When the Predators are Away…

crinoid-predation

Ancient fish feed on crinoids before the extinction. (Art by Robert Nicholls)

At the core of ecology is the perpetual battle between predators and their prey. The relationship typically works like a see-saw: when more predators come into an environment, the prey population drops, until the predators start going hungry and dying off, allowing the numbers of prey to rebound, and so on. Ecologists have observed these dynamics in the wild as new predators are added to ecosystems or eliminated through extinction or relocation. But for the first time, this kind of predator-prey relationship has been observed in the fossil record, thanks to a newspaper article and a 360-million-year-old mass extinction.

One year ago, Lauren Sallan and Michael Coates of the University of Chicago published a paper on the Hangenberg Event, a mass extinction event that brought the prehistoric period known as “the Age of Fishes” to a catastrophic end. To pinpoint when the extinction occurred, Sallan built a new database of vertebrate fossils during and after the Devonian period, which ran from 416 to 359 million years ago. For 15 years after the Hangenberg extinction, they discovered that the formerly thriving ancient fish of sea and freshwater largely disappeared during a period known to paleontologists as Romer’s Gap.

Sallan and Coates’ study was covered by websites, radio shows, and newspapers, including a Los Angeles Times story that ran concurrently in the Chicago Tribune, and was spotted and clipped by the father of Thomas Kammer, a geologist at West Virginia University. Kammer studies the fossils of crinoids, species similar to modern sea lilies and related to starfish, and had been stumped by a mystery in his own database – a sudden burst of abundance and diversity known as the Age of Crinoids. When Kammer was sent the newspaper article, a missing piece of the story clicked into place.

“I read the article and it was like one of these light-bulbs going on over your head,” Kammer said. “We’ve been puzzled for many years as to why there were so many species and specimens of crinoids. There had to be some underlying evolutionary and ecological reason for that.”

Perhaps, he thought, the crinoids’ time of dominance may have been a consequence of the mass extinction of fishes. Some fish feed on crinoids to this day, but proving that crinoids were a part of the diet of ancient fishes that lived hundreds of millions of years ago is much harder. As Kammer put it, “You don’t actually find the evidence of a fossil fish with a crinoid in its mouth very often.”

Kammer reached out to Sallan for a collaboration, an effort that was joined by Lewis Cook of WVU and William Ausich of Ohio State University. In a study published by Proceedings of the National Academy of Sciences, the team analyzed the two fossil datasets side by side, and found that the timing of the fishes’ abrupt decline and the crinoid’s rise were indeed related. What’s more, as fish populations slowly returned to their former prominence, the crinoid numbers dropped back down to their earlier levels.

“It really tells us about recovery from mass extinctions, especially mass extinctions that involved loss of predators,” Sallan said. “Even if you have a group like the crinoids which makes it through the extinction unscathed, the death of their predators affects them for a further 10 to 15 million years.”

However, that prosperous era for crinoids came with a price. Without predators constantly testing their defenses, crinoid species retained their previous style of armor, which had evolved to protect against ancient fishes with sharp “shearing” teeth. But when new fish species began to retake control of the world’s waterways, they employed stronger “crushing” teeth that easily handled the outdated armor of the crinoids – called a “legacy adaptation” by the authors of the new study.

“There’s a complete absence of predation pressure and the crinoids take off, but they retain their defenses as if they can’t get out of them,” Sallan said. “When a new form of predator appears, they can go directly for the best solution to cracking a crinoid, which is crushing. The Devonian-era armor of crinoids isn’t suited for defending against that attack, but they can’t lose it without losing all of their residual defenses.”

Recently, some scientists predicted that Earth is showing warning signs of another mass extinction event, due to the number of species that are endangered or recently extinct. Given the lofty position of humans in the food chain, this is certainly bad news for our species, while others might actually benefit from the disappearance of humans. But as the new study shows, in the long run, even the winners of a mass extinction end up losing out.

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Sallan, L., Kammer, T., Ausich, W., & Cook, L. (2011). Persistent predator-prey dynamics revealed by mass extinction Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1100631108

About Rob Mitchum (526 Articles)

Rob Mitchum is communications manager at the Computation Institute, a joint initiative between The University of Chicago and Argonne National Laboratory.

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