The first vertebrates were fish that appeared 500 million years ago around the beginning of the Cambrian period (on the geologic time scale). They are called agnathans (from the Greek a, meaning “without,” and gnath, meaning “jaw”)—small, jawless fish up to about 8 inches (20 centimeters) long. They have also been called ostracoderms (“shell skin”) because their bodies were covered with bony plates, most notably a head shield protecting the brain.