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Ordovician Period


Abstract

This page describes the Ordovician Period, including stratigraphy and the fossil record. A famous lagerstätte – the Soom Shale – is briefly discussed, followed by a sketched outline of some of the major fossil groups.

Keywords: Ordovician, Ordovician biota, Cambrian-Ordovician boundary, Ordovician-Silurian boundary, Soom Shale, trilobites, eurypterids, graptolites

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 500 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. The story told by that record, report the authors, runs contrary to the common wisdom. The record tells of the kind of large-scale climatic and ecological changes that have traditionally been linked to extinctions, not radiations. Yet, the record clearly shows that no major extinctions preceded the Ordovician radiation. They therefore conclude that global changes are just as likely to promote as diminish global biodiversity" (Droser et al. 1996).

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.

 
 

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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 mass 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

Fig. 1: Temporary Ordovician stratigraphic column.

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.

(Earth History Resource Site.)

High sea level. Cratons flooded.
North America covered by shallow epicontinental seas.
Several sea level fluctuations (transgressions and regressions)

Baltica nears Laurentia causing Taconic Orogeny & volcanism.

Collapse of Cambro-Ordovician carbonate platform and deposition of clastic wedge in eastern North America.

First "tectonic cycle" in Appalachians:

  • SS molasse (red SS)
  • SH flysch (black shale & turbidites)
  • LS shallow shelf deposits (carb. platform)

Evidence for volcanism found in GA.

(RCB paleogeographic reconstruction)

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.

(Earth History Resource Site.)

Glacial deposition in NW Africa

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.

(Earth History Resource Site.)

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.

(Earth History Resource Site.)

Lagerstätten

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

Soom Shale

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

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.

Annelids?
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.

(Earth History Resource Site.)

Eurypterids?

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.

(Earth History Resource Site.)

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).
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.

(Earth History Resource Site.)

"The oldest identifiable gnathostome [jawed fishes] fossils are extremely scrappy, consisting of isolated scales and teeth. Because paleontologists find similar scales and teeth in more recent fossils that are complete enough to reveal the presence of jaws, it is inferred that the early fishes from which these bony fragments came also had jaws. Among the most ancient scraps are supposed shark scales from the Silurian of Mongolia and the Ordovician of the USA, thought to be about 420 and 450 million years old respectively, resembling the simple skin denticles (small toothlike scales) of modern sharks" (Maisey 1996, p. 61).

"The most complete Ordovician vertebrate known to date is Sacabambaspis janvieri. This jawless fish, like heterostracans, possesses dorsal and ventral shields made up of large median plates. Anteriorly, the dorsal shield delimits an elliptical space that contains the eyeballs with their dermal sclerotic and scleral ossifications, and the paired olfactory capsules. The anterior part of the ventral shield is composed of square-shaped platelets aligned in rows to form an exoskeletal mouth apparatus resembling that of heterostracans. S. janvieri may have a lateral series of 20 branchial plates and its bone is cellular. All the Ordovician vertebrates were previously classified with the heterostracans. Our present knowledge of the anatomy of the Ordovician vertebrates indicates that the Australian and Bolivian forms are more closely related to each other, forming a monophyletic group (Arandaspidiformes), than to the North American genera which are more closely related to the heterostracans. The Ordovician vertebrates may thus be united with the heterostracans into a monophyletic group, the Pteraspidomorphi. This does not invalidate the myopterygian concept, but reduces the importance of the involved characteristics. The analysis suggests that the presence of two olfactory capsules is primitive for vertebrates, as is also a single median narial opening" (Gagnier 1995, Abstract).
During the Ordovician the first creeping lichens and bryophytes move onto land, the beginning of a great new experiment of life.

(Earth History Resource Site.)

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.

Possible causes which have been mooted include:

  • climatic cooling
  • major glaciation
  • sea level drop
  • Iapetus Ocean (proto-Atlantic) closed, eliminating habitats
  • Cambro-Ord platform collapsed
  • Taconic Orogeny

 

References

Clarkson, E.N.K. 1993: Invertebrate Paleontology and Evolution (3rd ed.) Chapman and Hall.

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, 127 pp.

Crowley, T.J.; North, G.R. 1991: Paleoclimatology. Oxford University Press.

Droser, Mary L.; Fortey, Richard; Li, Xing 1996: The Ordovician Radiation. American Scientist, March-April 1996.

Gagnier, Pierre-Yves 1995: Ordovician Vertebrates and Agnathan Phylogeny. Bulletin du Muséum National d’Histoire Naturelle, Paris, 4e sér., 17, Section C, 1-4: 1-37.

Lapworth, C. 1878: The Moffat Series. Quarterly Journal of the Geological Society of London, 34: 240-346.

Maisey, John G. 1996: Discovering Fossil Fishes. Holt, 223 pp.

Nielsen, Claus 2001: Animal Evolution (second edition). Oxford.

Willis, K.J.; McElwain, J.C. 2002: The evolution of plants. Oxford University Press. 378 pp.


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