[Taxacom] Minimum dates

David Campbell pleuronaia at gmail.com
Thu Jun 8 15:15:49 CDT 2017


Molecular divergence dates are often unreasonably old because of bad
calibration, in addition to the problematic nature of many algorithms used
in calculating molecular dates.  I've encountered some cases where the
oldest known fossil for a taxon was used to assign the date of divergence
between the extant examples chosen for the study.  Unless there is specific
evidence that the chosen extant examples represent the most divergent crown
groups, this will likely give too old a date.  The oldest fossil may go
back before the origin of the crown group, which again would make the
fossil calibration too old.  And fossils are often placed in paraphyletic
groups (not to mention the numerous polyphyletic wastebasket taxa used in
some of the data that have been entered into Paleobiology Database), so a
record of a fossil assigned to family X might actually be a common ancestor
of families X and Y.  Again, this means that the fossil calibration will
give too old a date.

On Thu, Jun 8, 2017 at 10:29 AM, John Grehan <calabar.john at gmail.com> wrote:

> 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
> origins.
>
> 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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-- 
Dr. David Campbell
Assistant Professor, Geology
Department of Natural Sciences
Box 7270
Gardner-Webb University
Boiling Springs NC 28017


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