[Taxacom] another putative arthropod outgroup

Stephen Thorpe stephen_thorpe at yahoo.co.nz
Fri Sep 28 21:59:08 CDT 2012


at any rate, is there any sound basis for thinking that the DNA associated with reduced morphologies won't itself be reduced/absent, so, unless there is some very specific other DNA shared with a crustacean ingroup (might be, but might not), we are no better off?



________________________________
From: Ken Kinman <kinman at hotmail.com>
To: Stephen Thorpe <stephen_thorpe at yahoo.co.nz>; "taxacom at mailman.nhm.ku.edu" <taxacom at mailman.nhm.ku.edu> 
Sent: Saturday, 29 September 2012 2:47 PM
Subject: RE: [Taxacom] another putative arthropod outgroup


Hi Stephen, 
      Well, just to be fair, I wouldn't call the morphological data in this case "extremely" limited.  It is actually quite impressive how much detail they can see in these Orsten fossils.  My concern in this case is not so much in how limited the morphological data is (even though much of it is from such fossils), but rather how limited the molecular data is on pentastomids.  Sometimes amazing that grant money can be found for sequencing very large numbers of specimens of some species (or even subspecies or populations) of certain taxa, but extremely little on a much higher level taxon like Pentastomida.

                      --------------Ken



________________________________
Date: Fri, 28 Sep 2012 19:05:25 -0700
From: stephen_thorpe at yahoo.co.nz
Subject: Re: [Taxacom] another putative arthropod outgroup
To: kinman at hotmail.com; taxacom at mailman.nhm.ku.edu


>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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