A planet in crisis
At this unassuming outcrop 42 km north of Yichang city, a bronze plaque marks a line in the rock. This is a Global Boundary Stratotype Section and Point (GSSP), a location that is the official, internationally agreed-upon reference for a major geological time boundary. The line at Wangjiawan separates the Ordovician Period from the Silurian Period, pinpointing the climax of the Late Ordovician Mass Extinction around 443 million years ago. This event was the first of the 'Big Five' mass extinctions and eliminated about 85% of all marine species.
The extinction unfolded in two distinct pulses, driven by a severe and rapid ice age. During the Late Ordovician, the supercontinent Gondwana drifted over the South Pole, triggering the formation of massive ice sheets. This Hirnantian glaciation locked up immense volumes of water, causing a sea-level drop of up to 100 meters. The draining of shallow epicontinental seas destroyed the habitats for a huge number of species adapted to warm, tropical waters. This first pulse of extinction devastated communities of trilobites, brachiopods, and corals. The second pulse occurred as the ice age abruptly ended. Glaciers melted, sea levels rose rapidly, and a flood of freshwater disrupted ocean circulation. This led to widespread anoxia, a severe depletion of oxygen in the oceans, which proved fatal for many of the disaster's initial survivors.
Reading the rock record
The Wangjiawan section was chosen as a GSSP because of its completeness and clarity . The rock layers, specifically the transition from the dark shales of the Wufeng Formation to the overlying Longmaxi Formation, provide a continuous, undisturbed record of the environmental and biological crisis. The site has many fossils that allow geologists to date the layers with precision.
The definitive marker for the base of the Silurian is the first appearance of a specific species of graptolite, Akidograptus ascensus. Graptolites were tiny, colonial marine animals whose fossilized remains resemble small pencil markings on the rock. They were abundant and evolved quickly, making them excellent index fossils for dating rock layers worldwide. At Wangjiawan, the first appearance of A. ascensus is clearly identifiable. The boundary is also defined by the disappearance of many older graptolite species, a clear record of the extinction's toll. The excellent preservation and continuous sequence of fossils here allow scientists to correlate this exact point in time with other rock formations globally, making Wangjiawan a fundamental benchmark for understanding this ancient catastrophe.