Ptychopariida is a large, heterogeneous order of trilobite containing some of the most primitive species known. The earliest species occurred in the second half of the Lower Cambrian, and the last species did not survive the Ordovician–Silurian extinction event. The thorax is large and is typically made up of eight or more segments. The thorax is usually much longer than the pygidium, which is usually small. In some species the pygidium is outlined with a flat border.