Common names cause a lot of grief, but they remain friendly, although often uninformative or misleading, easy to remember, and at times they are useful as shorthand references. They certainly have their place in teaching and outreach. Now here's the particular problem. Molecular data nests the horsetails within the lineage of what traditionally have been called ferns. OK, fair enough, but that does not make horsetails into ferns. In a classification of extant (living) ferns published in 2006 my colleagues label the whole lineage "ferns". Right now the Phactor does not wish to argue about whisk ferns (also called psilophytes) in this same context. But horsetails have been distinct from ferns back to the point when there were neither ferns nor horsetails, but merely a group of plants that were the likely common ancestors of ferns and horsetails, and even within this group their respective ancestors were different. Traditionally, the lower vascular plants (those without seeds) were called the pteridophytes - the fern-like plants. Now this is fine and everyone understood what was meant, but the beautiful people have decided that because pteridophytes used to include the clubmosses, the term must be avoided. True, the clubmosses are a separate lineage, so you must re-define pteridophyte to exclude the clubmosses, and this type of thing is done all the time. Pteridophyte always had a broader connotation than the term "fern"; it basically meant the "ferns and fern allies (which also included clubmosses)". When you say fern, the image your mind forms is not at all what you see when someone says "horsetail". Until people get used to the narrower definition of pteridophyte there will be some confusion, but certainly that produces no greater confusion than calling horsetails "ferns". Part of the trouble is that the plants traditionally considered "ferns" do not form a single lineage; no lineage of ferns exists unless you include the whisk ferns and horsetails, but now you've got pteridophytes. So the Phactor still sees considerable utility to pteridophyte to label the whole lineage, although now redefined to exclude clubmosses, because he cannot bring himself to call a horsetail a fern. If anyone wants this publication, go to this link (Kathleen Pryer's web page), scroll down to 2006; it's the 1st entry. What think you RE ferns and horsetails?
Book review: Leonard Mlodinow's "The Upright Thinkers:The Human Journey from Living in Trees to Understanding the Cosmos"
2 hours ago in The Curious Wavefunction