[Taxacom] Minimum dates

John Grehan calabar.john at gmail.com
Thu Jun 8 09:29:59 CDT 2017

In referring to dates extrapolated from fossils I am referring to the use
of fossils as calibrations for molecular divergence. So the molecular dates
that are then applied to the divergences are proxies for the absence of
fossils and therefore cannot represent actual or maximal dates. Sure,
perhaps some methods result in excessive antiquity (presumably for lineages
within the clade that was calibrated), but they are still minimal estimates
- i.e. they are not themselves falsifiers of any other evidence of earlier

John Grehan

On Wed, Jun 7, 2017 at 10:39 PM, Rob Smissen <
SmissenR at landcareresearch.co.nz> wrote:

> Dear John and all
> While the age of the oldest known fossil of a taxon provides a minimum age
> for that taxon (if correctly assigned and correctly dated), it is simply
> not true that all dates extrapolated from that fossil through molecular
> dating are also minimum dates. Other sources of error in the process can
> result in dates of exaggerated antiquity. These sources of error include
> accelerated rates of sequence evolution in some lineages and sorting of
> ancestral polymorphisms.
> Many molecular systematists may well misrepresent their dates, and I
> personally take them all with a large pinch of salt (and preferably some
> lime and a shot of tequila). However, no matter how many times it is
> repeated, the claim that all dates extrapolated from a fossil of one taxon
> to the age of another taxon are also minimum dates is simply not true. I
> could, through deliberate naivety and misapplication of methods, "show"
> that one herbaceous plant diverged from its paternal parent several million
> years ago. Such a date would not be a minimum date.
> Please think a bit harder about this, you have a point but it is not quite
> the one you keep making.
> Best wishes
> Rob
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