About 360 million years ago, Earth’s seas were filled with myriad fishes, including creatures the size of school buses. Then a mass extinction hit the Age of Fishes. It killed off most of the big guys, according to a new study, and effectively shrunk most vertebrate species to the size of a human forearm or smaller. The findings imply that our planet’s next mass extinction—which some believe is already underway—could similarly shrivel any species that remain.

The ancient extinction happened about 359 million years ago, at the end of the Devonian Period. A 100,000-year-long cold spell triggered the growth of glaciers almost down to tropical latitudes, says Lauren Sallan, a paleobiologist at the University of Pennsylvania. Sea level fell substantially, wiping out much of the shallow-water habitat surrounding major landmasses. Because few creatures had yet moved onto land, many ecosystems were devastated. About 96% of the world’s vertebrate species disappeared, making it one of Earth’s five largest die-offs.

When Sallan and study co-author Andrew Galimberti of Kalamazoo College in Michigan, who is now at Michigan State University's Kellogg Biological Station in Hickory Corners, looked at the fossil record, they found interesting trends in body size during this period. The team compiled a list of more than 1000 species of vertebrates that lived in the 96-million-year interval straddling the mass extinction. During the 60 million years before the die-off, various types of fish and sharks had grown steadily in size, with some fish reaching the length of a school bus.

But in the 36 million years that followed the end-of-the-Devonian die-offs, the vertebrate species that had survived—or had evolved in its aftermath—were much smaller, on average, than those that lived beforehand. Immediately after the extinction, most surviving species measured less than 40 centimeters in length, the team found. And as time passed, average size of species in the ecosystems dropped even further. Neither climate nor oxygen levels in the atmosphere played a role in the overall shrinkage, the researchers report online today in Science.

Instead, the researchers point to an advantage of small size: Among today’s creatures, diminutive species tend to grow and reproduce faster, so that they can produce a new generation before they are killed by predators. Also, when animals breed more quickly, their populations can adapt to changing conditions more rapidly. A turbulent world may have pressured animals to adapt to changing conditions as quickly as possible, and thus they got smaller over time, Sallan says. Although some species grew in size after the mass extinction, she notes, they typically belonged to groups that later died out.

Many previous studies probably haven’t noted this “Lilliput effect”—named for the island of tiny humans in Gulliver’s Travels—because they looked at only a few species over a short period of time, says Peter Wagner, a paleobiologist at the Smithsonian’s National Museum of Natural History in Washington, D.C. In contrast, he notes, Sallan and Galimberti’s analysis assesses trends in body size over a broader time scale and among a wider range of species.

The team’s findings may have implications for the modern world, which some scientists propose is experiencing a sixth mass extinction—especially in marine ecosystems, where overfishing is rife. The new research “suggests that any future major extinctions of fishes could result in a long-term decrease in survivor’s body sizes,” says John Long, a paleontologist at Flinders University in Adelaide, Australia. The Lilliput effect may extend to other major groups such as mammals as well, some scientists have suggested.