ABSTRACT
Despite their diversity, the fact that they share these four unique derived characters, or synapomorphies, suggests that chordates are a monophyletic group (Nielsen, 1995).
It may seem strange to refer to agnathan fish as “vertebrates” given that they do not possess bone and hence lack true vertebrae. However, these primitive fish exhibit the same basic set of specialized characters seen in all
vertebrates, including the cranium. The vertebrate head contains welldeveloped sensory organs (e.g., eyes, ears, gustatory papillae), as well as sensory and motor nerves with peripheral ganglia. These cranial nerves enable the CNS to communicate with the periphery through cavities that can be identified in fossilized crania. The general organization of the vertebrate brain is also quite stable and, although there are many variations, there is a fundamental theme that makes broad sections of the CNS easily identifiable across species. Figure 6.1 shows a prototypical vertebrate brain and the major planes of reference.
