On March 9, back in the Ordovician, we talked about life invading the land. But I think you need to not think of it as an invasion, but more of a gentle encroachment. There wasn’t much beyond moss-like plants and algae, maybe some fungus, and we really only know about them from spores.
It wasn’t until the Silurian that we find “real” plant and animal fossils that are definitely from land-dwellers, including air-breathing animals. Plants had to evolve mechanisms to maintain water once they were out of the sea. That included a skin of sorts, to keep the water in, and pores to manage the water content. By the Silurian, plants were developing vascular systems to send water (and the nutrients it carried) here and there throughout their bodies. Plants called Cooksonia and related varieties are the oldest such plants we know. They’ve been found in Silurian rocks of Wales and England.
The first true air-breathing animals were things like millipedes and centipedes, and possibly spiders and scorpions – all arthropods. The oldest of all is called Pneumodesmus, and it’s a probable millipede from Scotland that lived about 428 million years ago, about the middle of the Silurian Period. It was discovered in 2004 and there’s only one specimen – but it has structures that indicate it was an air-breather. It would take 50 million years before the first vertebrate came ashore to live – during the Devonian, which we’ll get to next month.
—Richard I. Gibson
Photo by Xenarachne via Wikipedia under Creative Commons license