Fossils: Remnants of an Ancient Sea (Part 1)

In the Cragileith area, there are 67 species of fossilized invertebrates in 11 major groups. Only a small fraction of any population will end up being fossilized, and a smaller fraction still are available to be observed by people. The fossils that are found at Craigleith therefore represent a very small sample of the life forms that lived there in the distant past. However, they can help us reconstruct the ancient geography and climate of Southern Ontario as it existed in the Ordovician period, around 450 million years ago.
Around 360 to 450 million years ago, the Niagara Escarpment was the edge of a warm, shallow sea that covered part of modern day Southern Ontario. At that time, the North American continent was much further south, so that Ontario had a sub-tropical climate. The shallow sea occupied a depression now known as the Michigan Basin. Most of the fossilized creatures would have lived in the quiet muddy bottom of the inland sea or in shoals.

The sandstones, shales, limestones, and dolostones that we see in Southern Ontario were originally deposited as sand and mud by rivers entering the shallow sea. As the continent slowly shifted to the North, the deposits hardened and were uplifted by geological forces to form the bedrock that still exists today.

Fossils were formed when organisms in the ancient sea died and sank to the sea floor, where they became buried in sediment. The hardest parts of these organisms – the skeletons and shells – were preserved as fossils in the sedimentary rocks (fossilization of soft-bodied organisms can happen, but it is much more rare). The Michigan Basin provides us with an almost complete fossil record of the Cambrian, Ordovician, Silurian, and Devonian periods, covering an astonishing 210 million years.

Most of the bedrock along the Escarpment is covered by glacial deposits, soil and vegetation. However, there are some areas where the bedrock is exposed and you can find a remarkable number and variety of fossils. The exposed limestone and shale of the Craigleith area – including the shale beaches of Craigleith Provincial Park – is one of the best examples of this.

The fossils of Craigleith range from tiny filamentous worms to giant look-alike ancestors of modern squids. Some of the species, such as Lingula, still exist today, unchanged after 500 million years. Others are mysterious and have no living relatives and obscure ancestry. Some fossils, such as trilobites, are very abundant, while others are exceptionally rare.

Searching for fossils is a fun (and some would say, nerdy!) experience, but please either take a sketch or a photo of the fossils and leave them where they are so they can be enjoyed by other visitors.

In the next post, we will take a closer look at the fossilized species found in the Craigleith area. Stay tuned!

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