Tectonic Events May Have Triggered the Cambrian Explosion

Major geological changes causing sea level rise at the start of the Cambrian period (540–490 million years ago) could have kick-started the Cambrian Explosion—a geological time period when most major phyla of life suddenly appeared in the fossil record. A paper published in the November issue of Geology (doi:10.1130/G35886.1) proposes a new geological mechanism for this event.

“It has long been thought that the transgression of the ocean waters over the continents starting in the Cambrian contributed to the sudden radiation of multicellular life by providing a myriad of new ecological niches for life to evolve as shallow seas spread across the continents,” Ian Dalziel, professor of geological sciences at the University of Texas at Austin and sole author of the paper, told Eos.

Current reconstructions of the continents during the Cambrian show that Laurentia (the geological core of current-day North America) and Gondwanaland (made up of the modern-day southern continents) were separated by an ocean. These reconstructions, however, are based on measurements of paleomagnetism—the only way to understand ancient continental drift because of the continuous recycling of the sea floor, Dalziel explained.

Paleomagnetism tells scientists about the original position of rock in terms of latitude and orientation but does not reveal anything about its longitude, so rocks could come from different ocean basins, Dalziel said.

By looking directly at the rock record from five different continents, Dalziel found geological evidence that Laurentia and Gondwanaland started to separate in the early Cambrian. This would have raised sea level as the mid-ocean ridges emerged, swelled, and pushed water over continental shelves—a process that could have triggered the evolution of complex life as the chemistry of the ocean changed.

In fact, Dalziel had proposed this mechanism 2 decades ago, but only in the last few years did he discover a missing puzzle piece.

While doing field work in a remote part of Antarctica, Dalziel found samples of rock—much older than the separation itself—that had isotopic signatures that match those found in the southern United States, implying that these two continents were connected longer ago than previously thought.

“When I first thought that Laurentia and Gondwanaland may have been together at the end of the Precambrian, we didn’t at the time know that these [Antarctic] rocks were a little piece of North America,” Dalziel said.

In addition, Dalziel found that from southern North America southward, rocks related to the opening of the ancient Iapetus Ocean (an Atlantic Ocean precursor) get younger, indicating that the landmasses “unzipped” southward, opening to form a new ocean.

The opening of the Pacific and the Iapetus ocean basins at the start of the Cambrian “would have had profound environmental consequences and would have triggered a rise in sea level and flooding of the continents as seen in the rock record,” Dalziel said.

The rifting of the two continents also would have increased oxygenation and caused the upwelling of phosphorus and other nutrients, somehow allowing complex, multicellular organisms to evolve and form protective shells. This ancient rifting could also explain the geographic diversity of organisms, especially trilobites, found in the ancient ocean floor, Dalziel explained.

“[The paper] provides a tangible hypothesis that links tectonic events specifically to changes that are recorded in the stratigraphic and biological records,” said Shanan Peters, a professor of geoscience at the University of Wisconsin, who was not involved in the research. The paper points out a possible tectonic driver, but more research is needed from the biological and geochemical perspectives to get a complete picture, Peters continued.

Dalziel is not claiming that he has solved the mystery of why the Cambrian Explosion occurred; rather, he has suggested a geologically plausible scenario that can now be tested.

“Scientists will now have a new and specific paleogeographic scenario to test and to consider regarding the environmental and biologic changes in the Cambrian,” he said.

—JoAnna Wendel, Staff Writer

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