The History of the Vertebrates

Abstract

The history of the vertebrates extends back in time nearly 500 millions of years. The first records are fragmentary and scattered in the rocks of the Ordovician period (Fig. 1). After a lengthy gap from the Middle Ordovician to the Late Silurian, the record is reasonably continuous to the present time. The remains of vertebrates, with relatively rare exceptions, consist of hard parts bones, calcified cartilage, teeth and scales. The masticatory and locomotor systems are especially well documented and a great deal of the evolutionary history of vertebrates is written in the changes of structures related to these two systems. Less well documented, but still of primary importance, are the structures involved in reception and integration of information: the brain, cranial nerves, and sensory organs.