Psilophytopsida

Psilophytopsida is a now obsolete class containing one order, Psilophytales, which was previously used to classify a number of extinct plants which are now placed elsewhere. The class was established in 1917, under the name Psilophyta, with only three genera (Rhynia, Horneophyton and Psilophyton)^[1] for a group of fossil plants from the Upper Silurian and Devonian periods which lack true roots and leaves, but have a vascular system within a branching cylindrical stem. The living Psilotaceae, the whisk-ferns, were sometimes added to the class, which was then usually called Psilopsida. This classification is no longer in use.^[2]

The class should not be confused with the current use of the name Psilotopsida, which refers to a class of living ferns, containing only Psilotaceae (whisk-ferns) and Ophioglossaceae (moon-worts and adder's-tongue ferns).^[3]

Description

The class was created in 1917 by Kidston and Lang for fossils found in the Rhynie Chert Bed.^[4] Three genera were initially included, Rhynia, Horneophyton and Psilophyton. All lacked leaves and true roots, consisting only of branched stems; however they were considered to contain vascular tissue.^[1]

Additional fossil genera were added later. As described by Sporne in 1966, Psilophytopsida consisted of four families:^[5]

• Rhyniaceae
  • Rhynia – now placed in the class Rhyniopsida^[6]
  • Horneophyton – now placed in the class Horneophytopsida^[6]
  • Cooksonia – now considered to be paraphyletic basal trachaeophytes^[6]
  • Yarravia – uncertain position^[6]
• Zosterophyllaceae – now placed in the class Zosterophyllopsida^[7]
  • Zosterophyllum
• Psilophytaceae
  • Psilophyton – now considered to be a basal euphyllophyte^[8]
• Asteroxylaceae – now placed in the order Drepanophycales^[9]
  • Asteroxylon

By 1975, it had become clear that the class had become increasingly unnatural, containing unrelated early vascular plants. It was split up by Banks into three subdivisions: Rhyniophytina, Zosterophyllophytina, and Trimerophytina. Later cladistic analyses of early land plants suggested that at least the rhyniophytes and the trimerophytes were not monophyletic. Separating out 'basal groups', such as the earliest land plants, is intrinsically difficult, since at this stage they contain many shared characters (plesiomorphies) which are not sufficient to distinguish them.^[10]

The current classification of former members of the class is largely due to Kenrick and Crane in 1997.^[6]

References

1. Crane, P.R.; Herendeen, P. & Friis, E.M. (2004), "Fossils and plant phylogeny", American Journal of Botany 91 (10): 1683–99, doi:10.3732/ajb.91.10.1683, PMID 21652317
2. Taylor, T.N.; Taylor, E.L. & Krings, M. (2009), Paleobotany : The Biology and Evolution of Fossil Plants (2nd ed.), Amsterdam; Boston: Academic Press, ISBN 978-0-12-373972-8, p. 226
3. Smith, A.R.; Pryer, K.M.; Schuettpelz, E.; Korall, P.; Schneider, H. & Wolf, P.G. (2006), "A classification for extant ferns" (PDF), Taxon 55 (3): 705–31, doi:10.2307/25065646, JSTOR 25065646, retrieved 2011-01-28 Cite uses deprecated parameter |coauthors= (help)
4. Kidston, R. & Lang, W.H. (1917), "On Old Red Sandstone plants showing structure, from the Rhynie Chert Bed, Aberdeenshire. Part I. Rhynia GwynneVaughani", Transactions of the Royal Society of Edinburgh 51 (761-84), cited in Crane, Herendeen & Friis 2004
5. Sporne, K.R. (1966), The Morphology of Pteridophytes (2nd ed.), London: Hutchinson, ISBN 978-0-09-104881-5, p. 28ff.
6. Kenrick, Paul & Crane, Peter R. (1997), The Origin and Early Diversification of Land Plants: A Cladistic Study, Washington, D.C.: Smithsonian Institution Press, ISBN 978-1-56098-730-7, pp. 139–40, 249
7. Stewart, W.N. & Rothwell, G.W. (1983), Paleobotany and the Evolution of Plants (2nd ed.), New York: Cambridge University Press, ISBN 978-0-521-23315-6, quoted in Waggoner, Ben (1996), Introduction to the Zosterophylls, archived from the original on 2011-01-26
8. Arens, N.C.; Strömberg, C. & Thompson, A. (1998), Lab VI The Origin of Seed Plants, archived from the original on 2011-01-26
9. Gensel, P.G. (1992), "Phylogenetic relationships of the zosterophylls and lycopsids: evidence from morphology, paleoecology, and cladistic methods of inference", Annals of the Missouri Botanical Garden 79: 450–73, doi:10.2307/2399750, JSTOR 2399750
10. Taylor, Taylor & Krings 2009, p. 227