[Taxacom] another putative arthropod outgroup

Stephen Thorpe stephen_thorpe at yahoo.co.nz
Fri Sep 28 21:05:25 CDT 2012


>The paleontological morphologists insist that it is unparsimonous to assume that pentastomids have secondarily lost so many crustacean morphologies<
 
It is a general problem with obligate parasite groups, that they are so derived and have lost so many characters that their relationships are obscure. Given that paleontological morphological data is extremely limited (both by the patchiness of the fossil record, and the fact that you can't see much on a fossil specimen), I would look to molecular data on this one (though there is still no guarantee of success). Whether it is "unparsimonious" or not depends on a whole phylogeny, not just part of it taken out of context ...
 
Stephen


________________________________
From: Ken Kinman <kinman at hotmail.com>
To: "taxacom at mailman.nhm.ku.edu" <taxacom at mailman.nhm.ku.edu> 
Sent: Saturday, 29 September 2012 1:47 PM
Subject: [Taxacom] another putative arthropod outgroup


Dear All,      Beside tardigrades and onychophorans, another taxon (Pentastomida) has also long been put forward as an outgroup to euarthropods (or arthropods in general, including fossil taxa).  However, molecular data (18S rRNA and mitochondrial data), along with very limited morphological data, indicates that pentastomids are actually highly modified (morphologically "simplified") maxillopodan crustaceans.        
      Anyone want to weigh in on whether morphologists (especially paleontologists) or molecularists are right on this one?  The paleontological morphologists insist that it is unparsimonous to assume that pentastomids have secondarily lost so many crustacean morphologies, even though they are highly derived due to their parasitic life styles (see weblink below).  The question is whether they are right, or whether the molecularists are just sorely in need of far more molecular data on the pentastomids.  Anyway, if pentastomids are secondarily simplified crustaceans, will tardigrades turn out to also be secondarily simplified arthropods (although perhaps from another branch of arthropods such as chelicerates)?  The debate continues.                      
              ---------------------Ken Kinman                        
http://www.core-orsten-research.de/Publications/PDF_Paper/ulm_team/2011%20CASTELLANI_ETAL.pdf                         
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