Ordovician period

Characteristics and paleogeography

Most Ordovician rocks were deposited in shallow, epicontinental seas that covered large parts of the continents. Plate motions drove the assembly and dispersal of microcontinents, creating continental shelves rich in fossils and carbonate platforms. Sea levels rose and fell repeatedly; these transgressions and regressions controlled sediment deposition and the distribution of habitats. Glaciation began near the end of the period and contributed to a global sea‑level fall.

Life and biodiversity

The Ordovician witnessed one of the greatest increases in marine biodiversity in Earth history. Many animal groups that originated in the Cambrian diversified and expanded into new ecological roles. Common fossil groups include brachiopods, bryozoans, trilobites, cephalopods, gastropods, bivalves and early crinoids. Planktonic organisms, including graptolites, became important components of the open ocean and are widely used for biostratigraphic correlation.

• Typical Ordovician fossils: brachiopods, bryozoans, trilobites, cephalopods.
• Ecological trend: greater tiering and complexity in marine communities.
• Important development: expansion of suspension feeders and reef builders.

End‑Ordovician extinction and environmental change

The close of the Ordovician was marked by a major biotic crisis known as the End–Ordovician extinction event, widely considered the second‑largest mass extinction of the Phanerozoic. A combination of climatic cooling, glacioeustatic sea‑level fall, and associated habitat loss led to substantial losses among marine taxa. This extinction is discussed alongside other extinction episodes in studies of biodiversity decline and recovery (extinction).

History of recognition and naming

The name "Ordovician" honors an ancient Welsh tribe, the Ordovices, and was introduced in the late 19th century to resolve stratigraphic confusion. Charles Lapworth proposed the Ordovician after studying fossil differences in disputed strata, arguing they represented a distinct interval between Cambrian and Silurian rocks. Although adoption was gradual in some regions, the division gained broad acceptance and was ratified as a formal period by international geological bodies in the early 20th century.

Significance and study

Ordovician strata are important to geologists and paleontologists because they record rapid biological diversification, shifts in global climate, and tectonic events that shaped later Paleozoic geography. Modern research uses fossils, sedimentary records and isotopic data to reconstruct Ordovician oceans, to understand the causes and consequences of the end‑Ordovician extinction, and to trace the evolutionary history of many marine groups. For further context and stratigraphic correlations see regional compilations and international chronostratigraphic charts (period overview, Paleozoic, Phanerozoic).

Key references and resources: general stratigraphy summaries (eon/era charts), regional fossil guides (Cambrian background), follow‑up periods (Silurian link), naming history (name origin) and primary literature on extinction dynamics (extinction studies, End–Ordovician).