<< If I have the following order of conodonts--world wide then the layering of
the geologic strata acts as a relative age marker.
conodont X
conodont w
conodont T
conodont s
conodont r
conodont m
conodont n
conodont g
conodont b
conodont a
If the order is invariably, worldwide in this pattern, then I KNOW that
Conodont X is younger than w because the layer containing X must have been
deposited AFTER the layer containing w.
So when I can attach a radiometric date to a given layer say the layer
containing conodont w (345 myr) and r (352 myr), then I know that s and T
must be between those ages. Similarly, for the amphibians, if I can find a
terrestrial or estuarine deposit between two worldwide biostratigraphic
horizon whose associated radiometric dates average at 364 and 360 myr
respectively, then I know that the deposit is somewhere around 362 myr old.
The datings are done on items like volcanic ash which is a geologically
instantaneous deposit, or a lava flow, which is also an almost instantaneous
deposit but not as much as a bentonite(ash) bed.
>>
Let me make a small methodological point here. While "conodont x" lies in a
geological layer that is "younger" or later than the layer in which
"conodont a" is found, nevertheless, "conodont x" is genealogically older
than "conodont a." Radiometric dates refer to the geological layers;
genealogical dates refer to the chronological order within the lineage.
Those who come first are younger geneaologically than those that come later,
given continuity in the lineage.
If this reasoning is correct, then the entire current biota is older than
previous biotas.
Cheers,
Bob