Ancient insect graveyards reveal an explosion in bug diversity 237 million years ago

Two enormous fossil troves in China have yielded clues to a mystery: how insects became the most diverse members of the animal kingdom. The discovery reveals an explosion of diversity after a mass extinction event 252 million years ago, coinciding with a similar diversification of the plants that many insects feed on.

The find is “quite spectacular,” says Conrad Labandeira, a paleoecologist at the Smithsonian Institution’s National Museum of Natural History in Washington, D.C., who was not involved with the study.

The mass extinction, known as the Permian-Triassic extinction, was a triple whammy of massive volcanic activity, global warming, and asteroid impacts that wiped out more than 90% of marine life and 70% of land animals with backbones. Many kinds of insects, including primitive beetles, “true bugs” with plant-piercing mouthparts, and giant cockroaches and dragonflies, flourished before this event, but researchers were not sure which survived the cataclysm. Fossils that might answer the question are scarce, partly because insects lack durable parts like shells and bones.

Paleontologists Zheng Daran and Wang Bo of the State Key Laboratory of Paleobiology and Stratigraphy in Nanjing, China, and colleagues identified five sites in northwestern China that were about the right age to yield clues. Two of them—ancient lakebeds filled with layers of sandstone and other sedimentary rocks, 2300 kilometers apart—proved to be insect bonanzas, with impressions of wings, larval cases, and other insect remains. The team collected 800 fossils from the two sites.

Working with Chang Su-Chin from The University of Hong Kong, Wang, Zheng, and their colleagues dated the sandstone, determining that the sites were 237 million to 238 million and about 230 million years old. The older site was more diverse, with insects from 28 insect families in 11 major insect groups, the team reports today in Science Advances . These include many of the more advanced insects, such as flies and beetles, which all have a four-stage life cycle instead of the three-stage one of more primitive species such as cockroaches. The younger site had 10 insect families in six orders, including many of the same kinds as the older site. “They have better preserved material” on insects than has been found elsewhere, Labandeira says.

Overall, the findings suggest some types of insects—including beetles and cockroaches—survived the great extinction event, whereas others evolved after, Labandeira says. His own work on ancient plant-insect interactions, including patterns of plant damage, support this result, showing that insect biodiversity during that time period stemmed from a mix of survivors of the extinction and newly evolving species.

The sites underscore that this burst of evolution took place much earlier than researchers had thought, particularly for water-loving insects. Among the remains are fossil dragonflies, caddisflies, water boatmen, and aquatic beetles. Until now, paleontologists had thought such aquatic insects didn’t diversify until 130 million years ago. These insects—which include both predators and plant eaters—helped make freshwater communities more complex and more productive, says Zheng, moving them toward the ecosystems we see today.

This earlier date for the expansion of insect diversity aligns it with the explosion in plant diversity happening at that time. As pollinators and pests, the insects helped spur further plant evolution and vice versa. And together, they helped yield the flora and fauna of today.