The beginning and end of each chunk of time in the geologic time scale is determined by when some species appeared or disappeared from the fossil record ... The end of the Paleozoic and beginning of ...

The Devonian, part of the Paleozoic era, is otherwise known as ... fish that lacked jaws developed a range of bizarre forms. Fossil specimens include species with horseshoe-shaped heads and ...

3 min read During the Ordovician period, part of the Paleozoic era ... known mainly from the tiny fossil teeth they left behind. The few complete fossils that have been found suggest they were ...