Friday, March 27, 2015

Trilobite Detritus

The Craigleith area of Ontario is well known for its abundance of fossils. In 1992 I visited this locality on the southern shore of the Georgian Bay to search for them. There was a railway cut through the shale near Collingwood. The first slab of rock that I cracked open revealed a cluster of trilobite carapaces. They date to the late Ordovician which is about 445 million years ago.

Closeup of trilobite tail pieces.

Trilobites are extinct marine arthropods. The carapace consists of three parts:
  • Cephalon (head shield)
  • Thorax (articulated body segments)
  • Pygidium (tail piece)
As the animal grows it sheds the exoskeleton by molting. The molts break into the three parts and fall to the sea floor. Wave action sorts the segments by size in different areas which later fossilize. In this example we see an accumulation of pygidia.

Trilobite carapaces from molting
Slab of shale with trilobite carapaces.

The slab in the photo above is about two feet square and contains many tail pieces. These are likely Pseudogygites latimarginatus which is one of the more common species found in the area. A reconstruction of the complete carapace is shown below.

Pseudogygites latimarginatus reconstruction
Source: The Ordovician trilobite Pseudogygites Kobayashi in eastern and Arctic North America

Trilobites are similar to the modern horseshoe crab which can be found along the east coast of North America. As a child I would frequently find these "living fossils" crawling in the sand on the beaches of Narragansett Bay in Rhode Island.


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