The fossil record of ferns and fern allies in Tasmania

There is an excellent fossil record of ferns in Australia, and it provides an important source of information on the phylogeny and biogeography of living ferns. The most important elements of the fossil record are spores, sterile and fertile frond compressions and impressions, and the relatively rare but highly informative petrified remains, usually rhizomes and stems. Because ferns are such an ancient group of plants, much of the record cannot be applied directly to the extant flora. Although this causes problems in interpreting the fossil record, and affinities often remain unclear, the record relevant to living taxa is still imposing.

Pteridophyte spores are well-preserved and frequently seen in a wide range of sediments and are of considerable taxonomic significance. Indeed, not only can spore wall ornamentation often be observed, ultrastructural features may also be visible. One limit to the taxonomic value of the spore record is the considerable convergence in spore morphology and anatomy in fossil and extant pteridopyhtes, especially ferns. The consequent problems with the identification of dispersed fossil spores are exacerbated by the frequent loss of the diagnostically important exospore.

Fragments of vegetative fronds are comparatively uncommon in the fossil record. However, while these can sometimes be assigned to genera or families, identification is often impossible, because available characters such as frond shape and venation show considerable convergence. Conversely, fertile fronds tend to be much more useful, and those with in situ spores are among the most confidently and precisely identified of all plant fossils, as well as being useful in confirming or disproving the identity of dispersed fossil spores (e.g. Jordan et al., 1996). Tree fern stems and rhizomes are beautifully petrified under some special conditions that are still not fully understood and these can often be confidently identified.

In this brief review we concentrate on fossils that bear directly on living ferns and their allies, and we provide examples that demonstrate particularly interesting biogeography or palaeoecology. While many of the early fern-like fossils appear quite similar to living forms (e.g. Baragwanathia W.H.Lang & Cookson and some extant Lycopodium species), these will not be dealt with here because they are too remote from the living flora to be significant. Although the dispersed spore record is difficult to summarise, we direct interested readers to the review articles of Dettmann (1994) and Macphail et al. (1994) on the dispersed spore record and the role of ferns and fern allies in the Australian flora over the last 100 million years or so.