The Paleozoic Fossils of Ontario Rocks from the Pre-Cambrian to the Carboniferous are found in Southern Ontario.
Cambrian deposits are spotty and unfossiliferous.
The most diverse fossil fauna is in the marine and intertidal rocks laid down in the Ordovician, Silurian and Devonian periods. Rocks of the Ordovician period outcrop in the Ottawa Valley and in a north-south belt between the base of the Niagara Escarpment and the Canadian Shield. Silurian strata make up most of the Niaraga Escarpment. Small outliers may also be found on the Ontario-Quebec border in northern Ontario. While Devonian strata underlie the southwestern part of the province, there are only a few natural outcrops.