How plants conquered the land (I) (II)
Fossils tell the story
About fourhundred and fifty million years ago, at the end of the Ordovician and in the beginning of the Silurian, the land was desolate and empty. Barren, hardly wethered rockgrounds, empty sand-, gravel- and clayplains, no green. Maybe some lichens. And at wet spots some algae with a couple of spider-like little creatures creeping around. In the neighbourhood of the mouth of rivers, where the water regularly flooded the land, it was probably green with algae. At such places, e.g. in Australia, traces of big seascorpions have been found. There was not much happening on the land. Life enacted itself nearly completely in the water.
The oldest indications for the existence of real land plants have been found in cores from boreholes in Oman. They contained fours of mutually connected spores (tetrads) enveloped by remains of the spore sac in which they had been formed. Research on the spore walls point to a relationship with the liverworts. The fossils have been found in the Middle Ordovician and are about 475 million years old.
The first fossils of macroscopic land plants have been found in the
Middle Silurian of Ireland.
They are about 425 million years old. They consist of small bifurcations
some centimeters in size. Only in the very last part of the Silurian fossils
of land-plants become
more common and also more
complete. The best known plant from that time is called Cooksonia.
It is named after Isabel Cookson, who occupied herself with intensive collecting
and describing plantfossils.
The evolution from algae to land plants must have
been a lengthy process.
Species of Cooksonia are found at several places on earth, e.g. in Wales, Scotland, England, Czechia and Canada. The finding of a fairly complete plant is a rare occurrence. I was already very glad with the bifurcated little stem with two sporangia between Cooksonia-chaff on the photo. Cooksonia has become extinct in the Early Devonian.
At the time of the first Cooksonias a completely different group of plants has evolved, which tried to colonize the land. These plants are still enigmatic for scientists. Research for the real nature and the ecology of these plants is still in full progress. We shall mention three of them (there are more): Nematothallus, Parka and Pachytheca.
of Nematothallus look like black patches
measuring 0,5 to 6 cm. They are irregular in shape and sometimes there is
still a thick cuticle on the plant. It is possible to make a microscopic
preparation of a piece of cuticle and in some cases it turns out to have
a cell structure. As this has not developed in the way of cell division in
higher plants it is called pseudo cell structure. Under the cuticle is a
little mat consisting of very thin threads or tubes. The diameter of such
a tube varies from 3 µms to 40 µms (1 µm = one thousandst
of a millimeter). Spores have also been found in the mats. Apparently
Nematothallus did consist of a thallus of fine threads (Gr. nemato
= thread) covered by a thick cuticle, sometimes with a pseudo cell structure.
Plants like these do not exist any more, however there are recent lichens
resembling Nematothallus in several aspects.
A second enigmatic plant is Parka. This is a flat, circular to irregular patch covered by discs about 2 mm wide. These are sporangia each containing some 35,000 spores. The diameter of this plant varies from 0,5 cm to 7 cm. Where the sporangia have disappeared from the fossils, and this is often the case, a kind of reticulum remains. For a long time Parka has been supposed to be a collection of some animal's eggs. Especially in the region of Forfar, north of Dundee in Scotland, Parka is very common. Click here for a more extensive description of Parka.
The third still unelucidated plant is Pachytheca. This fossil resembles a gleaming little globe with a diameter between 1 and 7 mms. In transverse the sphere has an outer layer with radial tubes and a nucleus in which the tubes are going in all directions. For a long time Pachytheca was thought to be part of a bigger plant, but nowadays it is firmly believed to be a complete organism. The fossils have been found in e.g. Belgium and Great-Britain. This plant too became extinct in the Early Devonian. Click here for a more extensive description of Pachytheca.
Probably these and similar alga-like plants can be considered as an evolutionary experiment to colonize the land. It seems having been a dead end. It is not unlikely that these plants have lost the increasing competition with the succesful higher plants.
At the photo below of a slab from Forfar in Scotland
four of these very old plants are represented: Nematothallus (1),
Parka (2), Pachytheca (3) and Zosterophyllum (4).
Use the cursor as a magnifying glass. The enlargement can be changed
by turning the mouse wheel.