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Palaeozoic |
Department of Geology, Southampton Oceanographic Centre, European Way, Southampton SO14 3ZH, UK
During the Devonian there is a correlation between certain periods of sedimentary perturbations, often associated with hypoxia, and extinction events in the evolution of the goniatite and clymeniid ammonoids. A review is given of the survival and diversification strategies associated with these events. Survivors generally show novelty recognizable even in early stages. Characters involved include protoconch size and form, ornament and whorl form and degree of enrolling of the first whorl or so which completes the ammonitella. In post-ammonitella stages to the adult, changes include the nature of coiling, which is often polyphase, ornament and whorl form. Principally the character of new groups is defined by sutural changes following the proseptum, and new features of sutural ontogeny are usually recognizable in the first few millimetres of growth. Far from being a character merely indicating the plasticity of early stages, as is often claimed, it is suggested that larval strategy is crucial for both survival and subsequent radiation. Since, after an extinction event, radiation is only possible for survivors, the available biotic parameters for evolution are the stress-controlled factors at extinction events.
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