[Taxacom] arthropod, panarthropod, and ecdysozoan phylogeny and evolution

Stephen Thorpe stephen_thorpe at yahoo.co.nz
Tue Sep 25 21:28:37 CDT 2012


there is a certain circularity of argument here: we cannot work out a phylogeny unless we make assumptions about that phylogeny (i.e., that onychophorans and tardigrades are arthropod ingroups!) Shouldn't that be a result of a phylogenetic analysis, and not a starting assumption??



________________________________
From: Ken Kinman <kinman at hotmail.com>
To: "taxacom at mailman.nhm.ku.edu" <taxacom at mailman.nhm.ku.edu> 
Sent: Wednesday, 26 September 2012 2:19 PM
Subject: [Taxacom] arthropod, panarthropod, and ecdysozoan phylogeny and evolution





Dear All,      If I recall correctly, it has been at least 15 years since the paper announcing molecular evidence of a new clade name Ecdysozoa (including arthropods and related phyla).  And yet, every year since then I have been amazed that there has been very little real progress in a convincing phylogeny of the groups within Ecdysozoa (particularly the closest relationships of arthropods to the other phyla, and also how the major clades within arthropods are themselves interrelated).  
      I still believe the reason for this is an obstinate clinging to the old notion that either onychophorans or tardigrades are the immediate outgroup of Arthropoda, although there has been plenty of debate which of them is supposedly the closest.  On the other hand, I still maintain that NEITHER of those two groups is a likely outgroup to Arthropoda, and that using EITHER of them as outgroups in molecular analyses continues to hinder a true understanding of arthropod interrelationships.  I continue to be amazed that there is little or no serious consideration given to the possibility that they are actually ingroups which evolved from early Oligostracan arthropods, and that using them as outgroups totally distorts any analysis of arthropod phylogeny (even given increasing molecular data to work with).  And more broadly, it impedes our understanding of metazoan phylogeny in general.  Not that I am disputing that onychophorans and tardigrades are
 indeed closely related to arthropods, but that they could be easily be descended from (not ancestral to) arthropods by what I refer to as de-arthropodization.  
      Anyway, I was looking at the abstract of a paper published in early 2012 (the paper itself is hidden behind a paywall), but the summary offers nothing new that I can see.  The same old hand wringing in frustration, but apparently still clinging to the old notion that either onychophorans or tardigrades should be used as the outgroups to arthropods.  Unfortunately, if that old notion is wrong (that they are actually ingroups), continuing to use them as outgroups just serves to perpetuate the agony and frustration for even more years.  I really wish some of these authors would start thinking outside of the box after 15 years (back when thinking outside of the box first revealed Ecdysozoa).  Seems like they have been spinning their wheels ever since.  Anyway, here is a weblink to the early 2012 paper(abstract only), and I doubt that the paper itself offers anything new of significance if they are still stuck in a rut using the same two
 outgroups that have yielded little progress over the last 15 years. I think it was perhaps Einstein's definition of insanity that might be relevant to all of this.  
http://www.annualreviews.org/doi/abs/10.1146/annurev-ento-120710-100659


                        
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