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Part 2: The geological record |
For a period of some 330 x 106 years* from the early Devonian to the late Cretaceous, ammonoid zones and subzones in marine rocks provide a time discrimination of the order of 0.25-1.0 x 106 years or a resolving power of 1:100 to under 1:500. Several time intervals show much better results. The progressive improvement of these techniques is illustrated.
The evolutionary tree of the Ammonoidea is as well documented as for any group. The record, with its periods of extinction and diversification at several taxonomic levels can be viewed as a bio-seismograph of the factors controlling evolution. The role of early stage modifications in evolutionary strategy is emphasized. A relationship between environmental change and critical events in ammonoid history is examined. Examples illustrate punctuated as well as gradualist mode confirming Darwin's view on the broad range of evolutionary mechanism. The ammonoids show no evidence supporting an hypothesis of regular diversity decay. The case is made for a more collaborative and systematic study of evolutionary fluctuations as a key to documentation of primary causations which may be global or cosmic.
