Insects are soft-bodied, fragile animals whose remains are difficult to preserve. Wings are often fossilized, but insect bodies are usually parts of a primitive prehistoric animal, making them difficult for scientists to study. One of the ways paleontologists learn about prehistoric insects is through trace fossils, which are found almost exclusively in the form of traces in fossil plants.

Ph.D., a doctoral candidate in the Harvard Department of Evolutionary Biology. “We have a lot of experience with fossil plants,” said Richard J. Knecht. “It is trace fossils that tell us more about the evolution and behavior of insects in the distant past than fossil bodies, because plants and the trace fossils on them are so well preserved. And traces, unlike the body, will not move over time and will always remain where they were created.”

In a new study published in the journal New Phytologies, researchers led by Knecht describe an endophytic trace fossil found in a Carboniferous seed fern frond; This is the earliest evidence of internal nutrition within the leaf. The 312-million-year-old Carboniferous fossil shows how internal feeding, known as leaf mining, may have arisen, suggesting that this behavior emerged about 70 million years earlier than previously thought.

“The way insects feed inside plants, burrow into leaves, tumor-like galls that allow the insect to take control of the plant’s developmental mechanisms, insect holes and passages in wood, and the countless ways insects penetrate and consume seeds include Conrad C., senior research geologist and curator of Arthropod Fossils at the Smithsonian National Museum of Natural History.” “Nutrition nourishes embryonic tissues, but the most surprising is mineralization,” Labandeira said.

“The oldest mines were recorded in the early Triassic, shortly after the great extinction at the end of the Permian period, but galls, drilling and seed hunting were common much earlier, in the Paleozoic. Why is there a delay in mining? “I think we have the answer now.”

Internal plant feeding is common among holometabolic insects; these are insects that undergo complete metamorphosis: Lepidoptera (butterflies), Rigidoptera (beetles), Diptera (flies), and Hymenoptera (wasps and sawflies). The larva penetrates the leaf, begins to feed on the inner tissues of the leaf, and leaves a mark. As the larva tunnels through the leaf, it also grows, goes through various stages of molting, and even leaves behind a feces known as frass.

“Fras is one of the things we look at when defining internal nutrition. Fras may even have different characteristics that could be useful in determining which animal produced it, Knecht said. The larva will continue to leave its mark inside the leaf until it pupates, hatches, cuts off the leaf and flies away.

The fossil was found in the Coal Formation of Rhode Island. The Rhode Island Formation was initially a swampy and swampy environment, providing an anoxic environment in which plant fossils are very well preserved; What paleontologists call Lagerstette is the place where extraordinary fossils are unearthed in extraordinary safety.

“The only things that don’t fossilize are the larvae,” Knecht said. “They’re very fragile and small. So observing something like this is really useful because it gives us information about larval behavior at a specific time in the late Paleozoic, when we know very little about larvae.”

Results of backscatter and energy dispersive spectrometry (EMC) SEM analysis using JEOL 7900F SEM. (A) Backscatter SEM image of a portion of an endophytic trace. Boxes (BE) indicate areas where element mapping is performed. B1-E1 show composite images of all elements located in their respective focus spheres. Maps of individual elements of BE domains can be found in the supplementary data. B2-E2 are maps of phosphorus elements (blue color) located in BE rectangles. Credit: Richard J. Knecht and Anshuman Swain.

The extraordinary security allowed the researchers to clearly see an endophyte signature that fit the patterns paleontologists were looking for in this behavior. For example, larvae that follow a circuitous path will avoid the edges of the leaf and the main veins. This behavior is known to be characteristic only of insects with full metabolism, including animals extant today.

“This discovery pushes this behavior back 70 million years,” Knecht said. This shows us two things: First, larval behavior; Since larvae are generally not preserved, we cannot see them in the fossil record. Secondly, at that time there was a complete metamorphosis, the evolution of holometabolism.”

The fossil, along with other fossils studied by Knecht, is housed in the Museum of Comparative Zoology at Harvard.