BOUNDARY STRATOTYPES, EVOLUTIONARY THEORY AND PALEOECOLOGY
However, speciation is known to occur mostly in small isolated populations that are rarely if ever preserved. Only the spread of the new species after barriers are removed is visible to us as a FAD. While these migrations are mostly rapid and instantaneous by stratigraphic standards they can still be time transgressive wherever other barriers exist. Thus, we do not see the origin of a new species but rather its introduction into a widespread environment. If one uses taxa that form new species through phyletic gradualism one encounters two other problems. If a species is changing slowly, any boundary is gradational and a question of definition. If a new species originates it will rarely if ever happen sympatrically, i.e. in the same environment where the original species survives, but rather allopatrically, i.e. in an area where all individuals of the species change. Again what we will see in the GSSP is a migration event. Therefore, single FADs carry with them uncertainty and one should look for a more robust method to achieve the best results.
It is suggested to record bio-events, i.e. FADs and LADs (last appearance datum) for as many groups as are present and do so for a significant stratigraphic distance below and above the GSSP. In this way the GSSP is defined as a point in a sequence. In reality some or many of these bio-events will not be present in a section elsewhere. However, those that are present are still forming a comparable sequence and allow correlation. This method will require the cooperation of specialists for all the groups present and increase the complexity of the work and the definition of GSSPs. The significant stratigraphic distance that has to be investigated below and above the GSSP will be different for different environments and the number of bio-events necessary differ for different taxonomic groups.