Ordovician
The Ordovician Period is a geologic period within the Paleozoic Era, spanning from 485.4 to 443.8 million years ago and lasting approximately 41.6 million years, during which marine life underwent significant diversification while continents began to converge toward the formation of supercontinents.[1] This period represents about 0.92% of Earth's total geologic history and follows the Cambrian Period, preceding the Silurian.[1] It is named after the Ordovices, an ancient Celtic tribe in Wales where rocks from this time were first studied extensively.[2] Geologically, the Ordovician featured widespread shallow seas covering much of the continents, with the supercontinent Gondwana positioned over the South Pole and other landmasses like Laurentia (proto-North America) near the equator.[2] Plate tectonics drove major events, including the collision of volcanic island arcs with eastern Laurentia to form the Taconic orogeny and mountains, and the onset of subduction that contributed to the later assembly of Pangaea.[3][4] Climate was initially warm and stable in the early to mid-period, supporting expansive marine environments, but shifted dramatically in the Late Ordovician to a global ice age as Gondwana's position triggered glaciation and sea-level drop.[1][3] Biologically, the Ordovician is renowned for the "Great Ordovician Biodiversification Event," where marine invertebrate genera expanded fourfold, accounting for 12% of all known Phanerozoic marine fauna and marking a transition from Cambrian-style ecosystems to more complex ones.[1] Key groups included trilobites, brachiopods, graptolites, crinoids, bryozoans, and the first coral reefs, alongside early vertebrates like ostracoderms (jawless armored fish) and conodonts.[2][3] On land, primitive plants resembling mosses and lycophytes emerged around 460 million years ago, while arthropods began colonizing terrestrial habitats.[1][3] The period ended with the Late Ordovician mass extinction, the first of the "Big Five" Phanerozoic extinctions, which eliminated about 85% of marine species, including 25% of families such as many trilobites, brachiopods, and graptolites, primarily due to cooling climates, glaciation, and habitat loss from falling sea levels.[1][2] This event reshaped marine ecosystems and set the stage for Silurian recovery.[3]Overview
Etymology and definition
The Ordovician Period derives its name from the Ordovices, an ancient Celtic tribe that inhabited north Wales during the Roman era, as proposed by British geologist Charles Lapworth in 1879 to resolve a longstanding stratigraphic dispute. This controversy arose in the mid-19th century between Adam Sedgwick, who classified certain Welsh rock sequences as Cambrian, and Roderick Murchison, who assigned them to his Silurian System, leading to overlapping definitions of these units. Lapworth's intervention separated the contested strata into a distinct Ordovician System, bridging the Cambrian and Silurian while honoring the regional geological heritage.[5][6] The Ordovician is defined as the second geological period of the Paleozoic Era, spanning from approximately 485.4 to 443.8 million years ago and succeeding the Cambrian Period while preceding the Silurian. It is characterized by a major episode of marine invertebrate diversification, often termed the Great Ordovician Biodiversification Event, during which shelly faunas such as brachiopods, trilobites, and early corals proliferated across shallow marine environments. This period marks a pivotal phase in Phanerozoic life history, with global biodiversity increasing dramatically in response to ecological opportunities in expanding epicontinental seas.[3][7][8] The base of the Ordovician System is formally defined by the Global Stratotype Section and Point (GSSP) at Green Point, western Newfoundland, Canada, within the mudstone facies of the Beach Formation. This boundary is marked by the first appearance datum of the conodont Iapetognathus fluctivagus, providing a precise biostratigraphic anchor for international correlation. The section's well-preserved graptolite and conodont assemblages facilitate reliable global synchronization of Ordovician strata.[9]Timeline and boundaries
The Ordovician Period spans from approximately 485.4 Ma to 443.8 Ma, encompassing a duration of about 41.6 million years.[10] This temporal framework is established through integration of biostratigraphy and radiometric dating, providing a precise calibration for the period's boundaries within the Phanerozoic Eon. The lower boundary of the Ordovician, marking the Cambrian-Ordovician transition at 485.4 Ma, is defined at the Global Boundary Stratotype Section and Point (GSSP) in the Green Point section of western Newfoundland, Canada.[9] This boundary is characterized by the extinction of late Cambrian trilobite taxa, including members of the suborder Eodiscina, alongside the initial radiation of Early Ordovician (Tremadocian) graptolites such as Staurograptus and Rhabdinopora. The primary marker for the GSSP is the first appearance datum of the conodont Iapetognathus fluctivagus, which correlates globally with these biotic turnover events.[9] The upper boundary occurs at the Ordovician-Silurian transition, dated to 443.8 Ma, and is delineated by the GSSP for the base of the Hirnantian Stage at the Wangjiawan section near Yichang, China.[11] This boundary coincides with the onset of the Hirnantian glaciation and the first major pulse of the Late Ordovician mass extinction, defined biostratigraphically by the first appearance of the graptolite Normalograptus extraordinarius.[12] Radiometric ages for these boundaries derive primarily from U-Pb dating of zircon crystals in volcanic ash beds (tuffs) interbedded within marine sedimentary sequences. High-precision techniques, such as chemical abrasion-isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS), have been applied to ash layers in key sections, including those in South China and Baltoscandia, yielding uncertainties as low as ±0.1 to ±1.0 Ma and anchoring the biostratigraphic framework to absolute time. For instance, ash beds near the Cambrian-Ordovician boundary in Newfoundland and Wales provide U-Pb ages that confirm the 485.4 Ma datum, while similar dating in the Wangjiawan section supports the 443.8 Ma upper limit. The Ordovician is subdivided into three epochs: the Early Ordovician (Tremadocian and Floian stages), spanning 485.4 to 477.7 Ma; the Middle Ordovician (Dapingian and Darriwilian stages), from 477.7 to 458.4 Ma; and the Late Ordovician (Sandbian, Katian, and Hirnantian stages), extending to 443.8 Ma.[10] These epoch divisions reflect major evolutionary and environmental transitions, with boundaries ratified through GSSPs that integrate graptolite, conodont, and trilobite zonations for global correlation.[13]Stratigraphy and subdivisions
Global stages
The Ordovician Period is formally subdivided into seven global stages by the International Commission on Stratigraphy (ICS), forming the basis for international chronostratigraphic correlation. These stages—Early Ordovician (Tremadocian, Floian, Dapingian), Middle Ordovician (Darriwilian, Sandbian), and Late Ordovician (Katian, Hirnantian)—are defined by Global Boundary Stratotype Sections and Points (GSSPs), with boundaries ratified through biostratigraphic criteria emphasizing graptolites, conodonts, and trilobites as primary index fossils.[10] Global correlations rely on integrated biozonations from these fossil groups, which provide high-resolution markers across diverse lithofacies and paleogeographic realms.[13] Recent updates to the ICS International Chronostratigraphic Chart (versions 2023–2024) have refined stage durations using astrochronology, particularly for the Middle Ordovician, alongside U-Pb radioisotopic dating to achieve uncertainties as low as ±0.7 Ma for some boundaries.[10][14] A 2025 study proposes further refinements for the Late Ordovician, dating the Katian-Hirnantian boundary at 442.65 +0.17/−0.23 Ma and the Ordovician-Silurian boundary at 442.33 +0.34/−0.33 Ma using high-precision U-Pb geochronology, potentially shortening the Hirnantian stage, though these await official ICS ratification.[15] This framework ensures precise temporal resolution for studying Ordovician events, such as biodiversification and mass extinctions. The following table summarizes the global stages, their durations, and representative key biostratigraphic markers (using official ICS ages as of 2024):| Stage | Duration (Ma) | Key Index Fossils and Biozones |
|---|---|---|
| Tremadocian | 485.4–477.7 | Graptolites: Rhabdinopora praeparabola (base-defining FAD), Adelograptus and Expansograptus zones; Conodonts: Iapetognathus fluctivagus (base at Green Point GSSP); Trilobites: Symphysurina spp. and Jujuyaspis borealis.[10][13] |
| Floian | 477.7–470.0 | Graptolites: Paratetragraptus approximatus (base at Diabasbrottet GSSP); Conodonts: Oelandodus elongatus–Acodus deltatus Subzone (Paroistodus proteus Zone); Trilobites: Megistaspis (Paramegistaspis) planilimbata Zone.[10][13] |
| Dapingian | 470.0–467.3 | Graptolites: Azygograptus ellesi (upper A. suecicus Zone), Isograptus victoriae victoriae Zone; Conodonts: Baltoniodus triangularis (base at Huanghuachang GSSP).[10][13] |
| Darriwilian | 467.3–458.4 | Graptolites: Levisograptus austrodentatus (base at Huangnitang GSSP); Conodonts: Lenodus antivariabilis and Histiodella sinuosa zones.[10][13] |
| Sandbian | 458.4–453.0 | Graptolites: Nemagraptus gracilis (base at Sularp Brook GSSP); Conodonts: Pygodus anserinus Zone, Amorphognathus inaequalis Subzone.[10][13] |
| Katian | 453.0–445.2 | Graptolites: Diplacanthograptus caudatus (base at Black Knob Ridge GSSP); Conodonts: Upper Amorphognathus tvaerensis Zone.[10][13] |
| Hirnantian | 445.2–443.8 | Graptolites: Normalograptus extraordinarius (base at Wangjiawan GSSP); Trilobites: Hirnantia–Dalmanitina fauna.[10][13][11] |