|Evolution of plants
ferns The oldest
Four very old plants Parka Myriapods Crock Hey (Seed) ferns Scorpion Lepidodendron Sigillaria Calamites
Wood of Calamites Cordaites The leaf of Neuropteris Little animals Graissessac Psaronius Permian of Lodève
Bayreuth Yorkshire Gymnosperm wood Tree fern Tempskya Palm wood Hardwood Manosque Links Eight fossils
Cooksonia, a very old land plant (2)
of xylem vessels with annular or spiral-shaped thickenings at the walls counts
as characteristic for higher plants. Algae and mosses do not have such
D. Edwards (1992) was the first one to publish photos of xylem vessels of Cooksonia (Photo with permission of Nature Publishing Group). Edwards found the tracheids in axes with attached sporangia of the species C. hemisphaerica en C. pertoni from Lower Devonian river sediments in Shropshire.
Yet there are several snags in the assumption that every plant of
Cooksonia had tracheids. In the first place no tracheids have been
found in Silurian Cooksonia fossils. So it is possible that the tracheids
developed only in the course of time.
Research on spores of C. pertoni from different places in Great
Britain has revealed an evolutionary trend. The existence of four different
types of spores in C. pertoni has been proved: two are common and
two rare. There appears to be a tendency from smooth spores in the Silurian
to more ornamented ones in the Early Devonian. Of course the advantage of
ornamented spores for the plant remains uncertain, but it could possibly
be an adaptation to drought.
younger Cooksonia-fossils have been found in North-Wales, at a locality
in the Lower Ludlow (420 million years). These
fossils are diminutive. My biggest fossil from this place is a branchlet
with two bifurcations measuring 15 mms. Axes like these are sometimes placed
in the artificial genus Hostinella. Some of the axes show a black
line, probably pointing at the presence of xylem vessels.
Outside Great Britain Silurian Cooksonia-fossils have been found
in Canada (not identifiable at species name) , Bolivia (resembles C.
caledonica), Czech Republic (C. hemisphaerica), Kazakhstan (not
identifiable), China (the same), Siberia (C. pertoni, C.
hemisphaerica), the state of New York (not yet identified) and perhaps
Cooksonia also occurs in younger deposits. C. caledonica
has been described on the basis of Lower Devonian fossils from the area
around Forfar (Scotland).
Way of growing
Salt marshes have also been mentioned as possible growing places for
Cooksonia probably grew as a small shrub.