Pre-Cambrian coelomate!

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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)

10 Comments

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

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.

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)

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

Syntax Error: not well-formed (invalid token) at line 3, column 83, byte 212 at /usr/local/lib/perl5/site_perl/5.16/mach/XML/Parser.pm line 187.

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.

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This page contains a single entry by PZ Myers published on June 7, 2004 9:30 AM.

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