# Psilophytopsida

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**Psilophytopsida** is a now obsolete [class](/source/Class_(biology)) containing one [order](/source/Order_(biology)), **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](/source/Fossil) plants from the Upper [Silurian](/source/Silurian) and [Devonian](/source/Devonian) periods which lack true [roots](/source/Root) and [leaves](/source/Leaf), but have a [vascular system](/source/Vascular_tissue) within a branching cylindrical stem. The living [Psilotaceae](/source/Whisk_fern), 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](/source/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](/source/Rhynie_chert).[4] Three genera were initially included, *[Rhynia](/source/Rhynia)*, *[Horneophyton](/source/Horneophyton)* and *[Psilophyton](/source/Psilophyton)*. All lacked leaves and true roots, consisting only of branched stems; however they were considered to contain [vascular tissue](/source/Vascular_tissue).[1]

Additional fossil genera were added later. As described by [Sporne](/source/Kenneth_Robert_Sporne) in 1966, Psilophytopsida consisted of four families:[5]

- Rhyniaceae - *Rhynia* – now placed in the class [Rhyniopsida](/source/Rhyniopsida)[6] - *Horneophyton* – now placed in the class [Horneophytopsida](/source/Horneophytopsida)[6] - *[Cooksonia](/source/Cooksonia)* – now considered to be [paraphyletic](/source/Paraphyletic) basal [trachaeophytes](/source/Vascular_plant)[6] - *[Yarravia](/source/Yarravia)* – uncertain position[6]
- Zosterophyllaceae – now placed in the class [Zosterophyllopsida](/source/Zosterophyllopsida)[7] - *[Zosterophyllum](/source/Zosterophyllum)*
- Psilophytaceae - *[Psilophyton](/source/Psilophyton)* – now considered to be a basal [euphyllophyte](/source/Euphyllophyte)[8]
- Asteroxylaceae – now placed in the order [Drepanophycales](/source/Drepanophycales)[9] - *[Asteroxylon](/source/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](/source/Monophyletic). Separating out 'basal groups', such as the earliest land plants, is intrinsically difficult, since at this stage they contain many shared characters ([plesiomorphies](/source/Plesiomorphy)) 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]

See also: [Polysporangiophyte](/source/Polysporangiophyte) and [Evolutionary history of plants](/source/Evolutionary_history_of_plants)

## 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](https://doi.org/10.3732/ajb.91.10.1683). [PMID 21652317](https://pubmed.ncbi.nlm.nih.gov/21652317)

1. 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

1. Smith, A.R.; Pryer, K.M.; Schuettpelz, E.; Korall, P.; Schneider, H.; Wolf, P.G. (2006), ["A classification for extant ferns"](http://www.pryerlab.net/publication/fichier749.pdf), *Taxon*. **55** (3): 705–31, [doi:10.2307/25065646](https://doi.org/10.2307/25065646). [JSTOR 25065646](https://www.jstor.org/stable/25065646), retrieved 2011-01-28

1. Kidston, R. & Lang, W.H. (1917), ["On Old Red Sandstone plants showing structure, from the Rhynie Chert Bed, Aberdeenshire. Part I. *Rhynia GwynneVaughani*"](https://zenodo.org/record/1428664), *Transactions of the Royal Society of Edinburgh*. **51** (761–84), [doi:10.1017/S0080456800008991](https://doi.org/10.1017/S0080456800008991), cited in Crane, Herendeen & Friis 2004

1. Sporne, K.R. (1966), *The Morphology of Pteridophytes*, 2nd ed., London: Hutchinson, ISBN 978-0-09-104881-5, p. 28ff.

1. 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

1. 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"](http://www.ucmp.berkeley.edu/plants/lycophyta/zostero.html), [archived](https://www.webcitation.org/5w28k2lf7?url=http://www.ucmp.berkeley.edu/plants/lycophyta/zostero.html) 2011-01-26 at the Wayback Machine

1. Arens, N.C.; Strömberg, C.; & Thompson, A. (1998), ["Lab VI The Origin of Seed Plants"](http://www.ucmp.berkeley.edu/IB181/VPL/Osp/Osp1.html), [archived](https://web.archive.org/web/20101203075141/http://www.ucmp.berkeley.edu/IB181/VPL/Osp/Osp1.html) 2010-12-03 at the Wayback Machine

1. Gensel, P.G. (1992), ["Phylogenetic relationships of the zosterophylls and lycopsids: evidence from morphology, paleoecology, and cladistic methods of inference"](http://www.biodiversitylibrary.org/pdf3/009299400054744.pdf), *Annals of the Missouri Botanical Garden*. **79** (3): 450–73, [doi:10.2307/2399750](https://doi.org/10.2307/2399750). [JSTOR 2399750](https://www.jstor.org/stable/2399750)

1. Taylor, Taylor & Krings 2009, p. 227

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Adapted from the Wikipedia article [Psilophytopsida](https://en.wikipedia.org/wiki/Psilophytopsida) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Psilophytopsida?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.