[Taxacom] Hedges /Kumar (eds) The Timetree of Life

[Michael Heads]

Hi Jason,
 
 
You don't have to 'guess' at a lower bound for clade age. Using the fossil record itself to assess the accuracy of the record probably isn't a good idea but there are all sorts of other data that can be used to date clades apart from fossils. 
 
For example, one of the two or three major phylogenetic/geographic breaks in Africa is at the Benue trough/Cameroon volcanic line, separating West Africa (Upper Guinea) from Central Africa (Lower Guinea). (There is no obvious ecological break, it's all rainforest. The main phylogenetic break in the Guinea forest should occur at the Dahomey Gap - savanna at Togo-Benin - but it doesn't). The volcanic line marks a major tectonic feature, a failed Cretaceous rift and inland sea that split Africa in half and was a precursor to the successful rift that formed the Atlantic.  
 
The question is, if plants and animals can disperse so easily across the Atlantic, why do they have such important phylogenetic breaks at the Atlantic and also at the Cameroon line? Chance dispersal will explain it of course, but the point is that you're not forced to just guess, there is a huge amount of good data you can actually analyze.
 
Michael      


Wellington, New Zealand.

My papers on biogeography are at: http://tiny.cc/RiUE0

--- On Wed, 18/5/11, Jason Mate <jfmate at hotmail.com> wrote:


From: Jason Mate <jfmate at hotmail.com>
Subject: Re: [Taxacom] Hedges /Kumar (eds) The Timetree of Life
To: "Taxacom" <taxacom at mailman.nhm.ku.edu>
Received: Wednesday, 18 May, 2011, 11:36 AM



Dear Michael,

I should have specified. I meant accurate calibration is impossible if I literally interpret your article (John seems to).
As for the thrust of the paper, I may have misinterpreted the ultimate aim of your paper. Nevertheless I think that the issue is blown out of proportion (maybe not in 2004-2005). In the historical sciences (evolutionary biology, cosmology, climatology, etc) there is an assumed element of uncertainty derived from the data, due to the nature of the data. As such all hypotheses derived are interpreted with caution since new data, which appears at random, can change all the assumptions. 

It is possible to guess at a lower bound if the fossil data used for calibrating the phylogenies is interpretated against other fossil data from related groups. So, although a single fossil provides no maximal age, its interpretation against other closely related fossils will provide you with a ballpark maximal age. In the specific case we are discussing, considering the oldest known fossil for the ingroup (37myr) and for rodents in general (KT) it seems reasonable to assume that the maximal age will fall within this range, therefore outside the opening of the South Atlantic and thus not supporting a vicariance scenario.

Best

Jason
                          
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