|Global bio-events in mid-Jurassic ammonites controlled by seaways|
Westermann, G.E.G. (1993). Global bio-events in mid-Jurassic ammonites controlled by seaways, in: House, M.R. (Ed.) The Ammonoidea: environment, ecology, and evolutionary change. pp. 187-226
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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VLIZ: Geology and Geophysics 
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In the Middle Jurassic Ammonitina the precise time and 'centre' of origin of family-group taxa are seldom known. Original populations were presumably small, hence poorly recorded, but more than one 'centre' may have existed for cosmopolitan taxa. The well-known faunal turnovers used to define major chronostratigraphical boundaries, therefore, rarely coincided with cladogenesis or extinction of the higher taxa, but usually merely reflected the regional appearances and disappearances of the taxa by immigration and emigration. Both bio-events tended to be more or less diachronic for cosmopolitan taxa. Sea-level changes, as evident from litho- and seismostratigraphy and often eustatic in origin, influenced dispersal of species and faunas (regional taxon/taxa appearance), which at eustatic peaks was most rapid; and probably also biogeographical contraction (disappearance) and perhaps extinction at eustatic minima. The direct causes of mid-Jurassic biogeographical change, however, were the blocking ('closing') and unblocking ('opening') to faunal migration of the seaways between the major oceans. In particular the Central Atlantic-Americas Seaway complex and the Norway-Greenland Seaway provided corridors for ammonite migration, i.e. the Hispanic and Viking Corridors, respectively. The seaways also greatly influenced the physico-chemical and trophic properties of the water masses of the western Tethys Ocean with its extensive epeiric seas, the eastern Pacific Ocean, and 'the epeiric Arctic Sea. Both the epeiric and mostly (but not entirely) oceanic seaways, in turn, were influenced by world-wide eustasy, as well as by regional cratonic uplift.