[Taxacom] 53 million year rabbit foot

Jason Mate jfmate at hotmail.com
Mon Nov 21 16:38:29 CST 2011










Woops! Now I get this email. Nevermind, I sent the previous reply to you only so no harm done. Let me rephrase as well. The stem is nonexistant, it is a conceptual box to classify fossils, so it has no (temporal and character) dimension until you have fossils (assuming that these fossils belong in the stem to begin with), yet it is ´real´ (surely extinction has pruned at least some of the clades leading to the crown). Once you have fossils you start getting some real data, but even then the stem is a construct that denotes a temporal and character ´distance´ between the basal and crown´s node. All this assumes of course that the fossil is a stem. Extinction happens all over such that  lineages between extant lineages can become extinct. So the current sample does influence your fossil placement, if nothing else because they represent (hopefully) an unbiased sampling of the group´s characters and they influence your analysis. Of course it may be very biased but the point is that the crown influences your analysis which in turn determines what is a crown and a stem. Furthermore, and this is the amusing thing about definitions, a stem belongs to a more inclusive crown, it all depends on your interests and point of view!
So, going back to our friend the  fossil rabbit, is it a Lagomorph or not? Just because it is a fossil it is not automatically a stem, but even if this is the case, how far apart from the crown and basal nodes?. Do I even have data to time these two other nodes? Otherwise you might as well collapse the stem (temporally), so the basal and crown nodes are effectively superimposed. Obviously once you have a good fossil record then you can do lots of things and your nodes (may) separate (hooray, a stem) but this is for the lucky few. Stem and crown are quaint philosophical concepts to those (most/many?) with few or any fossils.  If, like me, you work on terrestrial arthropods (i.e. Aphodines in my case) you are very fortunate to get the odd fossil that kind looks like this or that family, nothing else. 
Best
Jason
> Date: Mon, 21 Nov 2011 21:55:15 +0100
> From: isoran at gmx.de
> To: taxacom at mailman.nhm.ku.edu
> Subject: Re: [Taxacom] 53 million year rabbit foot
> 
> :S
> great... confusion worked... let me rephrase:
> 
> Dear Jason,
> 
> I rather think it is THE STEM age you can assign the fossil age to (as
> a minimum), and NOT THE CROWN age. (Why should the "sample" of extant
> spp. somehow have an influence to the placement of the fossil?)
> 
> Best
> 
> 
> Ingo
> 
> 
> 
> schrieb Ingo Michalak:
> > Dear Jason,
> >
> > I rather think it is the crown age you can assign the fossil age to (as
> > a minimum), and not the stem age. (Why should the "sample" of extant
> > spp. somehow have an influence to the placement of the fossil?)
> >
> > Best
> >
> >
> > Ingo
> >
> > schrieb Jason Mate:
> >>
> >>
> >>
> >> ´´I agree with you that a well-dated fossil puts a minimum age for the clade. But one has to be careful about assuming any phyletic relationships, for even the oldest members of a clade can exhibit unique specializations, thus likely representing any of a multitude of extinct lineages. Insular forms of Lagomorpha have existed at different times, and islands are a common feature since the break-up of Pangaea.´´
> >> We agree that we are in agreement but I just want so say that I make no assumptions regarding the fossil. As you have said yourself assigning a fossil (in this case a Lagomorph) to a particular group within the crown clade requires information which may or may not be available. That is why the most inclusive placement is the standard practice. If the fossil represents a Lagormoph sensu amplo then we can only speak for Lagomorpha as a whole and the fossil is a date for the crown node.
> >> BestJason
> >>
> >>
> >>    		 	   		
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