Fossil record

Trilobites left an amazing wealth of fossils during their 350 million years on Earth. These fossils are of some stratigraphic value during the Cambrian and Ordovician. From the fossil record it is possible to see their diversity increase and decrease until their eventual extinction.
During the Cambrian trilobites underwent several radiations and they suffered some setbacks. The number of families steadily increased through the Lower Cambrian then in the Mid Cambrian there is a small drop in numbers. They recovered and reached their most abundant in the Late Cambrian with a total of 63 families (see table).
Ordovician. Trilobites started to specialise and their numbers decreased with only 42 families left in the Late Ordovician. There was a dramatic drop in family numbers between the Late Ordovician and the Early Silurian. The family numbers dropped by over a half from 42 to just 19. The Ordovician extinction event took place between 440 and 450 million years ago. The Ordovician extinctions were due to an ice age causing global cooling and a loss of living space on the continental shelves due to the lowering of the sea level. It is the second most devastating extinction event in the history of Earth.
During the Silurian the number of families stayed fairly constant at 19, with a small drop to 17. During the Devonian, numbers slowly fell to 5. The Carboniferous saw numbers fall to just 4. During the Permian only 3 families survived, but they were unable to survive the Permian extinction event.

Websites produced by students on the MSc Palaeobiology programme in the Department of Earth Sciences at the University of Bristol for academic year 2003-4