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In an old-growth temperate forest in what is now Oregon, an insect dug deep into a sandbar near a creek. There, in a moist burrow, she laid dozens of oblong eggs, about fifty in total. Despite her careful work in building this underground nursery, none of the eggs would ever hatch. Instead, the eggs, locked in a pod, fossilized into a stony, mineralized mass. And now, 29 million years later, they provide a record of insect reproduction that could be unlike anything paleontologists had seen before.
Recently, micro-CT scans of the egg capsule revealed not only that it was millions of years old, but also that it was most likely made by a locust. The eggs and overall nest construction are very similar to the eggs and pods of modern grasshopper species. This newly documented knowledge paints a clearer picture of that ancient ecosystem and confirms that locusts were present and flourished there – and that some species of locusts buried their eggs underground.
Insect eggs are extremely rare in the fossil record, and intact egg cases are even rarer. This is likely the only fossilized locust egg pod ever recorded, and it provides a window into their reproduction dating back to the Oligocene epoch (33.9 million to 23 million years ago), researchers reported Monday in the journal Parks Stewardship Forum .
“This work is exciting because such exceptional preservation provides unique insights into one of the least understood life stages of insects, especially in the geological past,” said lead study author Jaemin Lee, an evolutionary ecologist and doctoral student at the University of California, Berkeley. told CNN in an email.
Undisturbed fossils
What makes this fossil even more remarkable is that it was found in a habitat that is typically not friendly to fossils, said study co-author Dr. Nick Famoso, paleontology program manager and museum curator at John Day Fossil Beds National Monument. The site, located in Mitchell, Oregon, is managed by the National Parks Service.
Delicate fossils like this specimen are usually preserved along with plant material in lake deposits. Such places are often anoxic, oxygen-poor and relatively static, Famoso explains. Fossils can form there in peace, unaffected by currents or bacteria. But millions of years ago, a river or stream flowed through this location. Nevertheless, the conditions surrounding this egg pod were just right to remain buried and fossilize undisturbed in near-perfect condition, despite the dynamic environment of flowing water nearby, Famoso said.
The eggs in this fossil are notable for their preservation, “both individually and in clusters,” said paleobiologist Dr. Ricardo Pérez-de la Fuente, deputy head of research at the University of Oxford’s Museum of Natural History in the United Kingdom, said in an email.
“They are the first to be recognized in the fossil record as belonging to orthopterans – grasshoppers and their relatives, which is remarkable,” says Pérez-de la Fuente, who was not involved in the study.
“The work also represents an important step towards formalizing the description of immature stages of insects, more specifically those of eggs,” said Pérez-de la Fuente. This branch of science, known as ootaxonomy, “can provide crucial data on the evolution, behavior and ecology of insects in deep time, but which is often neglected in paleontological studies.” Furthermore, he added, the pod and eggs may provide clues about the environment in which they were fossilized.
The eggs had an unusual curvature
Christopher Schierup, collections manager for the National Parks Service, discovered the egg case in the fossil beds in July 2012. Schierup was conducting a routine visual survey of the site when he spotted the object, which was embedded in a piece of rock that had become stuck. rolled down a hill, Famoso recalled.
“No tools were needed to get it out of the ground,” he said. Schierup wrapped the object in toilet paper and carefully returned to the visitors center where our laboratory is located, Famoso added.
Based on surface analysis of the fossil, the researchers initially thought they had found a cluster of ant eggs. But Famoso was skeptical because their curvature differed from the curves of ant eggs and pupae. His suspicions were confirmed by Lee, who first saw the object in 2022 during a visit to the John Day Fossil Beds. They took the specimen to the University of Oregon’s Knight Campus in Eugene, where micro-CT scans were performed by co-author Angela Lin, director of the X-ray Imaging Research Core Facility.
“Then we discovered there was a protein layer that held everything together,” Famoso said. This wasn’t just any group of eggs; it was a kind of underground egg pod, called an ootheca, in which the eggs were cradled by a protective layer that had mineralized into a stony crust.
“Underground egg cases are currently produced by only two groups of insects,” Lee said. These are grasshoppers (order Orthoptera, suborder Caelifera) and heelwalkers (order Mantophasmatodea).
Radial arrangement
Visible on the surface were 28 ellipsoid eggs, each measuring no more than 4.65 millimeters in length and 1.84 millimeters in width (this is similar to the eggs of modern locusts, although egg size may vary depending on the species). Scans revealed another twenty eggs buried in the matrix in four to five layers, arranged in a radial pattern. Some eggs were hollow, while others were filled with sediment, the study authors reported.
“The mineralization we could see in each of the eggs made it very clear that it was a fossil structure,” Famoso said.
Because fossil insect eggs are so scarce, not many specimens were available for comparison. So Lee consulted a global database of insect eggs, which contains more than 6,700 living species, to identify the eggs in the fossil pod.
“I compared the eggs’ defining characteristics, including the size, length-to-width ratio and curvature of the individual eggs, with those of the live eggs,” he said. “Such large, elliptically curved eggs with large clutch size (~50 eggs in total) are unknown in living insect groups other than grasshoppers and grasshoppers.”
This unusual find provides a never-before-seen look at reproduction in the ancient relatives of modern locusts. The virtually pristine specimen also speaks to the level of preservation in the national park’s fossil beds, Famoso added.
“Just being able to see that internal structure and really describe what these things look like – that was something that was really exciting for us,” Famoso said. “As far as we know, there is nothing like it anywhere else in the fossil record.”
Mindy Weisberger is a science writer and media producer whose work has appeared in the magazines LiveScience, Scientific American, and How It Works.
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