Biodiversity born of mass extinction

U. CHICAGO (US) — Fossils from more than 300 million years ago show that what was bad for fish was good for the fish’s food.

A new study, published online in the journal Proceedings of the National Academy of Sciences, reports that the mass extinction known as the Hangenberg event, which decimated ocean life 360 million years ago, produced a “natural experiment” in the fossil record that mirrors modern observations about predator-prey relationships.

“This is the first time that specific, long-term predator-prey interactions have been seen in the fossil record,” says Lauren Sallan, graduate student of organismal biology and anatomy at the University of Chicago and the study’s lead author. “It tells us a lot about the recovery from mass extinctions, especially mass extinctions that involved a loss of predators.”

The Devonian Period, known as the Age of Fishes, spanned from 416 to 359 million years ago, and was a time of astonishing diversity for marine vertebrate species. That thriving world was devastated by the Hangenberg event, a mass extinction of unknown origin that set the stage for modern biodiversity.

The next 15 million years in the fossil record are dominated by crinoids, species similar to modern sea lilies and related to starfish, that survived the event in such numbers that the period is known as the Age of the Crinoids—entire limestone deposits from the era are made up of crinoid fossils.

“We’ve been puzzled for many years as to why there were so many species and specimens of crinoids,” says study co-author Thomas Kammer, professor of geology at West Virginia University. “There had to be some underlying evolutionary and ecological reason for that.”

Researchers considered whether a change in the environment, such as water temperature or depth, or changes in predation could explain the crinoid explosion, but were hard to test using the fossil record, Kammer says. “You don’t actually find the evidence of a fossil fish with a crinoid in its mouth very often.”

Kammer and William Ausich, study co-author and professor of earth sciences at Ohio State University, assembled a database that was compared to data created by Sallan. When the data was placed side by side and analyzed, a clear relationship emerged.

As fish populations thrived in the Devonian, crinoid diversity and abundance remained low despite favorable conditions. But after the Hangenberg event devastated fish species, crinoids thrived, diversifying and multiplying. The data suggests the ripple effects of a mass extinction upon ecosystems can last millions of years.

“This study demonstrates a clear example of processes that operate in ecological time—predator-prey relationships—becoming significant in evolutionary time,” Ausich says. “This means that ecological patterns and processes taking place today may have very long-reaching evolutionary effects. And although that idea may not seem particularly surprising at first, respected evolutionary biologists have long argued that it was not possible.”

Eventually, the success of crinoids came to an end. As fish species recovered to previous levels, crinoid populations declined in tandem—further evidence for typical predator-prey dynamics known as Lotka-Volterra cycles in modern ecology.

Fossils even suggest that the long period of dominance had left the crinoids especially vulnerable to a new predator strategy. In the Devonian era, crinoids evolved hard armored shells to defend against fish with sharp “shearing” teeth. But when fish populations returned in force 15 million years after the Hangenberg event, those species used “crushing” teeth well suited for thwarting crinoid defenses.

The persistence of a once-beneficial trait that becomes obsolete should be called a “legacy adaptation,” the authors say. In the absence of a predator-prey arms race, a species’ inherited defenses may become outdated.

“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 says.

“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.”

The research was funded by the National Science Foundation, Paleontological Association, Paleontological Society, American Society of Ichthyologists and Herpetologists, and the Evolving Earth Foundation.

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