Trilobites became extinct in the Late Permian, so they have no modern forms.
The Permian extinction event caused ~95% of all marine life and ~75% of all
terrestrial life to become extinct. There have been many suggestions for the cause(s)
of the event, given in more detail here.
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