Re[2]: antiquity of segmentation

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Date
     I think that Bill is referring to Eddy De Robertis's commentary in the 
     May 1st issue of Nature (not Science), which discusses a paper in 
     Development (not Science) by Linda Z. Holland and colleagues.
     
     As for the calcichordate theory, I feel that there's a lot more to be 
     said in its favour than people think. I'm sure Dick Jefferies would be 
     happy to supply reprints to people who asked for them (rpsj@nhm.ac.uk) 
     though this book 'The Ancestry of the Vertebrates' is quite hard to 
     find these days, and may even be out of print. It does not, in any 
     case, refer to his most recent work on stem-group echinoderms and how 
     the calcichordate theory can be squared with morphogenetic fields. 
     Jefferies' student Paul Daley has done some excellent work in this 
     area.
     
     The calcichordate theory is in tune with a lot of recent genetic work, 
     particularly the 'posterior prevalence' phenomenon observed with Hox 
     gene expression -- it may also prompt a critical reevaluation of 
     Romer's interesting 1972 paper on the 'somatico-visceral animal'.
     
     Recent work by L. Z. Holland and others suggests that the tails of 
     larvacean tunicates are primitively segmented. This may imply that 
     that the present-day morphology is highly derived from a motile 
     ancestor  -- this might help support Jefferies' contention that the 
     calcichordates known as mitrates can be interpreted as free-living 
     armoured tunicates, and for his view that urichordates, not 
     cephalochordates, are the closest living sister group of craniates.
     
     Jefferies' latest paper in Lethaia is a rebuttal of Peterson's earlier 
     paper, a clever tour-de-force showing that the calcichordate theory 
     fails because Jefferies' interpretations of calcichordate anatomy fail 
     to support his own preferred phylogeny.
     
     However, Peterson seems to have used unordered multistate characters 
     rather than discrete one-zero  characters, thus playing down proposed 
     synapomorphies. Jefferies thus finds a flaw in Peterson's argument, 
     and, as a side issue, finds renewed justification for the 
     calcichordate theory.
     
Were I falsely modest I would refrain from mentioning my own book, 'Before The 
Backbone', which discusses much of the background. Happily I am a monster of 
vanity and arrogance and am pleased to share this information with fellow 
palaeonetters.

Henry Gee

______________________________ Reply Separator _________________________________
Subject: Re: antiquity of segmentation
Author:  paleonet@ucmp1.berkeley.edu at Internet
Date:    14/05/97 23:14


Bill Shear wrote:
     
> 
> Has anyone else been following this work?  Have the relevant genes been 
> searched for in, for instance, echinoderms or hemichordates?
> This may be naive; but are crinoid stem ossicles a product of 
segmentation?
     
Andrew Kelman
Australian Geological Survey Organisation 
AKelman@AGSO.GOV.AU