots (Lidgard & Crane 1990). These
were calculated as a percentage of the total
species diversity for each macroflora and
microflora. Within-flora diversity moderates
sample size effects (Spicer 1988) but the diver-
sity measure for groups is not independent, as
the values of a single flora must sum 100%.
Comparisons of absolute within-flora species
richness with within-flora diversity measures
demonstrate that within-flora diversity captures
patterns seen in absolute diversity and does not
induce spurious trends (Lidgard & Crane 1990).
Despite taxonomic biases in both the macro-
floral and microfloral record that make it
difficult to discriminate certain groups (e.g. ben-
nettites in the microflora, lycopods in the
macroflora), the datasets show similar patterns
through time. This gives us some confidence that
the observed patterns are a real reflection of
changes in vegetation composition.
Bennettitales and other non-coniferous
seed plants
Bennettites were initially reasonably diverse
(Aptian, 14%), but then rapidly disappear from
the floras becoming extinct by the Campanian
(Fig. 4A). Unfortunately bennettites do not
have a good pollen record; their monosulcate
grains lack distinguishing features at the light-
microscope level, which makes discrimination of
species difficult. Thus the pollen record appears
to be less diverse than the macrofloral record
(see Dettmann 1986b). Like the bennettites.
other non-coniferous seed plants (e.g.
Pachypteris, Ginkgoales) all show a progressive
decline from the Aptian to Albian, then recover
slightly in Cenomanian times before declining in
importance through the later part of the
Cretaceous (Fig. 4B).
The decline in bennettites is also reflected in
their abundance through the Cretaceous. They
are one of the most frequently encountered
foliage taxa in the Aptian floras, but by Albian
times, although still accounting for 3.5% of the
floristic diversity, they are very rare. Only a
handful of specimens are known from the
Albian and one from the Coniacian. This
suggests that not only did they become less
diverse but that they also became less important
ecologically within the vegetation. A similar
pattern is seen elsewhere globally. For example,
macrofloras in Australia have abundant Bennet-
titales in early Aptian floras (e.g. Douglas 1969)
but they disappear by Aptian-Albian times
(Drinnan & Chambers 1986). Recent work on
dispersed cuticles indicates that bennettites
remained a minor constituent of the Australian
vegetation at least until the Cenomanian (Pole
& Douglas 1999).
