The evolution of vertebrates

Precambrian fossils were not discovered until the 1940s and in the 19th century it was thought there were no fossils before the Cambrian. The reason for the great proliferation of fossil evidence since the Cambrian is that it was the time when hard parts - shells in molluscs and skeletons in arthropods and echinoderms - first evolved. Vertebrates are another main group of animals with skeletons. Vertebrates may have evolved as early as the Cambrian (a group called the conodonts, which are thought to be early vertebrates, exist in the Cambrian); but there are no fossil fish before the Ordovician and they do not become abundant until the Silurian.

The image opposite is of a fossilized pikaia gracilens , from the famous Burgess Shale collection. It is the earliest known ancestor of the chordates: the phylum which includes all vertebrates.