Question

It is very difficult to separate the impact of low soil nutrient levels from low water availability when examining photosynthetic organs in the Australian fossil record during the last 50 million years. Discuss how this can be done in either the Casuarinaceae or Proteaceae, and what the fossil record tells us about evolution in the family you choose.

Answer 1

The family casuarinaceae comprises 96 extant species in four genera that are confined to Malesian- Australian- Melanesian region, except for the essentially littoral species casuarina equisetifolia.The family is widespread in Australia, and ranges through most vegetation types, although it is restricted to one small population in rainforest.According to the fossil record, pollen of the casuarinaceae is readily identifiable but unfortunately the generic affinities cannot be determined.According to the data fossil pollen has been reliably reported from South Africa.It is also common in Australia and New Zealand from the paleocene onwards.The latest occurence of casuarinaceae pollen in New Zealand is early pleistocene.Macrofossils of casuarinaceae are relatively abundant and convey much more taxonomic information than pollen.In Australia the macro fossil record extends from early Eocene.The earliest records (Eocene- oligocene) are all gymnostoma.Three species have been identified although none of the fossil Gymnostoma has yet been named.

Casuarinaceae family was an excellent illustration of the effect of developing aridity in Southern Australia during the teritary.In the early teritary when the climate was uniformly wet, Gymnostoma was the only Casuarinaceae identified, and it occurs in many macrofossil deposits.Although Gymnostoma is quite sceleromorphic in its reduced and virtually nonfunctional leaves and woody photosynthetic stem, it is not particularly adapted to prevent water, loss.

Thus Gymnostoma can be considered to be a genus that may be adapted to oligotrophic soils, and in particular the characteristically phosphorus-poor.Austrialian soils, but is not particularly Xeromorphic.In contrast both casuarina and allocasuarina have well protected stomata in deep grooves along the stem which are often filled with trichomes.These genera are not only sceleromorphic but xeromorphic as well, i.e they have morphological adaptations to prevent excessive water loss.These adaptations must surely have evolved in reapores to continually dry conditions since they must severely disrupt the entry of CO2 into the stems.Such an adaptation must therefore confer a great benefit to the plant in order to overcome the disadvantage in terms of lost productivity, which we can determine with the help of fossil records.