Reef ecosystems diversified during the Ordovician. Large animal-built reefs already existed in the Cambrian, when archaeocyaths, sponge relatives rather than corals, were important reef builders.
Explore the Ordovician Earth in 3D →Around 480 million years ago, stromatoporoid sponges built reefs in South China. A 2025 study identified their calcium-phosphate skeletons, extending the stromatoporoid reef record by about 20 million years. These were neither modern coral reefs nor Earth’s first reefs.
Spores about 469 million years old document land plants long before forests. Studies combining fossils and molecular clocks suggest an older origin, possibly in the Cambrian to Early Ordovician. The oldest accepted fossil evidence does not pinpoint when the lineage began.
Around 444 million years ago, growing ice and falling sea level reduced shallow-sea habitats, while oxygen loss also threatened marine life. Extinction unfolded in pulses; researchers are still untangling how cooling, nutrient cycling and anoxia interacted.