Ordovician radiation

The Ordovician biodiversification event, or radiation, of 500 million years ago (mya) was one of the most significant events in the history of life, and in fact involved a greater increase in diversity at the family and genus level than did the more popularly known Cambrian radiation, which occurred 540 mya. (Recall that families comprise genera, which in turn comprise species.) During the Ordovician radiation the diversity of marine families nearly tripled, and those groups of animals that came to dominate the next 250 million years were established.

The animals that dominated during the Ordovician Period include familiar forms such as echinoderms (sea lilies and sea stars) and corals as well as groups not common today such as brachiopods and bryozoans. Brachiopods are invertebrate animals that superficially resemble clams in that they have two valves that enclose soft parts, but in fact are unrelated to clams. Unlike clams, brachiopods have a coiled feeding organ called a lophophore; their symmetry is also quite different. Bryozoans are common today as encrusters on seaweed and shells and are often referred to as lace animals or moss animals. Groups such as clams (bivalves) and snails (gastropods) also diversified but to a lesser extent; the major radiation of these groups did not occur until around 250 mya.

 

Diversity patterns

 

The great increase in marine biodiversity from the Cambrian to the Ordovician was recognized as early as 1860. The magnitude of Ordovician diversification was not fully appreciated, however, until the late 1970s and early 1980s. J. J. Sepkoski's compendium of marine families (1979) revealed a threefold increase in family-level diversity between the Late Cambrian and the Late Ordovician (Fig. 1). This expansion followed on the heels of an apparent Late Cambrian diversity plateau. The subsequent diversity plateau established in the Ordovician was roughly maintained (despite significant short-term fluctuations) until the catastrophic Permo-Triassic extinction 250 mya. Following the end-Permian extinction and subsequent Triassic recovery, familial diversity began a steady increase which has apparently continued up to the Recent epoch.

 

 

Fig. 1  Marine family diversity during the Phanerozoic eon (540 mya to present). The tinted area indicates diversity contributed by poorly preserved groups. Faunas characteristic of different time periods are indicated as follows: Cm, Cambrian; Pz, Paleozoic; Md, modern. Time periods are V, Vendian; C TeX , Cambrian; O, Ordovician; S, Silurian; D, Devonian; C, Carboniferous; P, Permian; Tr, Triassic; J, Jurassic; K, Cretaceous; T, Tertiary. (After J. J. Sepkoski, Jr., 1979)

 

 

 

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The increase in biodiversity during the Ordovician radiation was not gradual through the Ordovician (approximately 35 million years long) but was concentrated over approximately 10 million years, and the pattern of diversification had a strong and complicated biogeographic component—that is, it was the composite result of processes operating at a variety of taxonomic and geographic levels. Studies have shown that the timing, rate, and magnitude of diversification differed considerably among paleocontinents, and among individual basins within continents. It has been proposed that the differential diversification dynamics among regions and among clades (related groups of species) can, in part, be explained by temporal and geographic variation in plate tectonic activity, which has been tentatively correlated with diversity.

 

Ecosystem complexity

 

The Ordovician radiation was an ecological event of extreme magnitude. With a near tripling of biodiversity, significant ecological changes were inevitable, including changes in ecosystem structure and complexity.

The Ordovician radiation largely occurred in the marine ecosystem. However, evidence from spores and terrestrial trace fossils (preserved trails of animals moving through mud) suggests that the initial radiation of complex life onto land also occurred in the Ordovician.

There were numerous large-scale changes within all established ecosystems in the marine realm. One of the most significant changes occurred with the development of hardground and reeflike communities that were to dominate ecosystems for the subsequent 200 million years. Changes were also occurring in the soft-substrate environment of the continental shelves.

 

Hardground development

 

Hardgrounds are hardened sea floors that result from cement precipitation from seawater. They are not common today but were widespread in the Ordovician. In the Early Ordovician, hardgrounds were a major factor in the initial diversification of crinoids (stalked echinoderms, such as sea lilies) as well as encrusters and other fauna. A variety of bryozoan clades and functionally similar animals as well as boring sponges and worms diversified on hardground surfaces. The end result was a complex hardground ecosystem. The diversification in this habitat was so pronounced that it has been termed the hardground revolution.

 

Reef complexity

 

The Ordovician radiation also resulted in a significant increase in reef complexity. Middle Ordovician and later Paleozoic reefs contain a variety of sponges as well as encrusting organisms such as bryozoans. During the Middle Ordovician new framework organisms were added to the reef community. Framework organisms are those that give the reef its three-dimensional structure that rises above the sea floor. Examples include corals and skeletonized sponges known as stromatoporoids. The Ordovician advent of stromatoporoid reefs was of considerable significance as these dominated the reef ecosystem through the Devonian.

 

 

Adaptive strategies

 

Another way of examining ecological change is to look at adaptive strategies—how were animals making a living? Richard Bambach considered this for the Ordovician, using mode of life and feeding type as parameters to distinguish megaguilds (groups of organisms sharing the same adaptive strategy) [Fig. 2]. Bambach documented an increase in the number of megaguild occupations after the Cambrian. In fact, all of the adaptive strategies used by multicelled organisms living on or near the sea floor (benthic metazoans) for the remainder of the Paleozoic were in place by the end of the Ordovician. In particular, new general benthic adaptive strategies added to the Cambrian ensemble included mobile suspension feeders and mobile carnivores as well as infaunal (living beneath the sediment) shallow passive suspension feeders and deep active deposit feeders. The major addition was epifaunal (living on top of the sediment) suspension feeders, such as brachiopods, echinoderms, and corals (Fig. 1). These major groups were still present, and the composition of the guilds changed very little, in the post-Ordovician, in spite of two mass extinctions.

 

 

Fig. 2  Distribution of Ordovician taxa among benthic megaguilds defined by mode of life and feeding type. Those animals that were present in the Cambrian are capitalized. (After R. K. Bambach, 1983)

 

 

 

 

 

 

 

Dominance

 

Another measure of ecological change is dominance—which group is the most important taxonomically and which is the most abundant? In the Ordovician marine ecosystem, there was a switch from taxonomic dominance by trilobites to dominance by brachiopods and, to a lesser extent, crinoids. That is, there were more different kinds of brachiopods and crinoids than trilobites. This change was accompanied by a shift in actual physical dominance or abundance. That is, there were larger numbers of brachiopods and echinoderms than trilobites in the Ordovician. Brachiopods and echinoderms remained the most abundant kinds of animals in the marine realm until the Late Permian mass extinction. If one were to walk along the beach in the Ordovician, the beach would be littered with brachiopods and crinoid bits and pieces and very few trilobites.

 

Potential causal mechanisms

 

Compared to Cambrian ecological radiations, relatively little work has been done to explore the causal factors underlying the Ordovician diversifications. Recently, researchers have tended to view the diversifications as resulting from a complex mix of intrinsic biological and extrinsic physical factors.

For example, it has been noted that Cambrian oceans appear to have been characterized by mesotrophic-eutrophic (nutrient-moderate to nutrient-rich) conditions and that many Cambrian taxa were sessile (anchored rather than mobile), passive suspension feeders well adapted for such conditions. The diversification of animals in the Cambrian may have created more oligotrophic (nutrient-poor) conditions, thus leading to the rise, diversification, and dominance of the mobile, active-filtering Paleozoic fauna. A shift toward oligotrophic conditions may also have been key in setting the stage for the radiation of calcified algae.

Another body of speculation holds that the Ordovician radiations may be related to increasing continental nutrient flux resulting from increasing tectonism and volcanism. One suggestion is that the two major phases of diversification in the Phanerozoic oceans (Cambro-Ordovician and Mesozoic-Cenozoic) were generally correlated with intervals of elevated tectonism, which may have produced changes in substrate (changes from mud to sand or carbonate or vice versa), greater primary productivity (that is, production of complex organic molecules from inorganic compounds), and increased habitat partitioning leading to increased speciation. A tentative correlation between diversity and proximity to orogenic (mountain-building) belts has also been reported. These hypotheses and others will be tested in future years to provide a much better understanding of the potential causes of the Ordovician radiation.

 See also: Brachiopoda; Bryozoa; Deep-sea fauna; Echinodermata; Ordovician; Paleoecology; Paleozoic; Reef

Mary L. Droser

 

Bibliography

 

 

  • M. L. Droser, D. J. Bottjer, and P. M. Sheehan, Evaluating the ecological architecture of major events in the Phanerozoic history of marine invertebrate life, Geology, 25:167–170, 1997
  • M. L. Droser, R. A. Fortey, and X. Li, The Ordovician radiation, Amer. Scientist, 84:122–131, 1996
  • T. E. Guensburg and J. Sprinkle, Rise of echinoderms in the Paleozoic fauna: Significance of paleoenvironmental controls, Geology, 20:407–410, 1992
  • alifazeli=egeology.blogfa.com

 

Additional Readings

 

 

  • R. K. Bambach, Ecospace utilization and guilds in marine communities through the Phanerozoic, in M. J. S. Tevesz and P. L. McCall (eds.), Biotic Interactions in Recent and Fossil Benthic Communities, pp. 719–746, Plenum Press, New York, 1983
  • A. I. Miller and S. R. Connolly, Substrate affinities of higher taxa and the Ordovician radiation, Paleobiology, 27:768–778, 2001
  • A. I. Miller and S. G. Mao, Association of orogenic activity with the Ordovician radiation of marine life, Geology, 23:305–308, 1995
  • J. J. Sepkoski, Jr., A kinetic model of Phanerozoic taxonomic diversity, II: Early Phanerozoic families and multiple equilibria, Paleobiology, 5:222–252, 1979
  • G. J. Vermeij, Economics, volcanoes, and Phanerozoic revolutions, Paleobiology, 21:125–152, 1995
  • A. Y. Zhuravlev, Biotic diversity and structure during the Neoproterozoic-Ordovician transition, in A. Y. Zhuravlev and R. Riding, The Ecology of the Cambrian Radiation,
  • alifazeli=egeology.blogfa.com