New Ediacaran type fossil

Stromatoveris (click for bigger)

In the current issue of Science (5 May 2006) There is a description of an amazing new fossil from the Chenjiang lagerstatte in China (1).

I have written about the Ediacaran (Vendian) and the strange organisms before, and although many organisms remain mysterious this fossil seems to resolve one of the most puzzling aspects of the Ediacaran – how are they related to later organisms and their modern descendants.

One of the characteristic organisms of the Ediacaran is the ‘frondose’ fossils such as Charnia (also here) and this new fossil Stromatoveris is of the frondose type.

Although Stromatoveris seems to be of Ediacaran type it is found in early Cambrian sediments. This is strange as it is usually thought that the Ediacaran biota largely went extinct at the base of the Cambrian, although fossils such as Thaumaptilon walcotti have indicated the possibility of some survivors.

There are essentially two broad schools of opinion on the relationship of the Ediacarans, that they had a unique construction unrelated to any know type of organism or that they are related fairly closely to modern phyla. The remarkable preservation of Stromatoveris allows features to be detected that suggest that it is a relative of Ctenophores. Interestingly that does not rule out the possibility that other organisms, even ones that look superficially similar, are strange ‘giant protists’ or have a similarly unfamiliar type of construction.

Proposed phylogeny of Stromatoveris (click for bigger)

This has important implications for evolution, it seems that some modern phyla had diverged before the Cambrian, as has long been suspected on the basis of molecular dates. Another recent discovery, yet again using fossils from Chengjiang, links the Ediacaran Parvancorina with the arthropods such as trilobites (2) extending the range of another phylum back in time.

Refs:

1) Lower Cambrian Vendobionts from China and Early Diploblast Evolution.
D.-G. Shu, S. Conway Morris, J. Han, Y. Li, X.-L. Zhang, H. Hua, Z.-F. Zhang, J.-N. Liu, J.-F. Guo, Y. Yao, and K. Yasui.
Science 5 May 2006: 731-734.

2) A Parvancorina-like arthropod from the Cambrian of South China.
Lin, J.P., S.M. Gon III., J.G. Gehling, L.E. Babcock, Y.L. Zhao, X.L. Zhang, S-X. Hu, J.L. Yuan, M.Y. Yu, & J. Peng. 2006.
Historical Biology 18(1): 33–45.