TRILOBITES ARE AN EXTINCT GROUP OF MARINE ARTHROPODS THAT LIVED DURING THE PALEOZOIC ERA , RUOGHLY FROM THE CAMBRIAN PERIOD ABOUT 521 MILLION YEARS AGO TO THE END OF THE PERMIAN PERIOD ABOUT 252 MILLION YEARS AGO . THEY ARE AMONG THE MOST WELL KNOWN AND STUDIED FOSSILS , WHIT OVER 20,OOO SPECIES DESCRIBED
