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Ordovician Period


Abstract

This page describes the Ordovician Period, including stratigraphy, paleogeography, and famous lagerstätten, followed by a sketched outline of some of the major evolutionary events.

Keywords: Ordovician, Ordovician biota, fossil record, evolution

Introduction

The Ordovician Period is the second period of the Paleozoic Era. This period saw the origin and rapid evolution of many new types of invertebrate animals which replaced their Cambrian predecessors.

About 480 million years ago, in the Ordovician period, life forms diversified dramatically and gave rise to many of the marine forms familiar today. The fossil record of this period is amazingly intact in the Great Basin of California, Utah and Nevada and affords an almost unprecedented opportunity to learn about the conditions that favor innovation in biodiversity.

Primitive vascular plants appeared the land, until then almost totally barren.

The supercontinent of Gondwana drifted over the south pole, initiating a great Ice Age that gripped the earth at this time. The end of the period is marked by a major extinction event.

 
 

Related Topics


Further Reading

  • The Concise Geologic Time Scale (Ogg et al. 2008)

Related Pages

Other Web Sites

 
 

Stratigraphy

Type Section/Sections

The type section for the Ordovician, like that of the Cambrian and Silurian, is also in western Wales. The system was founded by Charles Lapworth in 1879, in part as a compromise to resolve the Murchison-Sedgwick conflict over their overlapping claims for their Silurian and Cambrian systems: In 1831 Sedgwick and Murchison commenced work on the stratigraphy of northern Wales. Sedgwick began at the bottom of the section and Murchison at the top. Sedgwick named his sequence of rocks the Cambrian and Murchison applied the name Silurian to the generally more fossiliferous upper formations. Eventually their sections overlapped, each claiming some of the same rocks for their systems, in what became a widespread and lengthy controversy until, forty four years later, Lapworth introduced the name Ordovician for the disputed sequence.

Despite this somewhat contrived basis, the Ordovician does comprise a distinct period in Earth history, with a distinctive biota bounded above and below by major extinction events.

The name “Ordovician” comes from Ordovices an ancient Celtic tribe that once inhabited the region in Wales where rock strata of this period occur.

The Ordovician was originally divided into two sub-periods, Bala and Dyfed, but more recently the Tremadoc epoch was removed from the Cambrian and a three-fold division of Ordovician strata instituted.

Lower (Cambrian-Ordovician) Boundary

The Green Point GSSP for the base of the Ordovician System, as well as the base of the Lower Ordovician Series and the lowest stage, was approved by the International Commission on Stratigraphy in December 1999 and ratified by the IUGS in January 2000.

By a unanimous vote taken in February 2000, the Subcommission selected the name Tremadocian for the lowest stage of the Ordovician System.

Upper (Ordovician-Silurian) Boundary

Near Moffat in South Scotland, an uninterrupted Ordovician-Silurian boundary sequence of deep water black shales is exposed at Dob’s Lin. Here Lapworth (1878) determined the stratigraphy based largely upon graptolites, the dominant macrofossils found in the exposure. Approximately 100 years later, in 1985, this locality was chosen as the International Stratotype for this transition.

The boundary is defined at the first appearance of the graptolite Akidograptus ascensus, at the base of the acuminatus Zone, 1.6m above the base of the Birkhill Shales. Follow the scree slope towards the top right hand margin, look for an outcrop which comprises a sequence of five pale-coloured clay bands set in grey mudstone (Upper Hartfell Shale). This is called the Anceps Zone. Above this the first appearance of black shales represents the bottom of the Birkhill Shales and the G. persulptus Zone. The Ordovician-Silurian boundary occurs 1.6m above this lithological boundary.

Chronology

Today, the International commission on Stratigraphy places the base of the Ordovician at 485.4 ± 1.9 Ma and the top at 443.8 ± 1.5 Ma (Cohen et al. 2015).

Paleogeography

Major Tectonic Events

In the latest Precambrian and early Paleozoic, the supercontinent Rodinia, centered about the south pole, broke apart as blocks drifted northward. Most notable of these blocks were the large continents North America [Laurentia], Baltica, and Siberia.

During the Ordovician ancient oceans separated the barren continents of Laurentia, Baltica, Siberia and Gondwana. The end of the Ordovician was one of the coldest times in Earth history. Ice covered much of the southern region of Gondwana.

Taconian Orogeny (~475 Ma) – Numerous plates and continental blocks approach North America from the south and east.

Land and Sea

During the Ordovician, Southern Europe, Africa, South America, Antarctica and Australia remained joined together into the supercontinent of Gondwanaland, which had moved down to the South Pole. North America straddled the equator, and was about 45 degrees clockwise from its present orientation. Western and Central Europe were separate from the rest of Eurasia, and were rotated about 90 degrees counterclockwise from their present orientation, and was in the southern tropics. North America is engaged in a slow collision with the microcontinent of Baltica, which forms the core of what is later to become Europe. The Iapetus Ocean continues to shrink as the previously passive margins of Baltica and North America converge. Where the Iapetus was, mountains are thrust up, remnant strata of which remain today in Greenland, Norway, Scotland, Ireland and north-eastern North America. Scotland and England are united into a single landmass.

Major transgression in middle Ordovician, therefore widespread shallow, warm epeiric seas.

Climate

The early Ordovician is thought to have been relatively warm; extensive coral reefs were developed in the tropics. Elevated carbon dioxide levels through the Cambrian and early Ordovician may have greenhouse conditions with global summer temperatures as high as 40°C (Crowley & North 1991). “However, by the late Ordovician (~458-443 Ma) there is evidence to suggest that global climates had become much more variable, and that certain regions were becoming cool and moist” (Willis & McIlwain 2002). There is evidence for a rapid drop in sea level in the Late Ordovician and climatic cooling leading to a major glaciation in Gondwanaland near end of the period.

Gondwana moved south and Africa, straddling the south pole, was extensively glaciated. There were even glaciers in what is now the Sahara. The glaciation caused global temperatures to drop as the period progressed, and the world entered an ice age, although conditions remain mild and equitable in the tropics.

Paleontology

General Characteristics

Essentially ‘modern’ (in the sense that problematica are mostly absent and all extant high level taxa represented) marine invertebrate fauna; however, land remains barren and vertebrates undeveloped.

“Diversity of marine life reached a peak during the Ordovician period. Most newly arising species represent modifications of existing forms, many of which made their first appearance during the preceding Cambrian period. Whereas many of the dominant Cambrian life forms waned or became extinct, some of the new groups that appeared during the Ordovician dominated the next 250 million years of the Paleozoic era. For example, the now-extinct graptolites first appeared in the Cambrian, but diversified dramatically during the Ordovician. In addition, many Ordovician brachiopods, mollusks, corals and stelleroids bear some resemblance to their modern descendants” (Droser et al. 1996).

The Ordovician was an age of evolutionary experimentation, in which new organisms evolve to replace those that died out at the end of the Cambrian. It was one of the largest adaptive radiations in the Earth’s history. The number of families of known marine invertebrates increases from about 200 at the end of the Cambrian to around 500 in the early Ordivician. The widespread shallow, warm continental seas were the perfect environment for many groups of organisms. Microorganisms such as colonial blue-green algae – stromatolites – are widespread. Foraminifera evolve. Acritarchs, although existing during the Precambrian, become more common. Stromatoporoids (possibly sponge-like organisms) also appear.

An interesting phenomenon is the sudden increase in filter feeding organisms. Cambrian animals were predominately crawling mud-grubbers and detrivores with a few swimming and burrowing predators thrown in. Filter feeders (such as Dinomischus and Lepidocystis) although an important part of the fauna, were not exceedingly common. In the Ordovician an increase in the amount of micro-plankton would be the obvious explanation for the sudden increase in number and diversity of filter-feeders. Groups absent or under-represented in the Cambrian suddenly become more important. We see the first appearance of the corals, including both rugose (solitary) and tabulate (colonial) forms, bivalve molluscs, and the planktonic graptolites (Graptoloidea).

The lophophorates (animals that suck food from the water using a special fringe of tentacles around the mouth) are more filter feeders that do well in the Ordovician. The Bryozoa appear in large numbers, and constitute the most predominant colonial animals of the time. Each bryozoan polyp is a tiny creature, not unlike a coral animal.

The hard-shelled brachiopods are also successful.

Biogeography

On the continental shelves of the isolated and drifting continents, marine organisms engage in evolutionary experiments. There are two main geographical provinces, a northern, equatorial tropical one, and a southern cool-water one centered around Gondwanaland.

In the case of trilobites, North America and northwestern margins of Europe, Spitzbergen, Siberia and north-east Russia are characterised by a diverse fauna that H. B. Whittington calls the Bathyurid fauna. This includes trilobites of the families Bathyuridae, Hystricuridae, Asaphidae, Komaspididae, Remopleurididae and Pliomeridae. A subprovince around Baltica bathyurids and pliomerids are rare and asaphids belong to different genera, indicated the Iapetus Ocean remained wide enough to act as a barrier to migration for these shallow water continental-shelf forms.

In the cooler waters of Gondwanaland are found the Selenopeltis and Hungaiid-Calymenid faunal provinces, including trilobites of the families Hungaiidae, Calymeniidea, Pliomeridae, Illaenidae, and endemic Asaphidae. The differences between Bathyurid fauna and Hungaiid-Calymenid faunal provinces were due to a combination of climate factors and geographical separation.

The same biogeographical distribution seems to apply to articulate brachiopods as well. The Balto-Scandian region is better defined in the brachiopod then the trilobite faunas, and for much of the period can be considered a distinct biogeographical province.

In the later part of the Ordovician genera of families previously limited to one faunal province appear in another, indicating a tendency towards migration and cosmopolitism. Changed in oceanic circulation (distribution of planktonic larvae etc) along with approaching continental masses would have made possible migrations of shallow water benthos.

Lagerstätten

Lagerstätten (sing. Lagerstätte) are fossil localities which are highly remarkable for for either their diversity or quality of preservation; sometimes both.

Soom Shale

The Soom Shale is a Late Ordovician lagerstätte – more to come.

’Beecher’s Bed’

‘Beecher’s Bed’ is an Early Ordovician lagerstätte located at Utica, New York State; pyritised trilobites with appendages; see e.g. Cisne 1972 – more to come.

Major Evolutionary Events

Early Ord – a time of adaptive radiation of many faunal groups, following the mass extinction of trilobites and nautiloids at end of Cambrian.

150 families -> 400 families

Representative fauna:

brachiopods, rugose & tabulate corals, bryozoans, crinoids, conodont animals, jawless fishes

Most marine organisms were epifaunal; few infaunal

Predators

Evolution of increasing effective predators.

Major predators were invertebrates

•nautiloids

•starfish

Decline of stromatolites

New kinds of reefs appeared

bryozoans, stromatoporoids, tabulate corals

The armoured placoderms began to differentiate from the main gnathostome lineage in the Ordovician, followed by the chondrichthyans (sharks and rays), also in the Ordovician.

Plants probably colonized the land; land plants were well established by the Silurian.

Major Taxa

Corals (subclass Zoantharia) were represented in the Cambrian by the Cothoniida and some tabulate-like corals, though the earliest undoubted Rugosa and Tabulata are Ordovician. By the Middle Ordovician the full range of colonial tabulate types had arisen, and they remained more numerous than the Rugosa throughout the Ordovician. However, the Tabulata were more profoundly affected by the Late Ordovician extinction.

Among the molluscs were newcomers such as bivalves, which were not common during this time, although the late Cambrian Archaeogastropods develop at a moderate pace. Virtually all the major bivalve stocks were established by the Middle Ordovician (Clarkson 1993, p. 205).

A much more spectacular success story were the nautiloid cephalopods. Small and rare in the late Cambrain, the nautiloids evolve quickly along many different lines. At least ten different orders flourished at this time, all but one appearing for the first time during the early or middle part of the Ordovician. This astonishing diversity included straight, curved, loosley coiled, and tightly coiled shelled types, and even one group (the Ascocerids) that in order to become lighter and more streamlined lost the a large part of their shell altogether. These carnivorous molluscs replaced the Cambrian Anomalocarids as the dominant life form and top predator of the world’s ocean. The biggest, such as the endocerids, attained huge size; with shells 3 to 5 meters or more in length they were the largest animal that, up until that time, had ever lived.

Ordovician trilobites were for the most part quite different from their Cambrian predecessors. Many evolved bizarre spines and nodules. Others, such as Aeglina prisca from the warm shallow seas of what is now Bohemia, were clearly pelagic nektonic swimming forms, and developed huge eyes with over a thousand facets, while still others went the other direction and lost their eyes altogether. Some trilobites developed shovel-like snouts for ploughing through mud, others fused the segments of their bodies. The curious Trinucleids developed a broad pitted margin around the head shield.

The ‘bryozoans’ are thought (e.g. Nielsen 2001, p. 232-234) to be a paraphyletic group, so the entoprocts and ectoprocts should be treated separately. In any event, it is only the ectoprocts which appear in the fossil record as early as the Ordovician; one of the few modern phyla which did not become established until after the Cambrian.

After humble Cambrian beginnings the articulate brachiopods greatly increase in diversity and abundance, with no fewer than fourteen new superfamilies. The Orthid and Strophomenid orders were especially diverse.

A number of early echinoderm experiments die out, others straggle on, while still others increase in diversity. The echinoderms included both a great many stalked (filter-feeding) and a few mobile (some predatory) forms. Among the bizarre forms were the carpoids, which were able to push themselves along the mud by means of a stout “tail.” During the Ordovician the Crinoids, rare during the Cambrian, suddenly appear and diversify in large numbers. Like the brachiopods these sessile benthic (attached bottom-living) invertebrates were to become an important group of filter feeders throughout the rest of the Paleozoic.

The echinoderm classes living today, except the holothuroids, are known from the early Ordovician, while all other classes of the Cambrian radiation became extinct during the Paleozoic. (After Nielsen 2001, pp. 419-420.)

The Ordovician was the high point of the graptolites, particularly the planktonic Graptoloidea. These evolved from benthic attached late Cambrian forms and diversified in earliest Ordovician (Tremadoc) time into a number of different planktonic types, including single-branched single and double rowed colonies, two-branched, four-branched, and spiral forms. In New Zealand, Ordovician anoxic deep sea black shales with abundant graptolites are well represented in North West Nelson (Cooper 1979).

The vertebrate ostracoderms remain rare, although several different groups evolve.

Their cousins the conodont animals, worm-like or eel-like organisms known mostly from numerous isolated denticles (which were used to support some kind of grasping or breathing structure in the mouth or throat) represent a major component, quite possibly predators and certainly nektonic/pelagic, in the marine food-chain.

Extinctions

Mass extinctions of tropical marine faunas occurred at the end of the Ordovician when 100 or more families became extinct, including more than half of the bryozoan and brachiopod species.

New Zealand Occurrences

The Ordovician is not widely represented in New Zealand, although there are a few occurences, mainly in the north west Nelson area (see Cooper 2004).

References

Cisne, J.L. 1972: Beecher's Trilobite Bed revisited: ecology of an Ordovician deepwater fauna. Postilla 160: 1-25.

Clarkson, E.N.K. 1993: Invertebrate Paleontology and Evolution (third edition). Chapman and Hall.

Cohen, K.M.; Finney, S.C.; Gibbard, P.L.; Fan, J.X. 2015: The ICS international chronostratigraphic chart v 2015/01. Episodes 36: 199-204.

Cooper, R.A. 1979: Ordovician Geology and Graptolite Faunas of the Aorangi Mine Area, North West Nelson, New Zealand. New Zealand Geological Survey Paleontological Bulletin 47: 1-127.

Cooper, R.A. (ed.) 2004: The New Zealand geologic timescale. Institute of Geological and Nuclear Sciences Monograph 22: 1-284.

Nielsen, C. 2001: Animal evolution: Interrelationships of the living phyla (second edition). Oxford University Press: 1-378.

Ogg, J.G.; Ogg, G.; Gradstein, F.M. 2008: The Concise Geologic Time Scale. Cambridge: 1-177.


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