The Panda's Thumb: Pre-Cambrian coelomate!

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Pre-Cambrian coelomate!

Posted by PZ Myers on June 7, 2004 09:30 AM

A hot-off-the-presses article in Science describes the discovery of bilaterian fossils in the Doushantuo Formation, from roughly 570 million years ago.

Ten phosphatized specimens of a small (<180 µm) animal displaying clear bilaterian features have been recovered from the Doushantuo Formation, China, 40 to 55 million years before the Cambrian. Seen in sections, this animal (Vernanimalcula guizhouena gen. et sp. nov.) had paired coeloms extending the length of the gut; paired external pits that could be sense organs; bilateral, anterior-posterior organization; a ventrally directed anterior mouth with thick walled pharynx; and a triploblastic structure. The structural complexity is that of an adult rather than larval form. These fossils provide the first evidence confirming the phylogenetic inference that Bilateria arose well before the Cambrian.

This is exciting news, not because it revolutionizes our understanding of evolutionary history, but precisely because it is nothing surprising at all—we expect, from molecular/phylogenetic evidence, that complex animal life arose long before the Cambrian 'explosion', and what these fossils represent is a satisfying confirmation of that expectation (and they neatly fit predictions about bilaterian evolution that Erwin and Davidson made in 2002). It is actually expected, though, that bilaterian coelomates are even older than the 570 million years of the Doushantuo Formation; the last common ancestor of protostomes (arthropods and others) and deuterostomes (vertebrates and others) is estimated to have lived somewhere between 600 and 1200 million years ago.

Continue reading "Pre-Cambrian coelomate!" (on Pharyngula)

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Comment #3524

Posted by RBH on June 7, 2004 10:31 PM (e) (s)

For the evo-devo types, the Erwin and Davidson paper PZ mentions in his posting has a very interesting re-orientation of what it is that’s conserved with respect to conserved genes like pax6:

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.

RBH

Comment #3526

Posted by Marty Erwin on June 8, 2004 01:16 AM (e) (s)

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

Comment #3528

Posted by God Fearing Atheist on June 8, 2004 01:46 AM (e) (s)

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.

Fish-face! I haven’t talked to you in forever.

Not knowing anything about these icky bugs, why is that? Calibration problems, I presume?

-GFA (Sean, Def, DoH, birdz)

Comment #3529

Posted by ~DS~ on June 8, 2004 04:51 AM (e) (s)

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

Comment #3580

Posted by Marty Erwin on June 9, 2004 04:18 AM (e) (s)

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.

Comment #3589

Posted by Russell on June 9, 2004 06:56 AM (e) (s)

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?

RBH

Comment #3623

Posted by PZ Myers on June 9, 2004 02:18 PM (e) (s)

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