By: Kristina Anderson, assistant curator
Today, planet Earth is believed to be around 4.6 billion years old. The geological time scale, which is in continual reconstruction, divides Earth history into broad spans of time. Many boundaries are marked by a biological event, such as the appearance of a new species. The Precambrian was originally defined as the time before life began. We now know this isn't true and even have examples within Ontario of life during the Precambrian.
Near Thunder Bay, the Canadian Shield has been altered very little since the deposition under water of iron-rich sediments two billion years ago. The residue of Gunflint chert found near Thunder Bay, which has been dissolved in hydrofluoric acid, contains microscopic remains of early organisms. Thin threads resembling present-day filamentous bacteria, blue-green algae and strange star-shaped, umbrella-shaped and parachute-shaped forms are found. The parachute-shaped organisms are found living today in ammonia-rich soil packed against walls of an ancient castle in Wales. Precambrian fossils, such as these have helped push the date of origin of life back as far as 3.5 billion years ago. Not all Precambrian fossils are microscopic. In the Sibley group near Thunder Bay large fossil stromatolites have been found. Stromatolites are layered, cone-shaped mounds both sedimentary and organic in composition. Stromatolites are still being formed by blue-green algae in the very salty waters of Shark Bay, Western Australia.
Most of Ontario's fossil record is found in the Paleozoic rocks that cover much of southern Ontario and the James Bay Lowlands. These rocks were deposited during the Ordovician, Silurian, and Devonian periods (450-350 million years ago) when Ontario was repeatedly covered by warm, shallow inland seas. The seas were fed by rivers draining from the bordering highlands of the Canadian Shield and acted as settling basins for thousands of meters of sand, mud, and clay. Eastern and northern boundaries of Paleozoic outcrop run roughly through the southern borders of Muskoka and Haliburton just east of Kingston (where Ordovician rocks lap onto Precambrian rocks). Ordovician rocks here abound with fossil snails, clams, squid-like nautiloids, trilobites, starfish, and sea lilies.
Late Ordovician and Silurian rocks form the Niagara escarpment that runs from Manitoulin Island to the Niagara Peninsula and on into New York State. The escarpment was formed by rivers draining highlands to southeast. Fossils are scarce here.
Silurian corals formed reefs that are now exposed as fossils on Manitoulin Island.
Devonian rocks on shore of Lake Huron near Kettle Point contain abundant corals and trilobites, sea lilies and other marine invertebrates. Even the bony skin plates of early armored fish have occasionally been found.
Unconsolidated deposits of Pleistocene Epoch are the next great chapter in Ontario's fossils. One of the most complete successions of interglacial sediment is exposed in Toronto Brick Pit. Fossils found here include wood, insects, freshwater clams and snails, antlers of deer, skulls of large bears, groundhogs, bison and giant beaver. Near Welland mastodons, wooly elephants, and pollen grains have been found.
Bivalve shells, Megalomus, internal filing. Guelph Formation, Guelph, Ontario.