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Charles Lapworth (1842-1920), an English geologist, mapped out complicated marine strata in the Southern Uplands of Scotland. His work led to the realization that the lower Paleozoic fauna could be divided into three stratigraphic units: Cambrian (oldest), middle (which he named the Ordovician), and Silurian (youngest) (Lapworth Museum Website). Lapworth is well known for determining the correct chronological sequence of graptolites in this same location, thereby establishing graptolites as an important index fossil (Palmer, 1999, pp 68-69). The Ordovician period was named for the Ordovices, a Celtic tribe living in Wales during the Roman conquest. The Ordovician period extends from 488.3 million years to 443.7 million years ago.
Primary Producers & Reefs
Life thrived in shallow seas and seems to have started its foothold on land. Cyanobacteria, green and red algae continue to be the dominant primary producers (photosynthesizers) (Knoll, Summons, Waldbauer, and Zumberge, 2007, p. 148). At the beginning of the Ordovician stromatolites continue to make up reef systems. As the Ordovician unfolds sponges (lithistid & stromatoporoid), bryozoans, coralline algae, and corals become major reef builders (Johnson & Stucky, 1995, p. 35) and (Webb, 2001, p. 174). As these new reefs built vertically, new habitats became available above the sea floor. Corals (both rugose and tabulate) become much more common during the Ordovician.
Invertebrates
Trilobites survived but would never regain their prominence after the final mass extinction of the Cambrian. (Stanley, 1987, p.65). At up to 16 inches, Isotelus is one of the largest trilobites known. Isotelus was a large predator in the Ordovician seas and is Ohio’s state fossil. Brachiopods survive and diversify; articulate brachipods displace inarticulate varieties as the dominant shelled organisms of the sea floor. Corals and echinoderms start to find success in environments previously inhabited by sponges (Nudds & Selden, 2008, p. 56). Overall, many animal groups that were rare in the Cambrian diversify and become well established during the Ordovician period. Bryozoans, sponges, crinoids, nautiloid cephalopods, conodontophoras, jawless fish, and planktonic graptolites become much more common. Graptolites are thought to be colonial hemichordates and are so widespread and varied during the Ordovician that they can be used as index fossils to date and correlate rock strata. Crinoids come to dominate the ocean floor. Echinoids in the form of Sea Urchins first appear in the Ordovician. These regular, round echinoids with pentameral symmetry had long spines and were adapted for grazing along the sea bottom (Prothero, 2004, p. 336). Eurypterids and starfish make their first appearance during the Ordovician period.
Jawless Fish
Jawless fish become much more common in the Ordovician. The armored agnathans or Ostracoderms make their first appearance during the Ordovician. These fish had no jaws or paired fins; their body was ell-like. Ostracoderms had skeletons made of cartilage, protective bony shields around the head and bony scales covering the body. The bony scales give these fossil agnathans their collective name ostracoderms or shelly skins. Ostracoderms can be divided into two groups the heterostracans (order Heterostraci), which appear in the Ordovician and the cephalaspids (order Osteostraci), which appear in the Silurian. Arandaspis is a heterostracan found near Alice Springs Australia and is named after the Aranda, an aboriginal tribe. Arandaspis had a streamlined body with a head sheild made of two large bony plates. All heterostracans possessed a bony head shield, which could grow as the fish aged (Dixon et. al, 1988, pp. 24-25). The rest of the body was covered in bony scales arranged in oblique rows. Its jawless mouth was near the ventral surface indicating it may have fed off the ocean bottom. Heterostracans would diversify and become common during the Silurian and Devonian. Today, only the hagfish and lamprey represent the class Agnatha.
The Soom Shale
The Soom Shale of Western Cape, South Africa is a lagerstatten representing the Ordovician (Selden & Nudds, p. 29). This deposit was particularly important in establishing the nature of conodonts (phosphatic tooth rows). The Soom shale conodont animal Promissum pulchrum revealed conodonts to be a chordate jawless eel-like fish. The Soom shale formed in a shallow sea filled with nekton (swimming organisms), but relatively devoid of benthos (bottom life). Eurypterids, conodonts and orthocones filled the predatory niches. Brachiopods and cornulitids lived on the bottom as filter feeders (Selden & Nudds, p. 36).
Beecher’s Trilobite Bed
Beecher’s Trilobite Bed is a fossil lagerstatten near Rome, New York. The trilobite bed is famous for producing trilobites in which the soft tissue anatomy has been preserved by pyrite replacement. The first examples of trilobite antennae along with ventral anatomy including legs, gills, and musculature have given paleontologist a window into the anatomy of once living trilobites. Beecher’s Trilobite bed is a 4 cm thick layer within the Frankfort Shale beds. Evidence suggests this layer was formed from a microturbidite. Turbidites are “marineslides”, which occur when sediment is pulled down a slope by gravity into deeper waters. Earthquakes can trigger Tubidites. Pyritized trilobites in Beecher’s Bed include: Triarthrus eatoni, Triarthrus beckii, Cryptolithus bellulus, and Stenoblepharum beecheri. Triarthrus trilobites are often found in dark mudstones rich in sulfur, which formed in anoxic (low oxygen) environments. Richard Forty believes these trilobites may have been the first to use sulfur bacteria as chemoautotrophic symbionts (Forty, 2001, pp. 70-71). Triarthrus trilobites had reduced mouthparts and filamentous structures in their ventral anatomy that could have been used to cultivate the sulfur bacteria. These bacteria were also responsible for facilitating the pyrite replacement of soft tissues after death. The shale surrounding Beecher’s Trilobite Bed contains trilobite parts, echinoderms, planktonic graptolites, articulate brachiopods, and orthocone (straight-shelled) cephalopods (Nudds & Selden, 2008, pp. 56-71).
Evidence of Life on Land
Tantalizing clues provide evidence that life started its invasion of land during the Ordovician. A variety of spores (reproductive structures from primitive land plants) make their first appearance in the mid-Ordovician (470 mya). One category of spore, the tetrads, possesses a mark that indicates cell division by meiosis. Along side these spores are bits and pieces of plant tissues. (Kenrick & Davis, 2004, pp. 19-21). Plant debris becomes more common during the late Ordovician and Silurian. In late Ordovician rock (450 mya) from England millipede-like trackways have been preserved (Palmer, 1999, p. 71).
Mass Extinction
The marine ecosystems experienced extinction on a global scale towards the end of the Ordovician period. The Ordovician extinction may be second only to the mass extinction that would end the Paleozoic Era. Heavy extinction occurred in the reef communities. Graptolites, bryozoans, brachiopods, nautiloids, and trilobites were especially hard hit. Nearly 25 percent of all animal families were wiped out (Selden & Nudds, 2004, p.36). Over fifty percent of trilobite families went extinct (Nudds & Selden, 2008, p. 69). There is evidence of glaciation and with this a lowering of sea level, and the expansion of cold water adapted species to lower latitudes as the Ordovician extinction event unfolded (Stanley, 1987, pp 71-75). An ocean turn over may have accompanied the cooling event bringing deep ocean water to the surface, which would have been toxic to the sensitive shallow marine benthic community. The Ordovician event took place over a span of 2 million years (Prothero, 2004, p. 90).
Bibliography |
Forty, R. (2001). Trilobite: Eyewitness to Evolution. New York: Vintage Books.
Kazlev, M.A. (2002). Palaeos Website. see: http://www.palaeos.com/Timescale/default.htm
Knoll, Summons, Waldbauer, and Zumberge. (2007). The Geological Succession of Primary Producers in the Oceans. In Falkowski, P.G. Knoll, A.H. [Eds] Evolution of Primary Producers in the Sea. (pp. 133-163). China: Elsevier Academic Press.
Lapworth Museum Website: http://www.lapworth.bham.ac.uk/about/Lapworth.htm
Nudds J.R. & Selden P.A. (2008). Fossil Ecosystems of North America: A Guide to the Sites and Their Extraordinary Biotas. Chicago: The University of Chicago Press.
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Stanley, S.M., (1987). Extinction. New York: Scientific American Books.
Selden P. & Nudds, J. (2004). Evolution of Fossil Ecosystems. Chicago: The University of Chicago Press.
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Division of Geologic Time see: http://pubs.usgs.gov/gip/geotime/divisions.html
http://www.uga.edu/~strata/
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