The enigmatic plant Parka decipiens

This plant grew in the Late Silurian and the Early Devonian, about 400 million years ago.
Especially in the old quarries in the neighbourhood of Forfar, north of Dundee in Scotland, it is a common fossil. But it also occurs in other places in the world.

The fossil looks like a little patch 0.5 - 7.5 cms in diameter showing a reticulate structure on the surface. Its form is circular, elliptic or irregular.
When the state of preservation is very good there are small coaly discs in the 'meshes' of the reticulum.
By treating these discs with nitric acid and other chemicals it is possible to make the contents visible: a mass of little objects (about 35 µms in diameter) called spores. It is not certain whether they are proper spores, for they lack the trilete mark, a Y-like scar, occurring on most of the real spores.
The spores are always completely flattened and sometimes they show cracks and folds. A sporangium contains about 35000 spores.

Complete specimena of Parka decipiens show a border of 0.2 - 1.2 mms in breadth. Actually the sporangia were formed in this border. Indeed incomplete sporangia can be seen lying in the border of some specimena. These sporangia were in the process of formation.

Some specimena show a 'holdfast' in the centre. Presumably that was the plant's attachment to the soil.
A small percentage of the Parka fossils show a radial pattern of striations. Probably these were situated at the underside of the plant and had something to do with the attachment to the soil.

There have been all kinds of ideas about the true nature of Parka decipiens. In the previous century Parka decipiens has successively been taken for the inflorescence of a plant, snail's eggs, frog-spawn, a bramble-like fruit and the egg-packet of a seascorpion. The spores were discovered in 1891 and from that moment it was clear that Parka was a plant.

Still Parka is not completely understood and the systematic place is uncertain. The plant resembles some extant liverworts, but also a couple of extant algae. The chemical compo­sition indicates a relationship with green algues. Probably Parka was a member of group of enigmatic plants, which tried to colonize the still barren land. Perhaps the higher plants, like Cooksonia, which developed at the same time, frustrated this attempt.

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