IN THE NEWS
May 1, 2013

PUBLISHED: Dr. Ed Landing Describes A Global Time Scale for the Origin of Animals

earth-science review
The article, "Cambrian Evolutionary Radiation: Context, correlation, and chronostratigraphy—overcoming deficiencies of the first appearance datum (FAD) concept" will soon be published in the journal, Earth-Science Reviews. It is currently available for preview online at: http://www.sciencedirect.com/.../.pdf

A Global Time Scale for the Origin of Animals

The Cambrian Evolutionary Radiation included the probable origin of many living marine animal groups and the "explosion" of marine animals with mineralized (limey or phosphatic) skeletons. This c. 30 million year-long interval began about 545 million years ago and began with the diversification of animals that were capable of burrowing into and eating bottom sediments. The last stage of the Radiation included the appearance of more "normal" marine animals such as reef-building sponge-relatives and trilobites.

The Cambrian Evolutionary Radiation did not reflect great "extrinsic" changes in the physical environment (as continent break-up, changes in oxygen level) as often suggested; it was probably "intrinsic" to life in that it featured the discovery of new habitats that older animals hadn't utilized (as burrowing into sediments, living in the intertidal environment) and should be seen as the evolution of more complex communities.

However, no global, standardized time division exists for the Cambrian Evolutionary Radiation. The problem is that fossils in this time interval provide a very poor basis for establishing time-lines for global correlation. Indeed, many fossils of this time represent organisms that migrated only slowly or simply never migrated between Cambrian continents. Thus, the fossil-based relative time scale that scientists use for time correlations in much younger rocks do not give any sort of acceptable precision in the early part of the Cambrian geological period.

A report co-authored by the Museum's State Paleontologist with colleagues US and European colleagues proposes that the 30 million years of the Cambrian Evolutionary Radiation should be subdivided into three, approximately ten million year-long units that can be correlated by globally extensive, very strong  changes in carbon isotope values. These changes in carbon isotope values are preserved in limestones laid down in the Cambrian oceans. (Limestones consist of carcium carbonate, and often preserve a record of atmospheric and marine carbon in the "carbonate" part of the lime molecule).

The  report also proposes a completely new paleogeographic map for the Cambrian. It also discusses the new evidence that shows the real problems that can arise from an uncritical use of fossils for interregional correlation. 

As of April 20, 2013, the paper is available on-line as an uncorrected galley on the Earth-Science Reviews (Cambridge University) web site: http://www.sciencedirect.com/science/article/pii/S0012825213000688
It will be published in print later in the year.

It can be cited as
Landing, E., G. Geyer, M. D. Brasier, and S. A. Bowring. 2013. Cambrian Evolutionary Radiation: context, correlations, and chronostratigraphy—overcoming deficiencies of the first appearance datum (FAD) concept. Earth-Science Reviews 121, 177 p., 10 figs., doi:10.10.1016/jearscirev.2013.03.008.

 


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