April 25. Cooksonia

We talked about early plants on April 11, and last month I mentioned that there are some fossil spores from the Ordovician that indicate there were some primitive plants on land that long ago. But really, even though there are some early to middle Silurian land plants known, there still wasn’t much, even towards the end of the Silurian, coming up at about 416 million years ago. 

Most of the land surface was bare rock or the kind of soil that forms from the physical weathering of rock. Plants seem to have mostly lived mostly along sea coasts, in wetlands, and it’s reasonable to suppose they might have grown along some rivers. But even with that limited range, they were still pretty much worldwide – from what is now Greenland to Siberia to Australia – all areas that were within the temperate or tropical zones during the Silurian.

Cooksonia

Cooksonia was the first plant on land that had an upright stalk, rather than mossy ground cover. It was a vascular plant, meaning it had a system for delivering nutrients and water around its body. It wasn’t very tall, though – just a few inches at most. It seems that the stalk was mostly there to help disperse the plant’s spores, which were in clusters on the tips of the branched stalks. Cooksonia was discovered in 1937 in very late Silurian rocks in England and Wales, but it’s since been found all over the world.

Cooksonia didn’t have leaves, or flowers, or a root system. William Lang, who described the first specimens, named it for Isabel Cookson, an Australian botanist and paleobotanist who worked with him in Britain. Her work on Silurian and Devonian plants was important in establishing theories of the evolution of land plants.

So, we’re getting there – the land isn’t just bare rock any more. Short stalked plants, mosses, and a millipede here and there to eat the dead plant detritus, and that’s about it. But it was a start.

—Richard I. Gibson

Drawing by Smith609 via Wikipedia, under Creative Commons license