July 22, 2004 | David F. Coppedge

New Book Reveals China’s Cambrian Explosion

Nature July 221 has a book review about the first volume in English of the Chengjiang biota of China, where tens of thousands of soft-bodied organisms are preserved in early Cambrian strata.  The book, The Cambrian Fossils of Chengjiang, China: The Flowering of Early Animal Life by Xian-Guang Hou et al., is praised by reviewer Zhe-Xi Luo, who puts a positive spin on the problem of the Cambrian explosion:

These beautiful and unique fossils have inspired new scientific insights and led to the clashing of ideas.  There is a great debate on the likely positions of Chengjiang animals such as the yunnanozoans in the deuterostome family tree.  Such debates will surely redefine the phylogenetic framework for establishing the earliest evolution of key features of chordates….
….it also provides an update on the fast-paced attempts to decipher the full evolutionary significance of this palaeontological treasure…..
….The early animal fossil record, however incomplete, can tell us about the early diversification of major animal lineages, a hot topic for molecular evolutionary studies, especially with regard to the timing of early animal evolution.  The Chengjiang fossils are the best source of evidence about the emergence of animal body plans, and have attracted interest from students of evolutionary development….
….the Early Cambrian sea of Chengjiang really is a cradle of early chordate evolution.

In the film Icons of Evolution, paleontologists onsite at the Chengjiang beds demonstrate that while soft-bodied fossils appear in the early Cambrian beds, including items as delicate as sponge embryos, no fossils appear in the preCambrian beds just below them, even though conditions were suitable for preservation.  The paleontologists also explain that all the phyla appear abruptly in the Cambrian beds.  Biodiversity actually decreases in the higher layers, contrary to the predictions of Darwin’s tree of life diagram.
    In the same issue, Andrew B. Smith2 comments on a fossil found in the same Chengjiang beds by D. -G. Shu and Simon Conway Morris et al.3 that they claim is a primitive echinoderm.  The phylogeny of echinoderms, which includes starfish and sea urchins, has long been a puzzle.  “If correct” about this fossil claimed to be 520 million years old, he asserts, “this links the echinoderms to an enigmatic group, the vetulicolians, remains of which are found in the same deposits of early Cambrian age.”  Making the connection with this enigmatic group poses a major difficulty, he says, “because of the difficulty of interpreting even their basic anatomical organization.”  Although echinoderms are placed within the deuterostomes (a “very diverse group” of animals with a mouth and anus which includes all the vertebrates), “in terms of morphology echinoderms have always stood apart because of their aberrant symmetry and lack of structures known as gill slits” unique to deuterostomes.  Starfish, with their five-fold symmetry radiating from a center, don’t fit the pattern of other deuterostomes.  Smith seems to remain unconvinced of the connection at this point:

There is now direct fossil evidence that all of the major deuterostome groups were established by about 520 million years ago.  Fossil vertebrates (yunnanozoans), tunicates (Shankouclava) and both asymmetric and radiate echinoderms (homalozoans, helicoplacoids) have all now been discovered in early Cambrian deposits.  Phlogites, a tentacle-bearing early Cambrian fossil of uncertain affinity, might even be a hemichordate or part of the common ancestral lineage of echinoderms and hemichordates.  So, if deuterostome divergence occurred around 575 million years ago, as recent molecular-clock studies suggest, there is a 50-million-year gap in the fossil record between the origin of deuterostomes and their appearance in the fossil record.  In the jigsaw of deuterostome evolution, vetulocystids represent another piece to be fitted into a puzzle where many of the pieces are still missing.

1Zhe-Xi Luo, “A window on early animal evolution,” Nature 430, 405 (22 July 2004); doi:10.1038/430405a.
2Andrew B. Smith, “Paleontology: Echinoderm roots,” Nature 430, 411 – 412 (22 July 2004); doi:10.1038/430411a.
3Shu, D. -G., Conway Morris, S., Han, J., Zhang, Z. -F. & Liu, J. -N. Nature 430, 422�428 (2004).

The spinmeisters of the Darwin Party, like this book reviewer, sound for all the world like a Stalinist explaining the benefits of the new Five-Year Plan.  Smith seems to be saying “not so fast” as he owns up to the mystery of the Cambrian explosion: all the major groups of animals, including vertebrates, appear suddenly in the early Cambrian without ancestors.  Think of all the changes that must take place to turn an organism with bilateral symmetry into one with pentaradiate symmetry like a starfish.  The first uncontested fossil echinoderm is already a full-fledged echinoderm.  Why is this so puzzling?  It’s only a puzzle if you’re trying to draw a mythical tree between the dots that is only a figment of philosophical imagination.
For an excellent overview of the Cambrian explosion and the challenge it poses to Darwinian evolution, see The Cambrian Explosion: Biology’s Big Bang by Meyer, Ross, Nelson and Chien (12/01/2003) available online at the Discovery Institute.

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Categories: Fossils, Marine Biology

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