certainly a polysporangiophyte, have been dated to the succeeding Wenlock epoch (). Taxonomy The concept of the polysporangiophytes, more formally called Polysporangiophyta, was first published in 1997 by Kenrick and Crane. (The taxobox at the right represents their view of the classification of the polysporangiophytes.) The defining feature of the clade is that the sporophyte branches and bears multiple sporangia. This distinguishes polysporangiophytes from liverworts, mosses and hornworts, which have unbranched sporophytes each with a single sporangium. Polysporangiophytes may or may not have vascular tissue – those that do are vascular plants or tracheophytes. Prior to that, most of the early polysporangiophytes had been placed in a single order, Psilophytales, in the class Psilophyta, established in 1917 by Kidston and Lang. The living Psilotaceae, the whisk-ferns, were sometimes added to the class, which was then usually called Psilopsida. As additional fossils were discovered and described, it became apparent that the Psilophyta were not a homogeneous group of plants. In 1975, Banks expanded on his earlier 1968 proposal that split it into three groups at the rank of subdivision. These groups have since been treated at the ranks of division, class and order. A variety of names have been used, which the table below summarizes. For Banks, rhyniophytes comprised simple leafless plants with terminal sporangia (e.g., Cooksonia, Rhynia) with centrarch xylem; zosterophylls comprised plants with lateral sporangia that split distally (away from their attachment) to release their spores, and had exarch strands of xylem (e.g., Gosslingia). Trimerophytes comprised plants with large clusters of downwards curving terminal sporangia that split along their length to release their spores and had centrarch xylem strands (e.g., Psilophyton). Research by Kenrick and Crane that established the polysporangiophytes concluded that none of Banks' three groups were monophyletic. The rhyniophytes included "protracheophytes", which were precursors to vascular plants (e.g., Horneophyton, Aglaophyton); basal tracheophytes (e.g., Stockmansella, Rhynia gwynne-vaughanii); and plants allied to the lineages that led to the living club-mosses and allies as well as ferns and seed plants (e.g., Cooksonia species). The zosterophylls did contain a monophyletic clade, but some genera previously included in the group fell outside this clade (e.g., Hicklingia, Nothia). The trimerophytes were paraphyletic stem groups to both the crown group ferns and the crown group seed plants. Many researchers have urged caution in the classification of early polysporangiophytes. Taylor et al. note that basal groups of early land plants are inherently difficult to characterize