The seemingly sudden spike in species diversity that occurred during the “Cambrian Explosion” around 542 million years ago has been of interest to scientists for quite some time, even leading Charles Darwin to dedicate a chapter of his book On the Origin of Species to his concerns about the proliferation of animals within the fossil record during that time.
UCSB associate professor of Precambrian and Cambrian paleontology Susannah Porter and graduate student John Moore were part of a research team that dated sedimentary layers by observing certain ratios of chemicals contained within them. The data were then used to correct the fossil record, revealing that instead of appearing suddenly, the species that came about during the “Cambrian Explosion” actually entered the record over a long period of time. The study was published in the Geological Society of America Bulletin.
According to Porter, a reason scientists may have been misled about the age of the fossils is they had not correctly established the timeline due to the inconsistency of sedimentary deposits in the area that many samples were collected from.
“When people thought there was a burst of diversification, it was because they focused on several rock outcrops in southeastern Siberia, where there is a lot of time missing,” Porter said. “This part of Siberia had periods where it was above sea level, so during that time interval you couldn’t find any of these fossils, which were all marine. When you do first find the fossils, it wasn’t because they had only evolved then, but rather it’s because the environmental conditions first allowed the fossils to be preserved during that time.”
Adam Maloof, assistant professor of geosciences at Princeton University and the first author of the study, said the researchers used the variation between two stable carbon isotopes — carbon-12 and carbon-13 — to determine the ages of the fossils in the rocks.
“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,” Maloof said in a press release. “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.”
The results of the timeline corrections revealed an earlier occurrence of new species as well as a gradual increase in their diversity over time, providing evidence refuting the theory of a sudden jump in the variety of species.
“There were two big surprises. First, how early the new animals appeared in the fossil record. I was shocked. Half of them are there by 535 million years ago,” Porter said. “The other big surprise was that there was a steady increase in diversity rather than a sudden burst.”
Moore said the Cambrian period was intriguing to scientists since it was the first time during which large amounts of animals that utilized minerals appeared in the fossil record.
“This time period is so interesting because it is the first time that biomineralized fossils become widespread and diverse,” Moore said. “Beforehand, such fossils were present but were few and far between.”
Porter said the team’s results go against the hypothesis that the diversity in the Cambrian period was a sudden burst.
“We question the evidence that it was a burst,” Porter said. “People have spoken of the ‘Cambrian Explosion,’ but we would play the devil’s advocate. This diversification was important, but we don’t think that it’s such a puzzling conundrum. With the different processes we have seen occur with other diversifications, we think it is explainable.”
While the study provides strong evidence toward the gradual increase hypothesis, Moore said the Cambrian period still has areas for exploration.
“We certainly haven’t solved the ‘problem’ of the Cambrian Explosion, but we’re helping to clarify the pattern,” Moore said.