THE LATE ORDOVICIAN EXTINCTION: ECOLOGICAL SIGNATURE OF A NON-IMPACT EVENT

Differing ecologic changes arising from extinction events with different causes provide a means of distinguishing causes of extinctions. The Late Ordovician Extinction (LOE) has two pulses that were associated with the onset and end of a major glaciation. The extinctions were caused by a combination of climatic change and loss of living space when extensive epicontinental seas were drained by glacioeustatic sea-level decline. Signatures of impact (Ir, Os, shocked quartz, etc.) have not been found.

Of the five great extinction events, the LOE ranks second in the magnitude of taxonomic loss, but it resulted in relatively little ecologic change. The asteroid impact at the Cretaceous-Tertiary Extinction (KTE) had taxonomic losses comparable to the LOE but produced much greater changes in the ecosystem. The primary killing agent in the KTE was darkness induced by impact ejecta—both particulate and gaseous. Organisms in food chains dependent on primary production were particularly sensitive to loss of sunlight. Organisms that could move into food chains based on detritus, or those able to withstand months without food, were buffered from loss of sunlight.

Extinctions in the LOE, though severe taxonomically, were spread across many groups and were not clustered in particular food chains as in the KTE. The lack of concentration of extinctions resulted in few niches being completely vacated. During the recovery survivors radiated into niches for which they were already adapted, and the new ecologic structures resembled pre-extinction ecology. The KTE was more selective, singling out herbivores and those carnivores specialized for feeding on the lost herbivores. Many niches were vacated and surviving groups evolved new adaptations that allowed them to move into the vacated niches. Using the KTE as a model, extensive reorganization of synecologic structures, especially in food chains based on primary production, may be a signature of impact related events.