Fossils unearthed in Grand Canyon reveal new details of evolutionary explosion of life

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3 days ago

Paleontologists have discovered remarkable fossils in the Grand Canyon that reveal fresh details about the emergence of complex life half a billion years ago.
The newfound remains of fauna from the region suggest that it offered ideal conditions for life to flourish and diversify, in a 'Goldilocks zone' between harsh extremes elsewhere. This evolutionary opportunity produced a multitude of early animals, including oddballs with peculiar adaptations for survival, according to new research.
During the Cambrian explosion, which played out in the coastal waters of Earth's oceans about 540 million years ago, most animal body types that exist today emerged in a relatively short time span, scientists believe.
Back then, the Grand Canyon was closer to the equator, and the region was covered by a warm, shallow sea teeming with burgeoning life — aquatic creatures resembling modern-day shrimp, pill bugs and slugs — all developing new ways to exploit the abundant resources.
Researchers turned to the Grand Canyon's layers of sedimentary rock to unlock secrets of this pivotal moment in the history of life, digging into the flaky, claylike shale of the Bright Angel Formation where most of the canyon's Cambrian-era fossils have been found.
The study team expected to recover mostly the fossilized remains of hard-shelled invertebrates typical of the region. Instead, the team unearthed something unusual: rocks containing well-preserved internal fragments of tiny soft-bodied mollusks, crustaceans, and priapulids, also known as penis worms.
'With these kinds of fossils, we can better study their morphology, their appearance, and their lifestyle in much greater resolution, which is not possible with the shelly parts,' said Giovanni Mussini, the first author of the study published Wednesday in the journal Science Advances. 'It's a new kind of window on Cambrian life in the Grand Canyon.'
Using high-powered microscopes, the team was able to investigate innovations such as miniature chains of teeth from rock-scraping mollusks and the hairy limbs and molars of filter-feeding crustaceans, providing a rare look into the biologically complex ways Cambrian animals adapted to capture and eat prey.
The 'Goldilocks zone' for innovation
For most of the planet's 4 billion-year history, simplicity reigned.
Single-celled microbes remained stationary on the ocean floor, thriving on chemical compounds such as carbon dioxide and sulfur molecules to break down food. What changed?
Scientists still debate what drove the Cambrian explosion, but the most popular theory is that oxygen in the Earth's atmosphere slowly began to increase about 550 million years ago, said Erik Sperling, an associate professor of Earth and planetary sciences at Stanford University.
Oxygen provided a much more efficient way to metabolize food, giving animals more energy to mobilize and hunt for prey, suggested Sperling, who was not involved in the new study.
'The (emergence of) predators kicked off these escalatory arms races, and then we basically got the explosion of different ways of doing business,' Sperling said.
During the Cambrian, the shallow sea covering the Grand Canyon was especially oxygen-rich thanks to its perfect, 'Goldilocks' depth, said Mussini, a doctoral student in Earth sciences at the University of Cambridge in the United Kingdom. Ranging from 40 to 50 meters (about 130 to 165 feet) in depth, the ecosystem was undisturbed by the shoreline's constant waves shifting around sediments, and sunlight was still able to reach photosynthesizing plants on the seafloor that could provide oxygen.
The abundance of food and favorable environmental conditions meant that animals could take more evolutionary risks to stay ahead of their competition, Mussini said.
'In a more resource-starved environment, animals can't afford to make that sort of physiological investment,' Mussini said in a news release from the University of Cambridge. 'It's got certain parallels with economics: invest and take risks in times of abundance; save and be conservative in times of scarcity.'
Many soft-bodied fossil finds before this one have come from regions with harsh environments such as Canada's Burgess Shale formation and China's Maotianshan Shales, noted Susannah Porter, a professor of Earth science at the University of California in Santa Barbara who was not involved in the study.
'It's not unlike if paleontologists far in the future only had great fossil records from Antarctica, where harsh cold environments forced people to adapt. … But then found great human fossils in New York City, where people flourished,' Porter explained. 'We have an opportunity to see different sorts of evolutionary pressures that aren't like, it's really cold, it's really hot, there's not a lot of water.'
Weird adaptations of Cambrian animals
While some of the feeding mechanisms uncovered in the Grand Canyon fossils are still around today, others are much more alien.
Among the most freakish: penis worms that turned their mouths inside out, revealing a throat lined with hairy teeth.
The worms, also known as cactus worms, are mostly extinct today, but were widespread during the Cambrian. The fossilized worm found in the Grand Canyon represents a previously unknown species.
Due to its relatively large size — about 3.9 inches (10 centimeters) — and distinct teeth, it was named Kraytdraco spectatus, after the fictional krayt dragon from the Star Wars universe, Mussini said. This particular penis worm appears to have had a gradient of hundreds of branching teeth used to sweep food into an extendable mouth.
'It's a bit hard to understand how exactly it was feeding,' Mussini said. 'But it was probably eating debris on the seafloor, scraping it away with some of the most robust teeth that it had, and then using these other, more delicate teeth to filter and retain it within this long, tube-like mouth.'
Rows of tiny molars, sternal parts and comblike limbs that once belonged to crustaceans were also among the findings, which all date back 507 million to 502 million years. Similar to today's brine shrimp, the crustaceans used these fine-haired limbs to capture floating food from the water and bring it to the mouth, where molars would then grind down the particles, Mussini explained. Nestled among the molars, researchers even found a few unlucky plankton.
Other creatures resembling their modern counterparts included sluglike mollusks. The fossils revealed chains of teeth that likely helped them scrape algae or bacteria from along the seafloor.
'For each of these animals, there's different components, but most of what we found directly relates to the way these animals were processing their food, which is one of the most exciting parts, because it tells us a lot about their lifestyle, and as a consequence, their ecological implications,' Mussini said.
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