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Anoxia, eustatic changes, and Upper Devonian to lowermost Carboniferous global ammonoid diversity
Becker, R.T. (1993). Anoxia, eustatic changes, and Upper Devonian to lowermost Carboniferous global ammonoid diversity, in: House, M.R. (Ed.) The Ammonoidea: Environment, ecology, and evolutionary change. The Systematics Association Special Volume, 47: pp. 115-164
In: House, M.R. (Ed.) (1993). The Ammonoidea: Environment, ecology, and evolutionary change. The Systematics Association Special Volume, 47. Clarendon Press: Oxford. ISBN 0-19-857765-6. 353 pp., more
In: The Systematics Association Special Volume, more
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  • Becker, R.T.
Abstract
    Detailed analysis of Upper Devonian ammonoid evolution and palaeobiogeography enables the minute recording of global palaeoceanographic changes such as anoxic events and eustatic fluctuations during a time of global greenhouse climates. Major improvements in biostratigraphy and taxonomy allow the distinction of 36 genozones building on and adding to existing zonal schemes. The ranges of 177 genus-level taxa are reviewed to estimate diversity changes, origination, and extinction rates. Within the Upper Devonian to earliest Carboniferous twenty evolutionary phases are distinguished and the Upper Kellwasser and Hangenberg mass extinctions are the most pronounced events. Transgressions generally led to diversification while regressions caused diversity reductions. This suggests great significance for the 'species-area effect' and the degree of viability of migrational routes to allow the spread and survival of allopatric taxa. Short-term global anoxic overturns did not affect the overall diversity but led to spreads of ammonoid biofacies into usually uninhabitated areas. Associated with hypoxic incursions are revitalizations of bradytelic lineages that must have survived in low-oxygen deeper-water refuges. Phases of the Frasnian-Famennian and Devonian-Carboniferous boundary biocrises show similarities with small-scale events. Judged from the viewpoint of ammonoid evolution, both mass extinctions appear to be multiphased sudden eustatic and anoxic events that prevented recoveries even for a considerable while after return to normal oceanographic conditions. The model developed requires common mechanisms to explain the observed short-term evolutionary disruptions of different magnitude. Ultimate causes may have differed much from observed recent to subrecent processes making actualistic comparisons unhelpful.
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