A little over 400 million years ago, the Earth was going through a bit of a rough patch. Sea levels were plunging, the ocean was starved of oxygen and the number of marine species going extinct skyrocketed.
From that chaos, a team of researchers including Wright and Ben Thuy, paleontology curator at Luxembourg’s National Museum of Natural History, has plucked a set of fossils that could help us understand how the animal kingdom responded to an upended world. In a new study published in the journal Communications Biology, the group describes two new species of ancient brittle stars, the slender-armed cousins of starfish, and suggests that they evolved as a direct result of environmental upheaval.
“Ecological stress can lead to evolutionary innovation,” Thuy said. Understanding that process could offer clues about how species might respond to human-caused turmoil today.
Brittle stars, which look like a stretched-out version of a typical starfish, have existed for millions of years. The species alive today occupy all of Earth’s oceans, gobbling up other animals’ waste as they wriggle across the seafloor.
“They move intriguingly fast for an animal that has no brain and has five arms to coordinate,” said Thuy, who led the study. Brittle stars can also regrow broken limbs, reproduce by splitting in half and emit spurts of blue or green light that scientists believe may help them ward off predators.
What’s more, all those talents offer plenty of avenues for investigation. “They’re like an unused playground,” Thuy said. “You have loads of research questions that have never been addressed before.”
So when fossil collector Manfred Kutscher, a co-author on the study, approached him with an array of brittle star fragments, Thuy was fascinated.
The fossils were miniscule, only a fraction of a millimeter long. They came from Gotland, an island off the coast of Sweden, embedded in rocks dating back between 420 and 428 million years ago.
Some of the vertebrae and arm pieces Thuy examined under the microscope surprised him. After comparing them with the thousands of other microfossils in the group, Thuy and his colleagues determined that they’d come across two entirely new species.
That meant the team needed to name them — and they got creative. A number of the researchers liked heavy metal music, so they dubbed the two species accordingly: Ophiopetagno paicei, after album-cover artist Joe Petagno and Deep Purple drummer Ian Paice; and Muldaster haakei, after Meshuggah drummer Tomas Haake.
Thuy, who’s paid homage to rockers in other species, too, said the names weave together two of his main sources of inspiration: music and science. “You can build a bridge to another world,” he said. “You can reach a completely different audience.”
Their multidisciplinary names aren’t the only things that set these brittle star species apart. The researchers had a hunch that M. haakei might actually be a direct descendant of O. paicei on the evolutionary tree — a rare find in the fossil record.
That’s when they enlisted Wright to run the statistical tests that would help reveal the relationship.
“I had no idea what the results were going to be,” Wright confessed. “Thinking about the vast number of species on planet Earth that must have existed in the history of life, how likely is it to find two species that are an ancestor-descendant pair?”
After running the analysis, Wright was floored. He tweaked every part of the program he could think of to see if it might spit out an evolutionary history that made the two brittle star species “cousins” instead of an ancestor and a descendant. “I re-ran the analysis multiple times to figure out how confident we should be about the results. I even tried inserting some crazy assumptions, but no matter what, I still couldn’t break it,” he said.
But an ancestor-descendant pair is more than just a cool paleontological discovery. If every fossil is a snapshot in an evolutionary movie, these two brittle stars are like a few milliseconds of film. They allow scientists to glimpse evolutionary change unfolding from one species to the next.
Thuy used another analogy. “Limitations are inherent to the fossil record, because it is a small window into what happened in the past,” he said. In this case, though, “the glass was exceptionally clear.”
Things didn’t look so good out the window. O. paicei, and then M. haakei, traversed the ocean floor in a world buffeted by environmental crises: fluctuating sea levels, changing ocean chemistry and mass extinctions.
That planetary shudder is known as the Mulde Event, one of several upheavals that happened in quick succession in geological terms. The turbulence may not have been enjoyable for the brittle stars, but it could be key to understanding the evolutionary change that took place between O. paicei and M. haakei.
For example, the fossils from adult specimens of M. haakei are up to four times smaller than those of the other brittle stars from around the same time, including its ancestor O. paicei. According to Wright, “miniaturization” like this is often seen in the fossil record after extinction events. “It could have to do with depleted resources, like food,” he said. But it’s not often that researchers are able to pinpoint the likely driving forces behind an adaptation that separates a descendant species from its ancestor.
Of course, this work doesn’t unequivocally prove that environmental distress actually triggered the evolutionary change in the brittle stars, Thuy said. It only shows a correlation in time between the two. But he and his colleagues believe the Mulde Event likely played a role.
Warnings from a wipeout
400 million years after these brittle stars roamed the seas, today’s species encounter considerable environmental distress of their own. Between climate change, habitat destruction and other turmoil, modern organisms face an onslaught that is on track to be much more extreme than the Mulde Event.
“It will likely have a much larger impact on the environment and on future biodiversity,” Wright said.
Studies like this one are a step toward understanding how species will inevitably adapt — or die out — in response.
“It’s always difficult to draw significance from paleontological data, because we’re talking about different time scales,” Thuy said. “Nevertheless, I would say that this really helps us understand how ecological factors influence evolutionary change.”
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