OFR 151.pdf - CRC LEME

7.7.2 Palaeobotany
Palaeobotanical trends in southern and central regions reflect the immediate or delayed but
ecologically opposed effects of background warming associated with the rapid northward drift
of the continent, and global cooling and strengthening of equator-to-pole thermal gradients
following the thermal isolation of Antarctica (Macphail et al. 1994). Whether the same is true
for northern regions is unknown.
In southern Australia, a significant number of long-ranging, widely distributed taxa became
extinct in the terminal Eocene. For example, a member of the peat moss family Sphagnaceae
[Stereisporites (Tripunctisporis) sp.], which first appears in the Late Maastrichtian, is last
recorded in the Upper N. asperus Zone in the Gippsland Basin. Here, and also in south-west
Western Australia, central Australia and southern Queensland, palynological dominance
became increasingly restricted to one or more of only four taxa: Nothofagus (Brassospora)
spp., Casuarinaceae, Podocarpaceae and, less frequently, Araucariaceae. This floristic
impoverishment is associated with the earliest consistent occurrence of pollen of sclerophyll
trees, shrubs and herbs that are common to abundant in present-day subhumid to semi-arid
vegetation. Examples include the daisy (Asteraceae), samphire (Chenopodiaceae-
Amarathaceae) and grass (Poaceae) families in the Oligocene, and wattles (Acacia) and
bloodwood eucalypts (Eucalyptus gummifera-type) in the Early Miocene.
Oligocene-Middle Miocene pollen sequences give the impression of being characterised by
fewer first appearances and extinctions than are Eocene pollen sequences although it is
recognised the difference may be due to an industry focus on short-ranging or
morphologically distinctive spore and pollen types. For example, many less distinctive types
that may have first appeared or become extinct at this time, remain undescribed and/or their
time distributions have never been systematically recorded. If correct, then Oligo-Miocene
vegetation is likely to have been far more heterogeneous than is indicated by a simple
comparison of correlative microfloras. A corollary is that comparisons of Oligo-Miocene
microfloras may underestimate the strength of environmental gradients across the continent.
7.7.3 Palaeoclimates
Many Oligo-Miocene floras are difficult to date or evaluate in climatic terms for three
reasons.
1. The zone index fossil of the Canthiumidites bellus Zone (Canthiumidites bellus) has
not been recorded in inland Tasmania, central Australia or Queensland and may have
appeared earlier in northeastern Australia than in the Gippsland Basin. This makes it difficult
to distinguish between Oligocene to late Early Miocene and late Early to Middle Miocene
floras in the former regions.
2. There is no reason to suppose that more than a moderate fraction of the Australian
Oligo-Miocene flora has been preserved, except possibly in the Murray Basin and Tasmania.
Moreover, if the Murray Basin record is typical of other inland regions, then community
composition and presumably structure will have been shaped as much by local (edaphic)
factors as by changes in regional climate.
3. Macrofossil data from Tasmania indicate that cool climate species had begun to
evolve within families that now are confined to warm temperate to tropical environments.
This group includes several extinct species of Araucaria (Macphail et al. 1991). The same
may be true for Nothofagus (Brassospora).
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