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The fossil history and phylogeny of the cycadales

The fossil history and phylogeny of the cycadales

Geophytology 17(2): 125-162

Fossil remains attributed to the Cycadales include stems, leaves, scales, fructifications, seeds, pollen grains, and no exceptional plant fossil, Leptocycas gracilis, showing leaves, scales and a cone attached to a stem. A critical assessment of Palaeozoic fossils attributed to the groups suggests that all of them are doubtful and some, like Phasmatocycas, even appear to be based on erroneous reconstructions. The oldest undoubted cycads like Antarcticycas schopfii and Leptocycas gracilis and some others occur in the Lower and Upper Triassic. A few of these early cycads had compound leaves but other contemporaneous ones had simple leaves so that neither of the two kinds of leaves can be regarded as ancestral. Mesozoic cycads are generally referred to extinct genera but a few forms resembling Encephalartos (or Zamia) can be traced back to the Triassic and a Cretaceous leaf is actually referred to Encephalartos. Cycas-like leaf forms called Cycadites, Pseudocycas and Paracycas go back only to the Jurassic but the oldest Cycas-like megasporophylls called Cycadospadix are Triassic. Tertiary cycad leaves are mostly assigned to extant genera on the basis of the close similarity of their external form although in some cases even their cuticles are comparable. The only notable exceptions among Tertiary levels which have been lately referred to extinct genera are Dioonopsis nipponica, Eostangeria saxonica and two species of Pterostoma. The last mentioned species are assigned here to Macrozamia as M. zamioides (Hill) comb. nov. and M. anastomosans (Hill) comb. nov. due to the resemblance of their cuticle as well as their external form with those of Macrozamia. The contrasting distribution of living and fossil cycads show that while all the 11 genera of extant cycads are confined to the tropical and subtropical regions of the earth, the fossils of the group show a much wider geographical scatter where their distribution extends into the present temperate regions and even into areas near the north or the south poles. The author believes that this poleward distribution may particularly suggest subsequent drifts of the fossil localities instead indicating a capacity of past cycads for cold tolerance. Diverse views about the phylogeny and relationships of the group are reassessed. It is pointed out that even though the numerous similarities between cycads and preceding pteridosperms point towards their pteridospermous origin yet so far no particular pteridosperms can be regarded as ancestral to them. The similarities of cycads with other groups of gymnosperms are also discussed and it is concluded that none of them can presently be regarded as closely related to the cycads.

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