[Taxacom] 53 million year rabbit foot

Kenneth Kinman kennethkinman at webtv.net
Mon Nov 21 21:56:20 CST 2011


Hi Ingo,                    
       Since Jason said that the stem lineage is real, I'm

pretty sure he meant to say that it was nonextant (rather than
"nonexistant").       

      Anyway, if a taxon is very rich in extant subtaxa, and

very poor in fossils, there are often no known fossils in the stems
(such as bacteria, many worm phyla, various dipteran taxa, etc.).
Without known fossils, many of their stems are empty sets at the
present, and many are likely to remain so for the foreseeable future.

     Sometimes the extant taxa were splitting off (radiating) very
quickly, and the stem age and crown age are virtually the same (meaning
that stem fossils may never be found, even if they were very rarely
preserved at all). So in such cases, one could perhaps actually argue
that the stem lineage is very close to being nonexistent (temporally
collapsing, as Jason noted).                
                -------------Ken                
---------------------------------------------------------

Ingo wrote: 
            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). 






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