The Palaeozoic Fauna is largely made up of marine invertebrates, in the earlier periods entirely so; i.e. so far as we have yet learned, though land life surely began before the oldest records of it yet discovered. Graptolites and Hydroid Corals, true Corals, Echinoderms (especially Crinoids, Cystideans, and Blastoids), long-hinged and hingeless Brachiopods, Mollusca (particularly the Nautiloid Cephalopods), and the crustacean groups of Trilo-bites and Eurypterida are the most abundant and characteristic types of animal life. Insects, centipedes, and spiders were common toward the end of the era. Cambrian rocks contain no fossil vertebrates, but they make their appearance in the Ordovician. For long ages the only vertebrates were fishes and certain low types allied to the fishes, but at the end of the Devonian and in the Carboniferous appeared the Amphibia, followed in the Permian by true Reptiles. Teleosts, such as make up by far the largest part of the modern fish-fauna, both marine and fresh-water, as well as birds and mammals, are entirely absent from the Palaeozoic.

The overwhelming majority of Palaeozoic species, and even genera, fail to pass over into the Mesozoic, and even in the larger groups which continued to flourish almost always a more or less complete change of structure occurs, so that Palaeozoic corals, Echinoderms, and fishes, for example, are very markedly distinct from those which succeeded them. The difference is generally in the direction of greater primitiveness of structure in the older forms, Palaeozoic types standing in somewhat the same relation to subsequent types as the embryo does to the adult.

In the vast periods of time included in the Palaeozoic era occurred some remarkable climatic vicissitudes, which will be more fully described in the succeeding sections. Times of widespread glaciation occurred in the Lower Cambrian of Norway and China, probably of Australia, and perhaps also of South Africa; in the Devonian of South Africa, and in the Permian of the latter region, India, Australia, and South America, perhaps also in Europe and North America.

For most of the era, however, the climate appears to have been mild and equable on the whole, very much the same kinds of animals and plants occurring in high as in low latitudes. In short, we can detect no evidence of climatic zones as being distinctly marked in those periods.