CambrianOrdovician

CambrianOrdovician is a nonstandard designation used to describe the consecutive Cambrian and Ordovician periods of the Early Paleozoic, spanning roughly from 541 to 443.8 million years ago. This composite interval captures the rise of complex marine ecosystems and the early diversification of multicellular life that shaped the trajectory of life on Earth.

During the Cambrian period, sea levels were broadly high and shallow seas covered much of the continents. The Cambrian Explosion brought rapid diversification and the appearance of many major animal groups with hard shells, including trilobites, brachiopods, mollusks, echinoderms, and the first reef-builders such as archaeocyaths. Biomineralization transformed ecological interactions and the fossil record, while terrestrial ecosystems remained sparse.

In the Ordovician, marine life continued to diversify, an event now called the Great Ordovician Biodiversification Event (GOBE). Ecological complexity increased across numerous groups, with expanding reef systems, diverse bryozoans, corals, and orthids, as well as early fishes and conodonts adding to the vertebrate presence. Global seas remained widespread and warm early on, but climatic cooling toward the end of the period, coupled with glaciation, led to sea-level fall and significant biotic turnover.

The Cambrian and Ordovician together established foundational patterns of Paleozoic marine ecosystems, including substantial biodiversity, reef construction, and early vertebrate evolution, setting the stage for further diversification in the subsequent Silurian and Devonian periods.