Some insect larvae live in leaves and tunnel around in them for food, leaving behind distinctive feeding paths and patterns of droppings. When the dinosaur-killing asteroid hit at the end of the Cretaceous, the leaf-mining insects in the western U.S. completely vanished as well. But just a million years later during the Paleocene, leaves began to show traces of mining from brand new insects. 

Now, ancient leaves with fossilized mines reveal a leaf-mining diversity higher than paleontologists ever recognized. And according to a new study published in PLoS One this week, leaf miners in the Great Plains suffered from “drastic extinction” followed by an influx of novel bugs in the Paleocene. The mines were not made by survivors from the Cretaceous.

A team led by Michael Donovan from Penn State University looked at 1,073 leaf fossils from Mexican Hat, a formation in southeastern Montana. They compared damage done to those leaves with that of 9,000 Cretaceous leaf fossils from the Hell Creek Formation of nearby North Dakota and 9,000 Paleocene leaf fossils from the Fort Union Formation in North Dakota, Montana, and Wyoming.

They found nine different mine damage types -- made by moth, wasp, and fly larvae -- and six of the damage types were unique to Mexican Hat. None of the newly discovered Mexican Hat mines can be linked back to Cretaceous mining fauna. 

Here, you can clearly see a nice mine made by a micromoth larva on Juglandiphyllites glabra, the earliest known member of the walnut family:

"These results show that the high insect damage diversity at Mexican Hat represents an influx of novel insect herbivores during the early Paleocene and not a refugium for Cretaceous leaf miners," says study coauthor Peter Wilf of Penn State in a news release. "The new herbivores included a startling diversity for any time period, and especially for the classic post-extinction disaster interval."

Leaf miners are typically host-specific, feeding on only a few plant species each. Insect extinction across the Cretaceous-Paleocene boundary may have been directly caused by the disappearance of host plants during post-impact conditions. Insect herbivores continuously need leaves to survive; plants, on the other hand, can remain dormant as seeds in the ground. 

The researchers think that the leaf miners seen in the Mexican Hat fossils appeared in the area because of a transient warming event (many of which occurred during the early Paleocene) and range expansions. "Insect herbivore extinction decreased with increasing distance from the asteroid impact site in Mexico,” Donovan says. “So pools of surviving insects would have existed elsewhere that could have provided a source for the insect influx that we observed at Mexican Hat."

Images: Michael Donovan/Penn State