[Taxacom] 53 million year rabbit foot

Ingo Michalak isoran at gmx.de
Mon Nov 21 18:27:42 CST 2011


Dear Jason,

my comments follow below


 >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.

I would agree on the last sentence and disagree with the first. I think 
it is very existant. It is the (estimated) time between the split of a 
certain group from its sister clade (stem age) to the first speciation 
event that led to two extant subclades (crown age).

 >All this assumes of course that the fossil is a stem.

It may be somewhere on the stem, on any other subclade's stem, or on an 
extinct lineage belonging to the Lagomorpha, which might have evolved at 
any time from the true stem age on. All you can say it belongs to 
Lagomorpha.

 >Extinction happens all over such that lineages between extant lineages 
can become extinct.

"between" (younger than crown age) and "below" (older than crown age).

 >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.

Agreed, I meant that the first split is only dependant on which lineages 
survive. Extinction happens all the time, as you said. Speciation does 
as well. So the crown age is more or less random (imagine in the near 
future the two or three basalmost species go extinct. In a new analysis 
with your approach the crown group should be again calibrated with the 
fossil age. The result would be older extant lineages. Does this make 
sense?)

 >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.

I can not see that taking the stem age instead is biased in this 
respective.

 >Furthermore, and this is the amusing thing about definitions, a stem 
belongs to a more inclusive crown[,...]!

Yes, I think we agree on speaking of crown and stem of Lagomorpha.

 >So, going back to our friend the fossil rabbit, is it a Lagomorph or not?

This can (hopefully) be answered by a (morphological) expert of the 
group. On such judgements molecular biologists are dependant.

 >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?.

If no assignment to any subclade can be made, this question cannot be 
answered. This is the reason to assign it to the whole group to be 
conservative (in which we agree).

Best

Ingo





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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