312-Million-Year-Old Fossil Reveals Earliest Proof of Leaf Miners, Sheds Light on Insect Behavior and Evolution

A group of researchers found an endophytic trace fossil on an ancient seed-fern leaf, representing the earliest indication of internal feeding within a leaf.

Studying Insects Through Trace Fossils

As soft-bodied animals, insects are fragile organisms whose remains are difficult to preserve. Their wings are usually fossilized, but their bodies are often found in bits and pieces of the original prehistoric animal, making it challenging for experts to study them. One method paleontologists use to learn about prehistoric insects is studying trace fossils, almost exclusively found as traces on fossil plants.

Experts have an excellent fossil plant record. Trace fossils tell more about the behavior and evolution of the insects than the body fossils because they are well preserved by the plants and the trace fossils. Unlike the body, the trace does not move over time and is always found in the site where it was made.

Understanding Insect Behavior and Evolution

A group of researchers led by Richard K. Knecht from the Department of Organismic and Evolutionary Biology at Harvard University investigated an endophytic trace fossil discovered on a seed-fern leaf. The trace fossil was found in the Carboniferous Rhode Island Formation, originally a swampy, water-logged environment that provided an anoxic setting for plant fossil preservation. Also known as a Lagerstätte, the site produces extraordinary fossils with exceptional preservation.

The discovery represents the earliest indication of internal feeding or leaf mining. The 312-million-year-old fossil proves that this behavior may have originated approximately 70 million years earlier than believed.

The earliest evidence of leaf-mining was recorded from the Early Triassic period, right after the great end-Permian extinction. However, galls, borings, and seed predation are assumed to have occurred earlier in the Paleozoic era. It appears there was a delay in developing the habit of leaf mining.

Insects have various ways of feeding internally within plants, such as mining the insides of leaves, taking control of the plant's developmental machinery through tumor-like galls, boring of wood, and other methods of invading seeds to consume nutritious embryonic tissues. Among these behaviors, leaf mining has been the most mysterious, according to research co-author Conrad C. Labandeira from the Smithsonian National Museum of Natural History.

Internal feeding on plants is common among holometabolous insects, including moths, beetles, flies, wasps, and sawflies. In this process, a larva bores into the leaf and begins to feed on the internal tissues of the leaf, leaving a trail behind. The larva grows as it tunnels within the leaf, going through various molting phases and leaving behind its droppings called frass.

Frass is one of the things experts look for when identifying internal feeding. It has various traits useful when defining the animal that makes it. During internal feeding, the larva continues to trail within the leaf until it pupates, hatches cuts itself out, and flies away.

The delicate and small insect larvae do not fossilize. According to Knecht, seeing something like this is crucial because it provides information about larval behavior at a specific period. This has become particularly helpful for scientists studying the late Paleozoic era, where very little is known about larvae.

The exceptional fossil preservation enabled the researchers to observe the endophytic trace that follows the patterns searched by paleontologists when defining leaf mining behavior. The findings of this study show the behavior of larvae not typically seen in fossil records and indicate that the evolution of holometabolism also existed at this time.