The horsetails and their fossil relatives have long been recognized as distinct from other seedless vascular plants. Before the advent of modern molecular studies, the relationship of this group to other living and fossil plants was considered problematic. Because of their unclear relationships, the rank botanists have assigned to the horsetails varies from order to division. When recognized as a separate division, the literature uses many possible names, including Arthrophyta, Sphenophyta, or Equisetophyta. Other authors have regarded the same group as a class, either within a division consisting of the vascular plants or, more recently, within an expanded fern group. When ranked as a class, the group has been termed the Equisetopsida or Sphenopsida.
Recent phylogenetic analysis has produced evidence that this group of plants belongs firmly within the fern clade of vascular plants. A 2006 classification by Smith et al. places the class Equisetopsida within an unranked clade of broadly defined ferns, as a sister to two classes more traditionally called ferns, Marattiopsida and Polypodiopsida.
The probable relationships within Equisetopsida are shown in the cladogram below:
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