Palaeos: Paleozoic		Late Ordovician epoch
Ordovician period		Late Ordovician: 2

The Late Ordovician: 2

Life

The GOBE

The Late Ordovician was the high point of the mass anti-extinction called the Great Ordovician Biodiversity Event (GOBE).  With rare exceptions (graptolites, rostroconchs), all animal groups present in the Late Ordovician attained their highest diversity in that epoch (Webby, 2004a) and global genus-level animal diversity came close to its Paleozoic maximum (Stanley, 1998).

There are any number of plausible reasons why this occurred.  Warm epicontinental seas have always promoted the expansion of animal life, and these conditions prevailed in Laurentia, Baltica, Avalonia, Australia, China, Siberia, Mongolia and Kazakh.  At the same time, the cool to cold waters circulating across the long coast of West Gondwana encouraged the evolution of different forms adapted to those conditions.  The continents were widely scattered, providing both a larger area of coastal and shelf environments and a large number of partially isolated ecosystems in which species would necessarily diverge through genetic drift.  

In addition, the diversity of benthic environments increased.  The shift to "calcite" seas meant an increase in the area of carbonate shelf, and the Late Ordovician has thus been described as the "Golden Age of hardground epizoonts." Taylor & Wilson (2003). At the same time, the intensity and depth of softground bioturbation was increasing, greatly expanding the volume of mud & crud environments for mud-sucking worms (or "softground benthic infauna," if you must) and their predators. Stanley (1998).  ATW080322

Dissecting the GOBE

For the general run of animal life, the GOBE consisted of two diversity peaks separated by a slight dip, followed by a crash in the Hirnantian ice age. The peaks occurred in the "mid Caradoc," which we translate as Late Sandbian, and the "mid Ashgill," which we suppose to be the mid to late Katian. Webby (2004a).  Webby asserts that the Sandbian peak was both lower and more gradual than the abrupt rise in the Katian Age.  That is, the Sandbian peak was merely a continuation of the Darriwilian trend line. The slight dip between the peaks may be associated with an event of uncertain nature which resulted in the loss of various groups of Laurentian brachiopods and ostracodes in northern Baltica.  Id.

Krug & Patzkowsky (2004) raise some serious questions about the methodology supporting this type of analysis. They attempt to correct for sampling bias and find, when this is done, that most of the temporal fine detail disappears.  Animal diversity beaks broadly into the first half of the Katian and then remains relatively constant, declining slightly through the rest of the Katian before crashing in the Hirnantian. Yet Cocks & Torsvik (2005) make the qualitative, but weighty, point that the later Katian was marked by an episode of warming "which led to the formation of substantial carbonate mud mounds (bioherms) with very diverse brachiopods, trilobites, molluscs, echinoderms and bryozoa."  Ultimately, we do not yet know enough about the fossil record, and the statistical techniques needed to analyze it.  The GOBE certainly happened, but its precise contours are still hazy.  ATW080322.

The Freeze

We have covered the timing and mechanics of the Hirnantian ice age above.  The effects of this cold snap on life were variable.  Plainly, many species became extinct -- 70% of all animal species, according to one estimate. Benton & Harper (1997).  Many common families of Ordovician brachiopods, echinoderms, ostracodes, and trilobites disappeared entirely.   Many others shifted their ranges.  Id.  The end of the cooling caused yet further evolutionary angst (Stanley, 1998; Krug & Patzkowsky, 2004), as groups which had just worked out how to adapt to colder conditions became frustrated with an environment that capriciously changed its mind and wanted everything back the way it had been before it installed a state-of-the-art South Polar ice cap.  We admit that we have seriously considered extinction ourselves, under similar circumstances.  

A 70% species-level extinction wouldn't involve much family level extinction at all, if the extinct species were randomly distributed.  Further, to use an industrial analogy, this extinction did not represent the kind of wholesale elimination of jobs groups which occurs when the local factory closes, or when technology replaces assembly lines with assembly language.  The end-Ordovician is more closely analogous with the case in which two local firms merge and become a regional producer.  Some jobs become redundant, some become more specialized, and some new ones are created.  Many individual lives and careers are drastically altered, for better or worse, but the overall economic dislocation is minor and temporary.

In one sense, this analogy is exact.  The physical merger of Avalonia, Baltica, and Laurentia wouldn't leave room for three different species of brachiopod that did exactly the same thing.  These species would specialize or become extinct.  To that extent, an end-Ordovician extinction would have occurred with or without an ice-age.  Similarly, any rapid series of environmental changes will favor generalists over specialists. By definition, generalists will overlap in ecospace more than specialists, and diversity will necessarily suffer.  As human beings, we are in a particularly poor position to complain about that result.  Finally, the ice-age almost completed the Ordovician trend toward replacing the old Cambrian fauna, e.g. trilobites and "inarticulate" brachiopods, with the characteristic animals of the Paleozoic, e.g. stromatoporoids, corals, conodonts, and brachiopods which, if no more eloquent than their ancestors, were none the less articulate enough for the needs of the Silurian.  ATW080323.