Insects that eat leaves produce very specific types of damage. One type is from leaf miners -- insect larvae that live in the leaves and tunnel for food, leaving distinctive feeding paths and patterns of droppings.
Donovan, Peter Wilf, professor of geosciences, Penn State, and colleagues looked at 1,073 leaf fossils from Mexican Hat for mines. They compared these with more than 9,000 leaves from the end of the Cretaceous, 65 million years ago, from the Hell Creek Formation in southwestern North Dakota, and with more than 9,000 Paleocene leaves from the Fort Union Formation in North Dakota, Montana and Wyoming. The researchers present their results in today's (July 24) issue of PLOS ONE.
"We decided to focus on leaf miners because they are typically host specific, feeding on only a few plant species each," said Donovan. "Each miner also leaves an identifiable mining pattern."
The researchers found nine different mine-damage types at Mexican Hat attributable to the larvae of moths, wasps and flies, and six of these damage types were unique to the site.
The researchers were unsure whether the high diversity of leaf miners at Mexican Hat compared to other early Paleocene sites, where there is little or no leaf mining, was caused by insects that survived the extinction event in refugia -- areas where organisms persist during adverse conditions -- or were due to range expansions of insects from somewhere else during the early Paleocene.
However, with further study, the researchers found no evidence of the survival of any leaf miners over the Cretaceous-Paleocene boundary, suggesting an even more total collapse of terrestrial food webs than has been recognized previously.
"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," said Wilf. "The new herbivores included a startling diversity for any time period, and especially for the classic post-extinction disaster interval."
Insect extinction across the Cretaceous-Paleocene boundary may have been directly caused by catastrophic conditions after the asteroid impact and by the disappearance of host plant species. While insect herbivores constantly need leaves to survive, plants can remain dormant as seeds in the ground until more auspicious circumstances occur.
The low-diversity flora at Mexican Hat is typical for the area in the early Paleocene, so what caused the high insect damage diversity?
Insect outbreaks are associated with a rapid population increase of a single insect species, so the high diversity of mining damage seen in the Mexican Hat fossils makes the possibility of an outbreak improbable.
The researchers hypothesized that the leaf miners that are seen in the Mexican Hat fossils appeared in that area because of a transient warming event, a number of which occurred during the early Paleocene.
This is a micromoth larva mine on Juglandiphyllites glabra, the earliest known member of the walnut family.
Other researchers on this project were Conrad C. Labandeira, Department of Paleobiology, National Museum of Natural History, Smithsonian Institution and Department of Entomology and BEES Program, University of Maryland, College Park; Kirk R. Johnson, National Museum of Natural History, Smithsonian Institution; and Daniel J. Peppe, Department of Geology, Baylor University.
Contacts and sources:
A'ndrea Elyse Messer