Rhyniophyta

From Palaeos

Status

The rhyniophytes are a paraphyletic taxon comprising the first vascular land plants (Rhyniopsida) and their morphologically similar, but avascular, cousins. They thus have a key position in the history of life. The coverage includes a brief discussion of their structure and importance on this page. Individual pages are included on two very early forms, Horneophyton and Aglaophyton, as well as on two well-known, derived members of the monophyletic Rhyniopsida, Cooksonia and Rhynia.

The flora that existed in the Upper Silurian to Lower Devonian was probably an assemblage of marsh inhabitants with structural features transitional between aquatic non-vascular and land vascular plants. Of these, the Rhyniophyta are a small but prominent group that include some of the earliest and most primitive terrestrial plants.

The Early Rhyniophytes

In these very primitive land plants in which there was little differentiation between the different parts. They had simple shoots rise from a creeping axis which hardly differs in structure from the upright shoots themselves. These shoots or stems often branched in a simple manner, forking into two, and then into two again, and sometimes the shoots terminated in spore capsules called sporangia. These ancient plants lacked leaves, seeds, and flowers. Instead of roots they had horizontal stems, connected with the soil by root hairs. The whole plant was generally less than 50 cm in height. At least one form, Taeniocrada, is thought to have been aquatic, although whether as a transitional form from fresh water to land or secondarily aquatic is not clear.

Included in the rhyniophytes are the ancestral genus Cooksonia, from the late Silurian to early Devonian (Wenlock to Frasnian). By the Early Devonian more advanced types like Rhynia had evolved. Fossils of Rhynia from Devonian chert of Rhynie in Aberdeenshire, Scotland are so perfectly preserved that the individual cells within the plant tissue are recognizable.

There are also a number of related forms, like Aglaophyton, that are apparently not even true tracheophytes.

Systematics

The following represents a suggested Linnean classification and time-line for the rhyniophytes and related forms. Not that the most primitive forms are not necessarily the oldest, indicating that either the evolutionary sequence presented here is incorrect, or that a lot of early plant evolution occurred in regions away from conditions allowing fossilization

Subphylum Rhyniophytina

Class not specified

Order not specified

Aglaophyton - Pragian

Class Horneophytopsida

Order Horneophytales

Family Horneophytaceae - Pragian

Class Rhyniopsida

Order not specified

Cooksonia - Wenlock to Frasnian

other rhyniophyte fossils

Order Rhyniales