Life in the Palaeozoic
This is a free, online textbook/course that starts "by looking at the Cambrian explosion, when many forms of animal life first appeared about 545 million years ago. Then we move on to study creatures living in the Ordovician seas, including the extinct trilobites. Next, we'll investigate the invasion of land by plants and invertebrates that occurred in the Silurian Period, and look at life in Silurian seas. You'll also learn about the Devonian Period, when vertebrates first moved onto land. The unit finishes with a brief outline of vertebrate evolution."