Fossil Vertebrates: Jawless Fish-Like Vertebrates

Jawless fishes (agnathans) have a fossil record that spans almost 500 million years, now extending back to the Early Cambrian of China (see Palaeozoic: Cambrian). They were particularly prominent during the Late Silurian and Devonian (see Palaeozoic: Devonian; Silurian) when they radiated to form a variety of diverse groups, most of them characterised by the presence of bony armour plating over the head and body. They are valuable in biostratigraphy, helping to determine the placement (see Stratigraphical Principles) of continents during the Palaeozoic and they chart the development of many features important in vertebrate history such as the appearance of bone, paired fins, lateral line systems, and an inner ear with two semicircular canals. During the last decade there has been a dramatic increase in the amount and quality of information about the origin of vertebrates and the diversification of the earliest forms. This has stemmed partly from new finds that have provided us with examples of early forms and also from new methods of analysis, which have enabled the development of a better understanding of relationships (see Micropalaeontological Techniques).