Pre-Cambrian coelomate!

If the canonical body parts of all bilaterians are truly homologous, as suggested by commonalties in their developmental regulatory programs, then the PDA [protostome-deuterostome ancestor] should have had all these same parts. But there is an alternative interpretation for the observations used to support morphogenetic conservation, which may apply to many of the most prominent examples. ... In development, morphogenetic regulatory programs for pattern formation precede the institution of cell differentiation programs, but it is likely to have been the reverse in the evolution of body parts. This would allow for the continuing selective advantage, at each evolutionary stage, afforded by the respective differentiated cell functions. ... The strong prediction is that the architecture of the gene networks controlling the formation of many analogous body parts in unrelated bilaterian clades will turn out to be clade-specific (except for assemblages of genes that always work together whatever the developmental context). However, the (much simpler) architecture of the regulatory apparatus for differentiated cell types will turn out to be conserved across Bilateria. Though yet fragmentary, there is already some convincing evidence on both scores (Davidson, 2001.

One of the problems I encounter here is the propensity to consider molecular clock dates as being informative in determining probable divergence dates of fossil lineages. The sheer scope of difference in dates (600-1200 Ma) is very significant and demonstrates to me that the accuracy and precision of molecular clocks is not adequate for determining a valid divergence date. Erwin and Davidson do clarify this point to some degree, noting that it is adequate to state that the last PDA (protostome-deuterostome ancestor) predates Kimberella.

Holland's recent paper in Science (v.304 p1255) notes the existence of bilateral symmetry in an anemone and in the same issue, Finnerty et al (v304 p1355) explain the expression of Hox and Dpp in an anemone, calling for a reevaluation of the origin of bilateral symmetry.

In 1997 Bengston and Zhou did SEM imaging of Duoshantuo embryos dated at >555 and

The sheer scope of difference in dates (600-1200 Ma) is very significant and demonstrates to me that the accuracy and precision of molecular clocks is not adequate for determining a valid divergence date.

Holy smokes it is Fish head. And DoE who still will not accept that Archuptrex ha s tru purching fete!!!one11!!won!!!

Apparently my initial response got clipped in posting so I'll continue it here.

In 1997 Bengston and Zhou did SEM imaging of Duoshantuo embryos dated at >555 and <600 Ma. Hagadron and Zhou also did microCAT scan imaging of the same fossils in 2002. Radial cell cleavage, a hallmark of deuterstome development, was apparent in those studies and initially suggested the presence of potential bilaterians in the Duoshantuo phosphate beds of Vendian age.

While sectioning is the traditional mode of examining such fossils (in the mode and method of Walcott) it is now possible to use microCAT techniques to derive equivalent information without destroying the integrity of the fossil and therefore preserving the fossil for future study. I am thrilled that these fossils have been discovered but point out that the indications of bilaterians in the early Vendian predate this finding.

In response to DoE's question about molecular clock dates, the answer is yes, calibration points appear to make a serious difference. I find the disparity between molecular clock dates and geochronologic dating of fossil lineages to be dismaying at present and suggest that more time spent in increasing the accuracy and precision of molecular clock techniques, as opposed to an orgy of publishing experimental results, might be useful.

Apparently my initial response got clipped in posting so I'll continue it here.

This is happening pretty regularly. Can we get Terminix to take a look at the blog software?

FYI- to the mods...Another poster recently asked me if there was a ban on posting. Claims he tried to post and got some kind of message saying new posts were not allowed or some such.

There is a 120 second timeout period in which one cannot post a second comment. This was implemented to help reduce the number of duplicate comments from hitting the "Post" button twice in a row.

PZ took the bait and has provided an elaboration on Pharygula of my evo-devo remark above. :)

RBH

Yeah, I'm so easily tricked into these things.