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No explosion of skeletal animals

UC SANTA BARBARA (US) — New research challenges the idea that a profusion of skeletal animals appeared on earth in a short burst beginning around 542 million years ago, during a time known as the “Cambrian Explosion.”

“We found that with improved dating and correlation of rock sequences, the short burst of appearances goes away,” says Susannah Porter, associate professor in the earth science department at the University of California, Santa Barbara. “Instead, appearances of the earliest skeleton-forming animals were drawn out over more than 20 million years.”

Skeletal animals became diverse much earlier than was thought, says graduate student John Moore, that with nearly half of the animal genera in the dataset appearing in the first 10 million years of the Cambrian Period.

The team’s findings are reported in the latest issue of the Geological Society of America Bulletin.

“The Cambrian diversification of animals was long thought to have begun with an explosive phase at the start of the Tommotian Stage, 17 million years above the base of the Cambrian,” says Adam Maloof, first author and assistant professor of geosciences at Princeton University. “To test this idea, we matched earliest Cambrian records of carbon isotope variability from Siberia, Mongolia, and China with a Moroccan record constrained by five radiometric ages from interbedded volcanic ashes.”

mellopegma_2

Mellopegma, an internal mold of a mollusc from the earliest Cambrian period. (Credit: Susannah Porter)

This time interval was from 542-520 million years ago.

This approach avoids the circularity associated with using fossils to correlate rocks, and then using those correlations to infer biological patterns, explains Porter.

In addition to improving the timeline of early animal evolution, the team generated proxy records for sea level, carbon cycling, and the chemistry of oxidation-reduction in the ocean—from the same sediments that contain the early animal fossils—says Porter.

The results indicate that early skeletal animals appeared during a 20-million-year interval of rising sea levels and increasingly oxidizing conditions at the sediment-water interface in shallow water environments.

Collaborating on the study were researchers from Massachusetts Institute of Technology, Princeton University, and Washington University in St. Louis.

The work was supported by the National Science Foundation.

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