[Laboratory VIII - Medullosans and Cycads]

The idea that cycads stem from Carboniferous so-called pteridosperms ("seed ferns" or seed plants with fern-like foliage) has long been popular with paleobotanists. Some authors have suggested a close relationship between cycads and Lyginopteris, but most favor an affinity to Medullosan seed plants. A few workers even suggest an immediate ancestor-descendant relationship between Medullosans and cycads, especially after it was discovered that their temporal ranges overlap (no gap in the fossil record!). Recent cladistic studies have generally supported the hypothesis that Medullosans and cycads are related (Doyle and Donoghue, 1986; Crane, 1985; but see Nixon et al., 1994 for a different result). In the cladogram in the Virtual Laboratory (modified from Crane, 1985), Medullosans and cycads form a clade on the basis of the double vascular supply to the ovules present in both groups, sister to the so called platysperms (plants with flattened ovules, e.g. conifers, Ginkgo, cordaites, glossopterids). Other arguments for their close relationship include the position of ovules on what appear to be modified leaves, their frond-like leaves and anomalous development of the secondary xylem. However, there are equally convincing arguments against this liaison. The most significant of which is the difficult in deriving the cycad microsporophyll from Medullosa pollen organs. As you have learned by now, justifying homologies is a thorny problem, and it seems to become more and more challenging as plants become increasingly complex through time!

The Pteridosperms

The pteridosperms or "seed ferns" or "seed plants with fern-like foliage" are generally considered to be an informal taxonomic group. Some workers expand the pteridosperms to include a variety of Mesozoic plants that bear their reproductive structures on their leaves, but otherwise are not closely related to Paleozoic, so-called seed ferns. Four lineages -- exemplified by common genera -- are generally recognized for the late Paleozoic pteridosperms.

There is certainly more diversity than captured in this simple treatment, but until sufficient material for phylogenetic analysis is available, the richness and relationships among the pteridosperms will remain somewhat hazy. In the meantime, we will look at Medullosa as an example of the group which has been treated phylogenetically.

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