OFR 151.pdf - CRC LEME

producing cicatrose to canaliculate trilete spores (Cicatricosisporites, Contignisporites,
Ruffordiaspora) had a cosmopolitan distribution that extended into very high latitudes whilst
their NLRs (Anemia, Mohria, Ruffordia) are confined to tropical America, Africa and
southern India (Dettmann and Clifford 1991, 1992). The same is true for the Araucariaceae,
which are mostly confined to seasonally wet and warm to hot (mesotherm-megatherm)
climates. Nevertheless, most of the plants that first appear during the Early Cretaceous
survived into latest Cretaceous or early Palaeogene time. Consequently presence/absence
data give the misleading impression that the Early Cretaceous vegetation was homogeneous
across the continent.
Contributing to this impression of apparent uniformity is the mixing of palynofloras within
the larger basins, most of which received water-transported miospores via rivers draining vast
tracts of the continent. Examples are the Canning-Officer and Carpentaria-Eromanga
Seaways and the Australo-Antarctic Rift System. In contrast, microfloras derived from coals
or lignites mostly represent local mire communities. Climatically-forced provincialism of the
flora had developed as early as the Valanginian [135 Ma]. For example, one clade of
brachyphyll araucarians (Balmeiopsis) is restricted to the drier, palaeo-northern side of the
continent (present-day northwestern to southwestern Australia) whilst the liverwort
(Foraminisporis wonthaggiensis) was restricted to the wetter, palaeo-southern side of the
continent (present-day northeastern to southeastern Australia).
Pollen data from northern and central Australia show that the primitive angiosperms
(angiospermids sensu Vasanthy et al. 1990) expanded over the continent during the mid
Cretaceous. Examples in the Eromanga Basin (times of first appearance in parentheses) are:
Clavatipollenites hughesii (Barremian/Early Aptian), Asteropollis asteroides (basal Albian),
Phimopollenites and Tricolpites (mid Albian) and tricolporate pollen types (Late Albian).
Angiospermid pollen first occur in significant numbers in the Surat Basin at the
Aptian/Albian boundary and show a marked increase in relative abundance during the Albian.
Since the highest relative abundance values follow regressive movements of the
palaeoshoreline, the earliest angiosperm communities are likely to have been riparian and/or
colonisers of newly exposed seafloor sediments.
5.3.2 Gymnosperms and cryptogams
Mesozoic forests and woodlands growing at polar latitudes in Australia were dominated by
gymnosperms and cryptogams. This woody biome has no living counterpart in the Southern
Hemisphere but a general resemblance to the boreal conifer forests and taiga of the Northern
Hemisphere is likely for bioclimatic and ecophysiological reasons. The name Austral Conifer
Forest is coined here to emphasise the presumed eco-physiognomic relationship. Some
Cretaceous plants were deciduous, e.g. the pteridosperm/pentoxylalean genus Taeniopteris,
but it is unlikely that the deciduous habit per se was advantageous at polar latitudes in the
Southern Hemisphere (cf. Wolfe 1987, Hill et al. 1999). Empirical evidence for this includes
the prominence of evergreen gymnosperms in coastal palaeo-southern Australia and their
relative rarity in coastal palaeo-northern Australia during the Late Cretaceous.
Creber and Chaloner (1985) note that the amount of light energy available at polar latitudes
during summer months is not dissimilar to that received annually in the middle latitudes.
Hence the light available within polar communities of widely spaced trees with tall conical
crowns may not have been substantially less than light availability within more dense plant
communities growing at lower latitudes. Similarly, a study of the response of selected
Southern Hemisphere tree species to prolonged darkness (Read and Francis 1992) suggests
that survival rates are higher under colder (>4 0 C) than under warmer temperature regimes.
Some species were able to survive 10 weeks of continuous darkness and Bond et al. (1999)
have shown that the foliage of one Northern Hemisphere pine (Tsuga) can survive under less
than 1% of full sun light in humid temperate climates.
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