304 BIOZONES greatest utility Care must be taken when defining such zones, that the recognition criteria employed are as explicit and objective as possible Nevertheless, the popularity of such zones in regional basin analyses (particularly in commercial biozonations) stands as testimony to their practical utility By convention, interval zones are named for the taxon used in its definition (e.g., Emiliana huxleyi Acme Zone, Tylosaurus Acme Zone) Other Types of Biozones Although the foregoing descriptions represent the most frequently used biozone types, other types exist These include such exotica as barren zones, coiling-direction zones, negative association zones, species pre-lap zone, species post-lap zone, etc In addition, biozone types exist that are not defined on fossils per se, but rather on the trace fossils left in the sediment as a result of animal and plant activity (e.g., track zones) All such zones are valid only to the extent that they are useful to the stratigrapher and geologist, and only to the extent that their definition is based on unambiguous observational evidence As was noted by Hedberg in 1971, ‘Time and usage will then be the surest means of determining whether it is desirable for such [zones] to persist’ Biozones and Biochronozones Ever since the days of Smith, Lyell, and Oppel, the purpose of establishing biozones has been to achieve chronostratigraphical correlations Other types of stratigraphic intervals (e.g., key beds, magnetochrons, isotope zones) may be based on boundaries whose emplacement is effectively synchronous over geological time-scale None is uniquely identifiable in the same manner as biozones, however Indeed, the use of biostratigraphic methods to place, in an approximate temporal framework, the observations upon which these other types of chronostratigraphic inference depend, is so common it is rarely even mentioned This failure to give biostratigraphical data due credit for the basic contribution it makes would not be so unfortunate, were it not for the fact that biostratigraphy has come to be viewed by many as a lacklustre and unimaginative field of study Nothing could be further from the truth Biostratigraphic data underpin not only most of what we know about geological time over the last 600 million years, but are critical to studies of palaeogeography, palaeoecology, extinctions, diversifications, and patterns of morphological evolution In order to remind oneself of the importance of biostratigraphical data, it is worth recalling that the lack of a detailed time-scale in which to place Proterozoic and Archaean events is not due to a lack of palaeomagnetic reversals, catastrophic depositional events, isotopic excursions, etc., but rather to the lack of fossil biotas on which to base a coherent, finely resolved, and widely accessible stratigraphy This having been said, the distinction between biozones and chronozones must always be borne in mind All biozone boundaries are diachronous by definition and, without recourse to radioisotopic Figure 10 Results of multivariate biostratigraphic analyses on simulated biostratigraphic data from various geological sections numbered to using the graphic correlation (A), as implemented by constrained optimization) and ranking and scaling (B) approaches These methods focus on the estimation of global, composite, taxon range biozones from datasets containing a series of biostratigraphic first and last appearance datums for two or more stratigraphic successions or cores Such approaches offer more consistency, objectivity, and higher reproducibility of results than either qualitative or pairwise quantitative approaches to biostratigraphic analysis In these diagrams, connecting lines between the eight sections represent best estimate lines of biochronostratigraphic correlation