[Taxacom] minimal estimates

[John Grehan]

In principle the first appearance of a fossil may approximate the origin of the group concerned, but there is no objective way to know. One may make a personal decision about that, but it is a personal judgement that may have no necessary relationship to actual history. But the main issue here from which this question arose is the possibility that other evidence of divergence may predict a date of origin predating the earliest fossil, and even if by a considerable time, the fossil does not falsify that prediction. And fossil ages cannot automatically falsify an earlier origin predicted from other sources such as tectonic correlation.
 
It would seem that divergence papers should inlcude some kind of reference as to why they consider a particular fossil reliably a member of the living group.
 
John Grehan 

________________________________

From: taxacom-bounces at mailman.nhm.ku.edu on behalf of David Campbell
Sent: Fri 5/20/2011 9:52 AM
To: taxacom at mailman.nhm.ku.edu
Subject: Re: [Taxacom] minimal estimates



Fossils may provide maximum dates as well as minimum dates, but to
figure out any credible date requires actually having some knowledge
of the fossil record, not merely grabbing some dates (or even less
plausibly, some published rate that magically becomes error-free) out
of a reference.  I read a paper not long ago that had merely grabbed
maximum dates out of the Paleobiology Database, yielding some serious
errors due to a misclassified homonym and a wastebasket
misidentification in a paper over 70 years old.  If the taxon of
interest has a very good fossil record (which can be analyzed), it's
likely that the first appearance is not too far after the actual first
appearance.  If the generaly phylogeny of the group is well-known and
well-fossilized, some maxima are readily possible.  E.g., extant
mammal taxa do not diverge in the Paleozoic.

Of course, there are the questions about the degree of linkage between
molecular and morphological divergence dates, calibration of the
fossil record (if your source for dates shows a big gap between the
earliest record and most other records, be suspicious-either the
record is sparse or there's an error).

--
Dr. David Campbell
The Paleontological Research Institution
1259 Trumansburg Road
Ithaca NY 14850

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