"On the Origin of Phyla" is essentially about the Cambrian radiation, the event that gave rise to most of the major animal groups. The book's particular strength is its integration of data from paleontology and biology.
The concept of a phylum is not straightforward. A phylum has been characterized as a group of species that share a unique body plan or organization that reveals no evidence of relationship to other phyla. This definition may be relatively easy to apply to living animals but often proves problematic for extinct ones. Phyla originated hundreds of millions of years ago, and extinction has weeded out intermediate forms, leaving significant differences between the living phyla. There is no difficulty, for example, in distinguishing between an arthropod (say, a spider), an echinoderm (a starfish, for example) and a chordate (such as your department chairperson). Fossils, however, particularly those of the Paleozoic Era (from 543 to 251 million years ago), may be more difficult to deal with. Their morphology may be unfamiliar, and there is the problem that information on the soft tissues has usually been lost through decay and is not preserved.
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Stephen Jay Gould's major treatment of the Cambrian radiation, Wonderful Life (1989), was based mainly on the famous fossils of the Burgess Shale. Gould considered many of the Burgess Shale animals to represent extinct phyla or body plans, and he argued that their large number was a measure of the impact of the Cambrian explosion. Just 15 years later, Valentine's book demonstrates that there has been a sea change in the focus on phyla, from considering them as a measure of separation to exploring relationships between them—something that is impossible to do, almost by definition, on the basis of morphological data, but has become a realistic goal in the age of molecular sequencing.
Fossils, however, remain central if we want to understand the early evolution of phyla,
we need the evidence provided by fossils from the Neoproterozoic Era (from 1 billion to 543 million years ago) and the Cambrian Period.Spriggina from the Ediacaran beds of South Australia (scale bar, 1 cm).
