The Golden Spike in Deep Time
At Fortune Head, on the southwestern edge of Newfoundland's Burin Peninsula, the low, rugged cliffs hold a globally unique geological marker. In 1992, the International Union of Geological Scientists designated a specific point in these rocks as the official start of the Cambrian Period, roughly 538.8 million years ago. This site is a Global Stratotype Section and Point (GSSP), often called a "Golden Spike," is the boundary between the Ediacaran Period and the Cambrian. It was chosen over sites in Siberia and China because of its clear rock layers and abundance of fossils.
The exposed rock sequence is part of the Chapel Island Formation, a series of sandstones, siltstones, and limestones over 1,000 meters thick. These sediments were deposited in the shallow coastal waters of the ancient Iapetus Ocean. This location records a shift in Earth's history: the Cambrian Explosion. This was a period of rapid evolutionary diversification when most major animal phyla first appear in the fossil record. The rocks here do not contain dinosaur bones, but instead hold subtler clues—the chemical fingerprints and trace fossils that record this biological revolution.
A Change in the Water
The leading hypothesis linked to Fortune Head is a deep change in ocean chemistry that triggered for the explosion of life. Geochemical analysis of the limestone and dolostone layers from this period suggests a massive influx of minerals into the oceans, particularly calcium. This surge in calcium ions is believed to have been driven by widespread continental weathering and erosion, a process associated with a feature known as the Great Unconformity. During this time, shallow seas advanced across the continents, eroding ancient crustal rock and washing its mineral components into the ocean.
This increased calcium concentration may have been a primary driver for biomineralization—the ability of organisms to create hard protective shells and skeletons. Before this, life was entirely soft-bodied. The newly available calcium carbonate in the seawater provided the raw material for animals to build external skeletons. Fossils show that the first organisms to do this, like the shell-bearing Cloudina and Namacalathus, appear just before the Cambrian boundary. Soon after, the first arthropods with hard exoskeletons emerged. The boundary itself at Fortune Head is defined by the first appearance of a complex burrowing trace fossil called Treptichnus pedum, indicating new behaviors in animals responding to their changing environment.