Wednesday, January 11, 2017

A new phylogenetic classification of ferns


Just a couple days ago a new, phylogenetic classification of ferns and lycophytes was published. In parallel with the well-established Angiosperm Phylogeny Group, the authors call themselves the Pteridophyte Phylogeny Group.

I was happy to see that publication for several reasons. The PPG includes several people I know or am friends with, so good for them. As mentioned, the classification is sensibly phylogenetic, so good for science. And personally I have a great (although non-research) interest in ferns, so I find it simply good to get this update.

So, what do we learn? What is the state of the art?

The lycophytes are well established as sister to all other vascular plants, and the main groups Lycopodiaceae, Isoetaceae and Selaginellaceae are not in any doubt whatsoever. What surprised me was the degree to which the Lycopodiaceae have been atomised into numerous mid-sized to monotypic genera. I have no idea what degree of divergence is recognised in this subdivision, but it seems a bit odd next to the Selaginellaceae with their single large genus.

The remaining groups are forming together what was so far known to me as the monilophytes, but here they are called Polypodiopsida. They are sister to the extant seed plants.

A monilophyte group that I am particularly fond of are the horsetails (genus Equisetum), here ranked all the way up to subclass. Pity there are none native to Australia. Next are the morphologically reduced Psilotales with their traditional genera Tmesipteris and Psilotum, again nothing unexpected.

Sister to them are the odd Ophioglossales, which, however, present the very same surprise as the Lycopodiaceae. I thought I knew them as featuring four easily explained genera: Ophioglossum with usually undivided fertile and sterile parts of the frond, Botrychium with pinnately divided leaves, and the somewhat palmately divided and monotypic Helminthostachys and Mankyua, the latter only described in 2002 and endemic to one Korean island (!). But the two larger genera are here divided into several genera, a tradition that I apparently was completely unaware of.

The last eusporangiate group are the Marattiaceae, a clade of large tropical ferns that had its heyday in the Carboniferous. As far as I can tell the classification is not much changed from what I read years ago.

Finally, we have the large diversity of the leptosporangiate clade. Many groups I am insufficiently familiar with to appreciate any potential changes in classification, but the higher level structure is well known. There is a grade of smaller groups - king ferns, filmy ferns, Gleicheniaceae & relatives, Schizaeaceae & Lygodiaceae, water ferns and tree ferns - and at its end the speciose Polypodiales clade, where it really gets complicated.

Apart from a few Polypodiales families that have piqued my interest, like Aspleniaceae or Dennstaedtiaceae, I really have no even half-informed opinion here. The point is that I can now use this new publication as a tour guide to figuring out where in the system I am when I next run into one of those generic, large-fronded, rosette-growing round ferns.

Reference

The Pteridophyte Phylogeny Group 2016. A community-derived classification for extant lycophytes and ferns. Journal of Systematics and Evolution 54: 563-603.

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