The vertebrates or Vertebrata (see below) form an ancient group with a history spanning some 545 million years.
On the one hand, they include the organisms most familiar to us, such as fish, birds, cats and dogs, as well as other humans; on the other, few people are aware of the great diversity in their form, structure, and habits.
Indeed, they include some of the largest and more complex organisms ever evolved. But vertebrates are part of a larger grouping of animals, and to understand their history and the development of their structure, they must be placed in phylogenetic context.In discussing vertebrates, several other groups of organisms are usually considered. A group of organisms is referred to as a taxon (plur., taxa). The taxa related to vertebrates include the Echinodermata (sand dollars, sea lilies, starfish, sea cucumbers, urchins), Hemichordata (acorn worms and pterobranchs), Urochordata (tunicates or sea squirts), and Cephalochordata (amphioxus). These are the typical nonvertebrate (or “invertebrate”) relatives of the group we are mainly interested in. The vertebrates themselves, or Vertebrata, are included in a larger taxon termed the Craniata. Within the Craniata and Vertebrata are many taxa. These taxa and the relationships among them (see Figure 1.1) are briefly outlined below to provide an organizational framework for undertaking the dissection of the vertebrates discussed in this manual. Before this, however, it is necessary to present an explanation of several important terms used in discussions of phylogeny.