Grand Canyon fossils reveal secrets that are forcing a rewrite of evolution textbooks

Our planet Earth has a mesmerizing history that spans across 4.6 billion years. For a majority of this immense timeframe, single-celled life reigned supreme.

But, about 500 million years ago, everything changed immensely. A dramatic occurrence took place known as the Cambrian “explosion.”

During this period, an enormous diversity of life forms made an appearance and found a place for themselves in the fossil record.

Imprinted in these preserved fossils, we can find traces of diverse animal groups that eventually evolved into animal species we see around us today, including us humans.

Understanding the Cambrian Explosion

The Cambrian Explosion was an incredibly important period in Earth’s history about 541 million years ago when life on Earth suddenly got a major upgrade.

Before the Cambrian, most organisms were simple and microscopic, but then things really took off.

Suddenly, the oceans were teeming with a dazzling variety of creatures, including the first animals with hard shells and complex body structures.

It’s like nature hit the fast-forward button, and you see the first fish, trilobites, and even some of the earliest ancestors of modern-day creatures making their debut.

Scientists are still trying to figure out exactly what triggered this burst of life, but they think it was a combination of factors like changes in the environment, the availability of oxygen, and maybe even genetic innovations that allowed organisms to develop new body plans.

Grand Canyon reveals Earth’s past

Now, an intriguing paper published by a team of researchers from The University of New Mexico (UNM) has added fresh insights into this fascinating event in Earth’s history.

“The Tonto Group of Grand Canyon holds a treasure trove of sedimentary layers and fossils chronicling the Cambrian Explosion some 500 million years ago, when the first animals with hard shells rapidly proliferated and sea levels rose to envelop continents with emerging marine life,” said Carol Dehler, professor at Utah State University.

Dehler served as the lead author for this paper, with UNM’s adjunct professor and paleontologist, Fred Sundberg, serving as the co-lead author.

New model of the Cambrian explosion

This research doesn’t just bring evolutionary advances to the table; it also has implications for geoscience education.

The study builds on Eddy McKee’s classic model for marine transgression (the spread of seas across continents), which was developed 50 years ago based on these same rocks.

McKee’s model, taught to geology students worldwide, suggests a gradual deepening and slow shifts in sedimentary environments.

“Our new model for deposition of the Tonto Group is much more nuanced, showing a mixture of marine and non-marine settings, breaks or unconformities when no sediment was being deposited, and a much faster tempo of evolution,” noted Professor Karlstrom.

“Even more than before, the Tonto Group of Grand Canyon remains one of the most important Cambrian-type sections in the world because of its complete exposure.”

Dating methods used for the study

The application of advanced techniques has provided new insights into the rate at which rock sedimentation occurred and has offered hints into how marine species like trilobites and other early animals diversified swiftly.

“Our new tandem U-Pb dating methods are refining precise ages for each layer in the succession and for the transitions between trilobite biozones,” said Schmitz.

“We are finding that different trilobite species radiated, then went extinct at a very fast, sub-million year, tempo.”

The dating methods used for the study involve extracting hundreds of sand-sized zircon crystals from a ground-up sedimentary rock sample.

The U-Pb ratios of these grains were first obtained using a preliminary method for swift dating, and then the youngest grains were precisely dated using a more elaborate laboratory technique.

New details about the Cambrian explosion

“Sedimentary rocks are hard to date,” said Crossey. “But the deposition of the sediment and fossils entombed within it, must be younger than the age of the youngest grain so with many dates we can bracket sedimentary ages”

The team’s new model, when compared to McKee’s model, provides key pathways that can enable students, researchers, and even anyone interested in geology to understand the Cambrian explosion on a deeper level.

“From the Tonto Group’s 500-meter-thick strata, we’re learning about sea-level rise and the effects of catastrophic tropical storms — probably more powerful than today’s devastating hurricanes,” Dehler continued.

“This occurred in a world without land plants and during a period of very hot temperatures when the Earth was ice-free. Sea levels were so high that sediments like the Tonto Group were deposited atop most continents in environments that allowed rapid expansion of animal diversity on Earth,” she concluded.

With advances like these, we can better understand and appreciate the complexity and beauty of the Cambrian explosion and Earth’s history, making us more connected to the past, and more conscious about how our actions today might shape the future.

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The team also included Karl Karlstrom and Laurie Crossey, distinguished professors at UNM, James Hagadorn from the Denver Museum of Nature and Science, Mark Schmitz from Boise State University, and Steve Rowland from the University of Nevada Las Vegas.

The study is published in the journal GSA Today.

Image Credit: UNM Newsroom

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