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Page K.N. (1995) Biohorizons and zonules: intra-subzonal units in Jurassic ammonite stratigraphy // Palaeontology. Vol. 38. Part 4. Pp. 801-814.
Intra-subzonal units, known collectively as ´horizons´, are employed in Jurassic ammonite stratigraphy. Two basic types of ´horizon´ are used by different authors. In France the unit is commonly employed as a sub-subzonal division which can be termed a ´zonule´. In Britain and Germany, however, the intra-subzonal unit typically used is more closely comparable to a faunal event. These faunal or ´biohorizons´ are analogous to the hemerae of S. S. Buckman. Zonules, therefore, have a significant time duration, whereas biohorizons have a negligible duration but typically a significant inter-biohorizon time interval. As intra-subzonal units ´horizons´ offer the potential for much finer scale correlation of marine Jurassic successions than is possible by any other means. The average zonule duration or biohorizon-plus-gap duration currently possible for the British Jurassic is around 200 000 years. Nevertheless, calculations of average duration on a stage by stage basis in the Jurassic suggest that at some levels this figure may be as little as 80 000 years.