Far back in its history, the Earth saw two massive spikes in biodiversity: the Cambrian Explosion, which took place between 540 million and 522 million years ago, and the Great Ordovician Biodiversification Event that followed the first burst 40 million years later.

The Cambrian Explosion brought an “evolutionary storm,” according to a 2016 article in Nature, in which soft-bodied animals and a seafloor of microbial slime gave way to the rise of many of the animal types we see today. With it came arthropods like Anomalocaris, nicknamed the “abnormal shrimp,” with compound eyes and undulating flaps for swimming. So did chordates, the phylum that houses vertebrates. During the second event, also known as the Ordovician Radiation, familial diversity nearly tripled. Ecosystems such as coral reefs began to take more modern form.

These two events were once lumped together, explained geochemist Ben Gill, but the emerging view is that the evolution of animals during these periods represent two distinct moments. That’s because scientists can spot a sort of lull in animal diversity from the end of the first event to the beginning of the second.

In that time, multiple extinction events took place, followed by small spurts of growth in biodiversity. Diversification and extinction canceled one another out, creating stasis. “Why didn’t we shoot all the way up to those modern-looking ecosystems?” asked Gill, an associate professor in the Department of Geosciences, part of the Virginia Tech College of Science. “Why do you have this interval where things level off and animals aren’t very diverse?”

A man with. reddish hair and a beard and glasses, poses in. checkered shirt near shrubbery and Hokie Stone walls.
Ben Gill. Photo by Steven Mackay for Virginia Tech.

With support from NASA’s exobiology program, Gill is leading a team of geologists and geochemists from Virginia Tech, Florida State University, Smith College, and Yale University to collect and study sedimentary rocks and fossils deposited globally under the ocean during the 40-million-year delay in diversification between the Cambrian Explosion and the Ordovician Radiation. From those rocks, they’ll gather paleontological and biogeochemical data to reconstruct that time period and probe the delay.

The NASA grant totals $1.24 million, with roughly $400,000 of that coming to Virginia Tech.

The team will explore the role of marine environmental instability at the time, which will involve examining another emergent theory about this point in Earth’s past: that widespread anoxia, or oxygen deficiency in the oceans, may have played a part in inhibiting growth in biodiversity during the time interval.

“If we went back about 15 years ago, people thought the Earth had the kind of levels of oxygen that we have today during this time interval,” said Gill, who is also an affiliated member of Virginia Tech’s Fralin Life Sciences Institute and Global Change Center. “Over the last 15 years, there’s been a greater appreciation that the oxygen content of the atmosphere didn’t really rise as rapidly to modern levels as we thought. Now, we’re trying to understand all the consequences of that.”

Gill’s working theory: “The ocean was probably not the best place to live in during that time,” he said. Those inhospitable conditions may include high heat and carbon dioxide, and low oxygen.

Recent studies have hypothesized that oxygen deficiency, or anoxia, was widespread in Cambrian oceans. Gill’s team will test that hypothesis through the team's work with sedimentary rocks. Researchers will look at how anoxia could have led to high extinction rates and affected animal ecologies that shape the structure of seafloor communities and their biodiversity. They’ll study skeletal carbonate production and bioturbation, an “ecosystem engineering” behavior of mixing up sediment, which is done by burrowing animals. Both ecologies leave breadcrumbs in rocks.

During the course of three years, the researchers will collect rock samples from areas of Nevada and Utah in the western United States, western Newfoundland, and Sweden. Each area holds sedimentary rock layers preserving sediment that was once on the seafloor — the “pages of the book” recording a given time interval, Gill said. Sara Pruss of Smith College will collect rock samples and count the fossilized skeletons inside them. Lidya Tarhan of Yale will examine the rocks for traces of bioturbation. Gill and collaborators Seth Young and Jeremy Owens of Florida State will analyze the samples’ chemistry, which reflect changes in the ocean’s chemical composition over time.


A ruler rests aside a stone with the fossilized outline of a circular sea creature etched inside it.
Gastropod fossil in the Antelope Valley Limestone exposed in the Arrow Canyon Range. These are one groups of organisms that diversified during the Ordovician Radiation that followed the lull in animal diversity in the later Cambrian and early Ordovician. Photo courtesy of Ben Gill.

The researchers aim to provide the first dataset to integrate observations from the fossil record with indicators of the environment to reconstruct the 40-million-year period between events.

How biodiversity did or didn’t thrive over time and the connection to conditions like oxygen are a subject of interest to NASA’s exobiology program. “As we look for extraterrestrial life, we need to understand how life on Earth evolved,” Gill said. “We need to know the broad conditions that life can exist under or has existed under on our planet. The planet was very different at different times. You can use that to search for criteria for life on other planets, especially the more earth-like exoplanets.”

The team’s findings also have implications not only for understanding Earth’s past, but for what we can learn as we look ahead at the effects of climate change, Gill said. We’re seeing increases in oxygen deficiency in the oceans right now, linked to global warming and algal blooms. “A lot of this work in the past, you can use it as: the Earth has run the experiment,” he said. “The environment has changed in some way, and we see the whole record of what happened in the wake of that.”

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