Richard J. Aldridge

Department of Geology, University of Leicester, Leicester LE1 7RH, U.K. email: ra12@le.ac.uk

Bioevents have commonly been recognized through direct analysis of the fossil record. This analysis may take the form of documenting the standing diversity of a group or groups of fossils through a period of geological history or may be compiled by looking at patterns of biotic change through a number of coeval successions. Investigations on a more ambitious scale include the classic compilations by Jack Sepkoski (1982 etc.) and others of global standing diversities of skeletonized organisms through Phanerozoic time.

To obtain a fuller understanding of bioevents identified by these methods, it is necessary to extend the database beyond the preserved record of skeletonized macrofossils. For example, the role of phytoplankton has been widely under-emphasised in studies of extinction events, especially in the Palaeozoic. Changes in planktonic productivity may be expected to have had a major effect on the entire marine ecosystem, but there are few detailed investigations of planktonic turnover during bioevents. A key problem, of course, is that phytoplankton are not preserved directly, and patterns of their distribution have to be read through the filtered record provided by their cysts. This record is not easy to interpret, but cysts are extremely common and very variable, and their patterrns of abundance and diversity provide a message that we should try to read. Comparisons of cyst abundance and diversity curves with carbon isotope patterns, which may also hold an indirect record of planktonic productivity, should allow hypotheses of plankton change to be developed and tested.

An example of such a study comes from the Early Silurian Ireviken Event (Gelsthorpe 2002). This bioevent was primarily recognized on the basis of a distinctive turnover in conodont faunas, accompanied by extinction in several lineages; the conodont extinctions are clearly stepped with a series of at least eight extinction horizons identified (Aldridge et al., 1993). Some other groups appear to show contemporaneous changes, including the acritarchs and prasinophyte algae (Le Hérissé 1989). Detailed sampling of key sections on the island of Gotland, Sweden, has been undertaken to compare the pattern of palynomorph turnover with that of the conodonts. The hypothesis being tested was that palynomorph extinctions were coincident with, or slightly pre-dated, the conodont extinctions. The results are surprising, with originations exceeding extinctions and no coincidence of the pattern of palynomorph disappearances with those of the conodonts. This bioevent is clearly more complicated than initially modelled, and new ideas have to be incorporated into its interpretation.

Another problem of determining the changes in the standing biota across bioevents is the common occurrence of gaps in the fossil record through such intervals. Specific Lazarus taxa may or may not be identified, but determination of the extent of the missing data requires a broader analysis. In an attempt to address this problem for conodont lineages across the Permo-Triassic extinction event, an international team is currently undertaking a phylogenetic analysis of the global conodont record through these two systems to identify the nature and extent of ghost lineages. The objectives are to produce a more complete appreciation of the diversity of conodont taxa before, during and after the event, to determine palaeogeographical controls on the distribution of the surviving species and to examine the success/failure of different trophic guilds.

From investigations such as those exemplified above we can develop more detailed databases for total, global biotic changes through bioevents. These databases provide the evidence on which to test and develop causal models for extinctions, and models of evolutionary/ecological patterns and processes through extinction/recovery phases.

References

Aldridge, R. J., Jeppsson, L. and Dorning, K. J. 1993. Early Silurian oceanic episodes and events. Journal of the Geological Society, 150, 501-513.

Gelsthorpe, D. N. 2002. Microplankton changes through the Early Silurian Ireviken Extinction Event on Gotland, Sweden. Unpublished PhD Thesis, University of Leicester.

Le Hérissé, A. 1989. Acritarches et kystes d’algues Prasinophycées du Silurien de Gotland, Sùede. Palaleontographica Italica, 76, 57-302.

Sepkoski, J. J. Jr. 1982. A compendium of fossil marine families. Milwaukee Public Museum Contributions in Biology and Geology, 83, 1-156.