[Taxacom] Homalozoan classification (still stem-group echinoderms?)

Ken Kinman kinman at hotmail.com
Tue May 1 21:23:56 CDT 2007


Dear All,
     Due to the continued bitter controversy over homalozoans (early 
echinoderms or early "carpoid" chordates?), I haven't really been inclined 
to attempt an updated classification since my 1994 book.  However, I have 
seen nothing since that changes my mind about them being stem-group 
echinoderms.

     Among those who do regard them as stem-echinoderms, there is a growing 
consensus that homalozoans are a paraphyletic taxon.  In 1994 I classified 
Class Homalozoea (sensu stricto) as separate from Class Helicoplacoidea, but 
it is now pretty clear that even Class Homalozoea (sensu stricto) is still 
paraphyletic, so I am going to now formally recognize (and code) them as 
such (including helicoplacoids as derived members that are sister group to 
crown group echinoderms).

     I am also adding the more recently discovered Order Vetulocystida at 
the base of Class Homalozoea.  I am still including Cambroclavida (incertae 
sedis) even though some workers now place them near acanthocephalans (not 
echinoderms).  I don't think anyone really knows where cambroclaves will end 
up.  However, I am going to now omit the problematic Order Cymbionitida 
(haplozoans), because they probably fit better in with my broad Class 
Crinoidea (including blastozoans).  Whether a broad Class Crinoidea is 
holophyletic is something I will have to tackle next.  Anyway, here is my 
updated and recoded classification of Class Homalozoea (including 
helicoplacoids, but excluding haplozoans):

          PHYLUM ECHINODERMATA

1  Homalozoea% (Cambrian-Carboniferous)
      1  Vetulocystida
      ?  Cambroclavida (affinities very uncertain)
      2  Stylophorida
      3  Solutida
      4  Cinctida
      5  Ctenocystida
      6  Helicoplacida
      B  Polyplacida
      7  {{crown group echinoderms}}

_1_ Crinoidea
2  Edrioasteroidea
3  Asteroidea, etc.
-----------------------------------
NOTES:  Cinctida and Solutida are here no longer considered sister taxa, 
although they are closely related and still form an informal paraphyletic 
grouping.  Also note that "Heterostelea" might belong in Homalozoea, but I 
am still betting that they will end up in one of the crown group classes.  
As always, I am open to suggestions, especially relating to such 
controversial fossil problematica.
      ----Ken Kinman

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