BIOZONES 303 Figure One dimensional (A) and two dimensional (B) representations of an interval biozone In this example the zone begins at the geographic acme of the hypothetical species Exus alphus and encompasses all strata between this extinction datum and the overlying Exus betus geographic expansion datum Globally, this interval between taxon range biozones defines a spatio temporal region within which the interval biozone’s inferred Total Stratigraphic Range (vertical) and Total Geographic Range (horizontal) are defined (dashed box) In local sections however, the biozone will be represented by a one dimensional interval whose boundaries are included within this global biozone Note the strong diachrony of the biozone boundaries Intervals of strata that lie outside the biozones’ geographical boundaries cannot be referred to the biozone per se, but can be placed within the corresponding biochrono zone ( time interval represented by the Total Stratigraphic Range) Many other definitional geometries are also possible (e.g., interval between two species’ last appearances, interval between two species’ first appearances) Figure One dimensional (A) and two dimensional (B) representations of an acme biozone In this example the zone begins at the first substantial increase in the relative abundance of the species (symbolized by dark shading) of the hypothetical species Exus alphus and encompasses all strata in which the species is regarded as abundant Globally, these abundance based data define a spatio temporal region within which the interval biozone’s inferred Total Stratigraphic Range (vertical) and Total Geographic Range (horizontal) are defined (dashed box) In local sections, however, the biozone will be represented by a one dimensional interval whose boundaries are included within this global biozone Note the strong diachrony of the biozone boundaries Intervals of strata that lie outside the biozones’ geographical boundaries cannot be referred to the biozone per se, but can be placed within the corresponding biochronozone (= time interval represented by the Total Stratigraphic Range) Many other definitions of acme are also possible (e.g., increased size, increased ornamentation)