OFR 151.pdf - CRC LEME

1. The usual practice of equating the relative abundance of macrofossils or microfossils
with the physical or numerical prominence of the parent plants involves several
questionable assumptions. For example, many tall or common (phytosociologically
important) genera and some plant families are severely under-represented by pollen
or spores. This class includes almost all warm temperate to tropical rainforest trees,
many sclerophyll shrubs and most herbs. Some important rainforest trees are ‘blindspots’
in the fossil record in that their pollen are seldom preserved, and macrofossils
are the only direct evidence of their past existence. Examples are Lauraceae and
Juncaceae. Similarly, plants that have non-dehiscent foliage or are confined to
dry/interfluve habitats, are unlikely to be represented in the macrofossil record
whether or not they are well represented by pollen or spores.
2. Unless unequivocal evidence such as tree-trunks in growth position is present, plant
community structure is deduced by analogy when the floristic composition of a fossil
community appears to agree with a modern one. However, such analogies can be
misleading given the wide range of life forms present in most plant families and
genera and the observation that many tall tree species can survive as low shrubs in
unfavourable habitats. Modern Tasmanian examples are the Huon Pine
(Lagarostrobos franklinii) and Myrtle (Nothofagus cunninghamii), both of which
occur as low-growing shrubs in the alpine zone and along riverbanks but also grow
into 30-40 m tall trees in lowland temperate rainforest.
3. The assumptions that the fossil plants had approximately the same ecological
preferences as their modern equivalent(s) is difficult to confirm for Tertiary taxa, and
highly suspect for the few Cretaceous taxa with known close descendants. The
clearest evidence found so far for this assertion comes from an Early Quaternary site
in western Tasmania, where macrofossils of the present-day alpine creeping pine
Microcachrys tetragona are preserved in association with other shrubs that are
confined to warm temperate and subtropical rainforest (Macphail et al. 1993, Jordan
1997a, 1997b). In other instances, the present day geographic range of a species or
genus appears to reflect past rather than modern day climates, and therefore is of
limited palaeoclimatic value (Coates and Kirkpatrick 1999).
In spite of the above caveats, palaeobotanical reconstructions of Cretaceous and Tertiary
climates are almost always based upon the present-day spatial distribution of their NLRs,
augmented by palaeogeographic and sedimentological data. For example, Hill and Scriven
(1997) have found a moderately good correspondence between the (hypothesised
temperature-forced) distribution of commonly-occurring macrofossil taxa in Tasmania during
the Oligo-Miocene and the altitudinal distribution of their NLRs. Horrell (1991) has claimed
that modern NLR data allows the equivalents of ten modern plant biomes to be recognised
across the globe during the Late Maastrichtian, although it is noted that biozones
characterised by year-round high humidity or semi-arid to arid conditions had to be defined
by geological evidence, viz. the distribution of thick coal and evaporate sequences,
respectively.
More generally, the value of past plant communities as proxy-climatic evidence is reduced by
the low taxonomic resolution of pollen or spores produced by the dominant species. For
example, it is not possible to use pollen to distinguish between shrub species that dominate
modern xerophytic vegetation, e.g. Acacia, Casuarinaceae, Gyrostemonaceae or Myrtaceae,
and their tree-sized relatives that occur in modern and Tertiary wet forest types (Macphail et
al. 1994). In such cases, the probable source may be able to be ‘identified’ by the presence of
associated taxa that produce distinctive pollen or spores (cf. Martin 1997a). These associates
need not be higher plants; Lange (1976, 1978a) has used the remains of Tertiary epiphyllous
fungi as evidence for the former presence of rainforest in arid northern South Australia.
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