Australia: The Land Where Time Began

A biography of the Australian continent 

Early Tertiary Macrofloras

Some generalisations have been made based on specific floras from specific times during the early Tertiary, though the individual floras have not been proven to be coeval. Several macrofloras are known from the Middle Eocene. At this time it has been generalised, based on physiognomic and systematic analysis, that both mesothermic and megathermic forests were common in the southeast quarter of the continent. (sensu Nix, 1982), though they were not uniform, or even very similar at any one time throughout a region, or in one region through time. A mosaic pattern of vegetation is found in the fossil record at places such as Anglesea in Victoria (Christophel et al., 1987; Barrett & Christophel, 1990), that is reminiscent of the extant rainforests of northern Australia, based on deposits in which the lenses studied were close in time, some may even have been coeval.

Some taxa are present, or even common, at most sites. An example is the Lauraceae that form a significant proportion of the flora at 4 of the 5 sites studied in detail. At Nerriga and Anglesea they are very common and reasonably common at the Golden Grove and Maslin Bay sites. At Nelly Creek they are rare. The Proteaceae are very diverse and common at Maslin Bay and Anglesea, less common at Golden Grove and Nelly Creek, and are not known from the macrofossil flora of Nerriga. Within the Protaceae, macrofossils of Musgraveinanthus (Musgravinae) have been found at Golden Grove and Anglesea. The pollen grain of Banksieaeidites arcuatus found in Musgravianthus alcoensis (Christophel, 1984), being found at all sites suggest it had a widespread distribution. Banksieacephyllum and/or Banksiearformi have also been found at Anglesea, Golden Grove and Maslin Bay, though not being found at Nelly Creek, are known to be common in neighbouring silcrete deposits (Greenwood et al., 1990). Golden Grove is the only site not known to contain fossils of Gymnostoma. Golden Grove and Anglesea have specimens that have been attributed to Elaeocarpus/Sloanea (Elaeocarpaceae), where they are common, are also present at Maslin Bay and possibly at Nerriga. All localities studied contain specimens of Brachychiton, and Myrtaceae and Lygodium have been found in most sites, and Zamiaceae has been found in deposits at Anglesea, Nerriga and Moranbah.

When the distributions of taxa found in macrofossil deposits are compared with 5 major extant community types, as in Table 1.11 of Hill ed. (1994), the Proteaceae and Myrtaceae form important parts of both the extant and macrofossil communities listed. The Lauraceae are present in all extant rainforest floras in the list. If Cassytha, a leafless parasite is included, they are also present in the sclerophyllous community.

It is apparent from the total diversity of most of the fossil floras that a majority of taxa have not yet been identified, at least some of the extant families may yet be found in the fossil record. The leaves of families such as Euphorbiaceae, Rutaceae and Sapindaceae have venation and cuticle patterns that are relatively indistinct, distinctive features having not yet been found, so their presence not being detected may not mean they are not present, merely that can't yet be identified. A couple of taxa, Mimoceae, especially Acacia, and Moraceae, especially Ficus.

Conspicuous phyllodes or bipinnate compound leaves are features of all extant Acacias. Fossils of this foliar form have not been found in any Eocene deposits. The reason for this absence from the fossil record of the Eocene is unknown, given that species of this genus are found in extant rainforests. The strong Gondwanan distribution of the group should ensure that it was present prior to the Miocene, and pollen of the group has been found in pre-Miocene deposits (Macphail et al. in Hill ed., 1994).

The other taxon apparently missing from the fossil record, the Moraceae, Ficus has veins in which the lower angle of the basal pair of secondary veins make its leaves conspicuous, yet it has not been definitely reported from Eocene deposits.

Overall, the Middle Eocene macrofossils show a diverse flora, in which major families, and even genera, still common in extant forest systems, were present and diversifying. The plant communities of the Middle Eocene were unique, though they had some similarity to modern communities, being composed of mixtures of plants, some of which eventually went extinct, or restricted to remnant rainforest refuges, some evolving into extant taxa that are not found in the fossil record. 

Sources & Further reading

1. D.C. Christophel in Hill, Robert S., (ed.), 1994, History of the Australian Vegetation, Cambridge University Press.
2. Nix, H., 1982, Environment determinants of biogeography and evolution in Terra Australis. In Evolution of the Flora and Fauna of Arid Australia, ed. W.r. Barker & P.J.M. Greenslade, p. 47-66. Freeville: Peacock Publications.
3. Christophel, D.C., 1984, Early Tertiary Proteaceae: the first floral evidence for the Musgraveinae. Australian Journal of Botan, 32, 177-86.
4. Christophel, D.C. & Greenwood, D.R., 1987, A megafossil flora from the Eocene of Golden Grove, South Australia. Transactions of the Royal Society of South Australia, 111, 155-62.
5. Barrett, D.J. & Christophel, D.C., 1990, The spatial and temporal components of tho Australian Tertiary plant megafossil deposits. In Proceedings of the Third IOP Conference, ed. J.G. Douglas & D.C. Christophel, pp. 43-9. Melbourne: A-Z Press.