OFR 151.pdf - CRC LEME

If these considerations apply to the Early Cretaceous vegetation, then inland (continental)
habitats may have been more suitable for tall gymnosperms than the milder climates found
nearer the coast, and communities growing close to the coast or along shorelines are more
likely to have been dominated by shrub and heath species of cryptogams. For the same
reason, relatively open canopy communities found at higher latitudes may have been replaced
by relatively closed canopy forests in lower latitude regions where sun angles are higher.
5.3.2 Angiosperms
The earliest unequivocal evidence of angiosperms are pollen grains of magnoliid dicotyledons
and monocotyledons found in Hauterivian deposits in Israel and southern England (references
in McLoughlin et al. 1995a). Macro and microfossil data show a major diversification of
angiosperms in the Northern Hemisphere during the mid Cretaceous, with angiosperms first
becoming abundant at lower latitudes and only subsequently becoming dominant at middle
and high palaeolatitudes during the early Late Cretaceous. The same trend is apparent in the
Southern Hemisphere (Drinnan and Crane, 1990). For example, angiospermid pollen, which
first occurs in the Surat Basin in the Early Albian, becomes relatively common and
widespread across the continent by Late Albian time (Burger 1980).
5.5.3 Climatic indicator taxa
Few Cretaceous species have NLRs with narrow ecological ranges. However, by comparing
regional differences in the relative abundance of their macrofossils or miospores against
palaeolatitude, it is possible to ‘second-guess’ the ecological preferences of some major plant
groups. For example, observations by Dettmann et al. (1992), Dettmann (1994) and Pole and
Douglas (1999) indicate that Podocarpaceae were more prominent in the Eromanga Basin,
than in the south-east during Albian to Cenomanian time – suggesting podocarps were
tolerant of strong seasonal contrasts in temperature as well as prolonged winter darkness.
Ferns such as Gleicheniaceae and Schizaeaceae display the same bias but in this case, a not
unreasonable explanation is that the diminished marine influence in the Albian allowed
extensive fern heath to develop over emergent seafloor. The same studies indicate that
Gingko, cycads and tree-ferns were characteristic of lower (warmer) palaeolatitudes and the
same is probably true for most fern allies except for Lycopodiaceae-dominated communities,
which were better developed at higher (colder) palaeolatitudes.
The fossil data may provide moderately reliable evidence for humidity but palaeotemperature
preferences remain little more than speculations based on palaeogeography. For example, the
literature consensus is that cheirolepidiacean woodland occupied (relatively cooler and
possibly drier) upland areas rather than (relatively warmer and possibly wetter) coastal
habitats despite Cheirolepidiaceae pollen (Corollinia) being most abundant in near shore
marine sediments. Conversely, Backhouse (1988) has suggested an association with welldrained
soils and warm (possibly seasonally arid) conditions. The relative abundance of
Sphagnum spores (Stereisporites spp.) is amongst the more reliable palaeobotanical indicators
of past climates in that Sphagnum (peat moss) bogs can only extend beyond the confines of
topographic depressions under cool-cold and possibly uniformly wet conditions.
Other forcing factors may have included herbivory by dinosaurs (cf. Wing and Tiffney 1987,
Archibald 1996), volcanism (Kerr 2000) and wildfires ignited by lightning strikes (Jones
1993). Evidence for palaeo-wildfires include a statistically significant correlation between the
relative abundance of one pteridosperm clade (Bennettitales) and charcoal in the Eromanga
Basin (Pole and Douglas 1999), implying that bennettitaleans were fire-tolerant or pioneers
on burnt sites. Microfossils preserved in a charcoal conglomerate in the Murray Basin record
the replacement of a gymnosperm-dominated community by ferns and fern allies (Macphail
and Truswell 1989). In Argentina, volcanic ash-fall is suggested to have forced changes in
the growth and distribution of Early Cretaceous gymnosperm-fern communities
(Archangelsky et al. 1995).
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