302 BIOZONES of fossils that actually lived together and (ideally) interacted with one another to the extent that the grouping itself achieved a degree of stability – and therefore recognizability – as a result of these interactions Because properly defined assemblage zones reflect ecological relations, this type of zone tends to be restricted to particular facies The ability of such zones to track spatial shifts in environments through time provides them with a distinctive utility in terms of palaeoenvironmental analyses However, this utility comes at a price, and that price is a relatively reduced ability to achieve long-distance chronostratigraphical correlations For this reason, assemblage zones of benthic organisms tend to be most useful on local and regional scales Assemblage zones of planktonic organisms perform well in terms of chronological correlations, but the degree to which such organismal groupings are maintained by close inter-specific interactions is debatable Like Oppel zones, the specific criteria used to recognize assemblage zone boundaries are flexible Not all of the groups present in the zone’s ‘type area’ need be present to recognise the zone in remote locations Unlike Oppel zones though, there is an objective and independent rationale underlying this definitional latitude In the case of assemblage zones, one seeks to recognise a set of dependent ecological relations among species and between organisms and their environment that transcend mere faunal and/or floral lists The objective reality of such patterns in nature is well established by numerous studies of modern faunas and floras and is reasonably well understood from a theoretical point of view Oppel zones, on the other hand, are unified only in the vague sense that the species used to recognize the zone are thought to be useful in chronostratigraphical analysis Although there is certainly ample justification for suspecting that, in many cases, the biostratigraphic ranges of the species in different regions and habitats will coincide, there is much less justification for regarding these organisms as part of transcendent causal association than is the case with assemblage zones As a result of the flexible manner in which assemblage zones are defined, the same groups can be used to define different assemblage zones (e.g., a coral– bryozoan assemblage zone and a coral–foraminiferal assemblage zone can have zone-defining taxa in common) and different members of the same ecological association can be used to define different assemblage zones Assemblage zones have been used frequently in areas where suitably short-ranging taxa are not present or have not been studied By convention, the name of an assemblage zone should be based on two or more taxa that figure prominently in the zone’s definition (e.g., Eponides– Planorbulinella Assemblage Zone, Eodicynodon Assemblage Zone) Interval Zone An interval zone (also referred to as an interbiohorizon zone, gap zone, or a partial-range zone) is defined as a body of strata delineated by the region between two distinctive biostratigraphic horizons, but that has no distinctive biostratigraphic identity of its own (Figure 8) The boundaries of interval zones can be marked by a wide variety of criteria These zones typically represent the undefined regions between other types of zones; especially taxon range zones An interval zone’s existence assumes a complementary relation with the underlying and overlying biostratigraphically defined horizons that serve as their inferior and superior boundaries As with all other types of biozones, the traditional one-dimensional concept of biozone geometry (Figure 8A) can mask the more complex geometries evident in two and three-dimensional conceptualizations (Figure 8B) In particular, interval zones are confined geographically to only those regions in which the defining biozones overlap So long as one’s region of interest is confined to the geographical area encompassed jointly by the zone’s defining taxa, recognition of the zone can be made with confidence Outside this geographic envelope, though, recognition of an interval zone becomes problematic, if not impossible By convention, interval zones are either named for the taxa used to define their boundaries (e.g., Globigerinoides sicanus–Orbulina suturalis Interval Zone) or for a taxon that occurs in the interval, but is not itself used in the zone definition (e.g., Globigerina ciperoensis Zone) Acme Zone An acme zone (also referred to as a peak zone, flood zone, or epibole) is defined as a body of strata delineated by the region of ‘maximal development’ of a taxon (e.g., species, genus, family), but not its total range (Figure 9) In this context, the term ‘maximal development’ is meant to be used flexibly In some cases it might refer to an initial increase and subsequent decrease in the relative abundance of a taxon that takes place within the confines of its biostratigraphic-geographic range In others, it might refer to an increase/decrease in body size, an increase or decrease in diversity, etc Since these aspects of a taxon’s evolutionary/ecological history tend to be strongly associated with local and regional conditions, it is on these spatial scales that acme zones have their