Trimerophytopsida

Trimerophytopsida Temporal range: Devonian
Fossil of Psilophyton dawsonii
Scientific classification
Kingdom:	Plantae
Division:	Tracheophyta
Class:	†Trimerophytopsida
Order:	†Trimerophytales
Genera
†Eophyllophyton †Psilophyton †Trimerophyton

Trimerophytopsida (or Trimeropsida) is a class of early vascular plants from the Devonian, informally called trimerophytes. It contains genera such as Psilophyton. This group is probably paraphyletic, and is believed to be the ancestral group from which both the ferns and seed plants evolved. Different authors have treated the group at different taxonomic ranks using the names Trimerophyta, Trimerophytophyta, Trimerophytina, Trimerophytophytina and Trimerophytales.

Taxonomy

At first most of the early land plants other than the bryophytes (i.e. the polysporangiophytes) were placed in a single class Psilophyta, established in 1917 by Kidston and Lang. As additional fossils were discovered and described, it became apparent that the Psilophyta were not a homogeneous group of plants. In 1968 Banks first proposed splitting this taxon into three groups, which he put at the rank of subdivision; he clarified his proposal in 1975. One of the three groups was the Trimerophytina. The subdivision is based on the type genus Trimerophyton, which might be expected to produce 'Trimerophytophytina' as the name of the subdivision, but the International Code of Nomenclature for algae, fungi, and plants allows the 'phyton' part of a genus name optionally to be omitted before '-ophyta', '-ophytina' and '-opsida'.

The group has also since been treated as a division under the name Trimerophyta or Trimerophytophyta, as a class under the name Trimeropsida or Trimerophytopsida (as here), and as an order under the name Trimerophytales.

See also

• Euphyllophyta