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2015

After a Mass Extinction, Only the Small Survive
New York Times, November 12, 2015
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From sharks to giraffes, many of Earth’s biggest and most magnificent species are threatened with extinction. A new study of the fossil record indicates that once large vertebrates disappear, evolution cannot quickly restore them — for tens of millions of years, most animals remain small.

The study, published Thursday in Science, emerged from research carried out by Lauren Sallan, a paleontologist at the University of Pennsylvania.

Studying fish that lived during the Mississippian Period, from 359 million to 323 million years ago, she noticed that they were substantially smaller than their ancestors.

“It piqued my curiosity,” Dr. Sallan said in an interview. “Why are these fish so small?” (She earned a nickname from her fellow paleontologists: the Sardine Queen.)

Other paleontologists had previously noticed that some groups of species seemed to shrink in size over time. It’s called the Lilliput Effect, after the fictional island in Jonathan Swift’s “Gulliver’s Travels” inhabited by tiny people.

Researchers found that the Lilliput Effect often occurred after abrupt and widespread extinctions. Dr. Sallan’s fish appeared to fit the pattern: They shrank after a mass extinction at the end of the Devonian Period, 359 million years ago.

Scientists suspect that this mass extinction was triggered by a global chill that brought glaciers all the way to the tropics. The deep freeze was brutal both for fishes and for the early land vertebrates (known as tetrapods) that were just starting to clamber around on feet with toes. An estimated 96 percent of all vertebrates became extinct at the beginning of the Mississippian Period.

Still, scientists have been uncertain about how important the Lilliput Effect was. The cases documented by paleontologists had been limited to small groups of species and typically the researchers only looked at fossils over a limited stretch of time, before and after mass extinctions.

Some thought these Great Shrinkings might be illusory; small-bodied fossils, they noted, might be more likely to be preserved than big ones. Dr. Sallan decided to put the Lilliput Effect to a stronger test.

Working with Andrew K. Gallimberti, then an undergraduate at Kalamazoo College, Dr. Sallan studied every known vertebrate species that lived in both the Devonian and Mississippian. With online databases and digitized fossil scans, Dr. Sallan and Mr. Gallimberti estimated the body lengths of 1,120 fossilized animals.

After the mass extinction 359 million years ago, the scientists found, vertebrates were smaller on average than before, and they stayed that way for the next 36 million years.

Peter J. Wagner, a curator of paleobiology at the Smithsonian Institution who was not involved in the study, said one of its strengths was the broad stretch of time it covered.

Previous studies were “more like snapshots of what things were like before and afterwards,” he said. “This study goes much further.” Dr. Wagner, who wrote a commentary in Science accompanying the new study, said the work was also important because it dissected the Lilliput Effect, examining trends in different groups of species.

Some groups simply stayed at the same diminished size during the Mississippian, while others steadily shrank. Sharks, for example, dwindled from over a yard long to about just a few inches. Our own tetrapod relatives shrank from the size of dogs to the size of cats or smaller.

That’s not to say that all vertebrates got tiny — a few actually got bigger. One group of monstrous fish called rhizodontids, for example, included some species that grew to nearly the size of a killer whale.

But these groups of big vertebrates didn’t produce many new species. Instead, they dwindled in diversity until they became extinct altogether. That’s why we watch “Shark Week” instead of “Rhizodontid Week.”

Dr. Sallan said that these patterns could help explain the Lilliput Effect. Small vertebrates may have had an evolutionary edge that made them less likely to become extinct at the end of the Devonian. Perhaps they grew quickly and reproduced early in life, making them less vulnerable to ecological disruptions.

But even after the deep freeze was over, large species didn’t bounce back for a long time. Dr. Sallan suspects that this was because the world’s ecosystems remained decimated for millions of years, posing an enduring challenge to the survival of vertebrates. “They had to do more with less,” said Dr. Sallan.

Small vertebrates recolonized the planet, adapting to the ecological niches left empty by the mass extinctions. Filling those niches took an unimaginably long period of time, Dr. Sallan suggested. Only after easier niches were full did some vertebrates start evolving new patterns of growth to adapt to different niches, reaching bigger sizes in the process.

Dr. Sallan sees a worrying parallel between the extinction of large vertebrates 359 million years ago and their slow disappearances today.

If they become extinct in large numbers, she said, there’s no guarantee that evolution will bring back big species in short order.

“It’s likely that the global ecosystem will snap into this alternate state,” said Dr. Sallan — one dominated by small vertebrates. “They’ll be happy to stay there for millions of years.”

Copyright 2015 The New York Times Company. Reproduced with permission.
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