A Golden Spike in Time
In the Three Gorges region of Hubei Province, a specific point in a rock outcrop defines the official beginning of the Cambrian Period. Geologists call this a Global Boundary Stratotype Section and Point (GSSP). It is the internationally agreed-upon reference section for the boundary between the Ediacaran and Cambrian periods, which occurred approximately 538.8 million years ago. This location, known as the Wangcun section, is marked by a "golden spike" to signify its global importance. The layers of carbonate rock here provide a continuous record of a major shift in Earth's chemistry and biology.
The rock layers belong to the Dengying Formation, a thick sequence of carbonates deposited in a shallow sea. This formation contains the important chemical and fossil evidence that geologists use to correlate rock layers across the globe. By studying these rocks, scientists can pinpoint the precise moment that corresponds to the dawn of the animal kingdom as we know it. The selection of this site by the International Commission on Stratigraphy means that any other rock exposure in the world showing the Ediacaran-Cambrian transition is compared against this standard reference in Hubei.
Chemical Fingerprints and First Steps
The primary marker for the start of the Cambrian is not a fossil, but a chemical signature in the rock. This is the field of chemostratigraphy. At the boundary level, there is a sharp and distinct negative anomaly in the ratio of carbon isotopes, known as the Basal Cambrian Carbon isotope Excursion (BACE). This sudden drop in the carbon-13 isotope (δ¹³C) indicates a major, worldwide perturbation of the carbon cycle. This geochemical event provides a precise and correlatable time marker.
Just above this chemical line in the stone, the first definitive evidence of complex animal behavior appears. The boundary is biologically defined by the first appearance of the trace fossil Treptichnus pedum. This fossil is not of an animal itself, but of the elaborate, branching burrow system it left behind as it systematically mined the sediment for food. The appearance of Treptichnus pedum signals a new stage in life's history, where animals began to actively engineer their environment. Soon after this point, the first "small shelly fossils" appear, marking the initial phase of the Cambrian Explosion when animals began to evolve skeletons.