The exploion of small shelly fossils and the classical explosion of trilobites, in other words, did not exceed 5 million years, and today this does look as the maximum time span of those great transformations.
By far the most important discovery, however, has come from biological studies, and more precisely from the comparative anatomy of fossils. This analysis proved that Cambrian animals invented the body plans of all the animals that have appeared on Earth ever since.
The Cambrian explosion was traditionally defined as the appearence of the first skeleton-bearing metazoa, but now we can characterize it with a vastly more general event, and say that it was the geologically sudden appearence of all known animal phyla. After the Cambrian explosion, in other words, many lower animal taxa came into being – new classes, new orders, new families, new genera and countless new species – but our planet has never seen again a new phylum.

Body plans and phylotypic stages

The concepts of body plan and phylum were introduced in biology (with French names) by Georges Cuvier in the first decades of the 19th century, together with other important concepts of comparative anatomy, the science of which Cuvier is rightly considered the founding father. A body plan is a set of anatomical characters that describe the spatial organization of the body’s organs, and a phylum is a group of animals which share the same body plan. The animals of the phylum Arthropoda (invertebrates), for example, have an external skeleton, a ventral nervous system and a dorsal heart, while those of the phylum Chordata (vertebrates) have internal skeleton, dorsal nervous system and ventral heart (Figure 7-2).
For a strange coincidence of history, the concept of phylum was also discovered in the very same years by Ernst von Baer, with a totally different method from Cuvier’s. Instead of adult animals, von Baer was studying the embryos of different classes of vertebrates, and discovered that there is an early stage of development where the embryos of pisces, amphibians, reptiles, birds and mammals are practically indistinguishable (it seems that the discovery happened because one evening von Baer forgot to label the bottles of his samples, and the next day could no longer recognize the class of the embryos).