Up until now, all the life we’ve been talking about has been in the sea. OK, maybe some trilobites ran up onto a wet beach above the water’s edge, and maybe some of the shelly critters lived in the intertidal zone, just as such animals do today, but they were fundamentally dependent on ocean water for their existence. And algae and bacteria can live almost anywhere today, so maybe they were exploiting some of the niches on land at a pretty early date.
It probably won’t surprise you to hear that the first big life on land was plants – moss-like plants, to be specific. The earliest evidence for them is spores in the fossil record of the Middle Ordovician, about 470 million years ago or maybe a few million years older. The spores are similar to the spores of modern liverworts, which are mossy plants. Spores from more complex vascular plants are found in Upper Ordovician rocks. Vascular plants have tissues for shipping fluids and nutrients around their bodies, and include modern trees and flowering plants.
Once plants were on land, they began the chemical weathering of rocks, which until then had largely been subjected mainly to physical weathering, breaking apart by freezing and thawing and such. Certainly there were some chemical reactions among rocks, water, and the atmosphere, but plants must have accelerated that chemical alteration considerably.
Chemical weathering of rocks removes things like calcium and other nutrients that plants can use – and combined with photosynthesis, which converts carbon dioxide to carbon and oxygen – might have removed enough CO2 from the earth’s Ordovician atmosphere to lead to a decrease in average temperature. So much so, according to some speculations, that it might have contributed to the onset of the Late Ordovician ice age. We’ll talk about that glacial period towards the end of the Ordovician, later in March. But never underestimate the power of plants acting over a long period of time!
We don’t find actual body fossils of plants in the geologic record until the Silurian, but the fossil spores in Ordovician rocks are pretty conclusive evidence that there were plants on land by Middle Ordovician time.
—Richard I. Gibson
Liverworts image from Ernst Haeckel's Kunstformen der Natur, 1904 (public domain)
Moss froze the planet?