between subclass Equisetidae and the other fern subclasses according to the Pteridophyte Phylogeny Group. A 2018 study by Elgorriaga et al. suggests the relationships within the Equisetidae are as shown in the following cladogram. According to the study, the age of the crown group of Equisetum dates at least to the Early Cretaceous, and most probably up to the Jurassic. Subdivision Subclass Equisetidae contains a single extant order, Equisetales. This order consists of a single monotypic family, Equisetaceae, with one genus Equisetum. Equisetum has about 20 species. Fossil record The extant horsetails represent a tiny fraction of horsetail diversity in the past. There were three orders of the Equisetidae. The Pseudoborniales first appeared in the late Devonian. The Sphenophyllales were a dominant member of the Carboniferous understory, and prospered until the mid and early Permian. The Equisetales existed alongside the Sphenophyllales, but diversified as that group disappeared into extinction, gradually dwindling in diversity to today's single genus Equisetum. The organisms first appear in the fossil record during the late Devonian, a time when land plants were undergoing a rapid diversification, with roots, seeds and leaves having only just evolved. (See Evolutionary history of plants) However, plants had already been on the land for almost a hundred million years, with the first evidence of land plants dating to . References Plant subclasses Extant Late Devonian first appearances Late Devonian plants