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In an ancient temperate forest in what is now Oregon, an insect dug deep into a sand bank near a lake. Then, in a damp hole, she laid dozens of long eggs, about 50 in all. Despite her careful work building this underground nursery, none of the eggs would ever hatch. Instead, the eggs, enclosed within a pod, fossilize into a stony, mineral mass. And now, 29 million years later, they stand as a record of insect reproduction that may be unlike anything paleontologists have seen before.
Recently, a micro-CT scan of the egg case showed not only that it was millions of years old, but also that it was probably made by a grasshopper. The eggs and overall nest construction closely resemble the eggs and pods of modern wren species. This newly documented information paints a clearer picture of that ancient ecosystem, confirming that grasshoppers were present and thriving there — and that certain types of grasshoppers buried their eggs underground.
Insect eggs are extremely rare in the fossil record, and intact egg cases are even rarer. This is probably the only fossilized wren egg pod on record, and provides insight into their reproduction dating back to the Oligocene Epoch (33.9 million to 23 million years ago), researchers reported Monday in the Parks Stewardship Forum magazine.
“This work is exciting because such exceptional preservation provides unique insights into one of the least understood life stages of insects, particularly in the geological past,” lead study author Jaemin Lee, an evolutionary ecologist and doctoral student at the University of California, Berkeley, told CNN in an email.
Fossilization without destruction
What’s even more remarkable about this fossil is that it was found in a habitat that isn’t usually kind to fossils, said study 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 Park Service.
Delicate fossils such as this specimen are usually preserved in lake deposits alongside plant material. Such places are usually anoxic, or poorly oxygenated, and relatively static, Famoso explained. Fossils can form there in peace, untouched by currents or bacteria. But millions of years ago, a river or stream ran through this spot. However, the conditions surrounding this egg pod were just right for it to remain buried and fossilize undisturbed in near-perfect condition, despite the dynamic environment of nearby flowing water, which Famoso said.
The eggs in this fossil stand out for their preservation, “individually and in clusters,” said paleobiologist Dr. Ricardo Pérez-de la Fuente, deputy head of research at Oxford University’s Museum of Natural History in the United Kingdom, in an email statement.
“They are the first to be identified as belonging to orthopterans — grasshoppers and their relatives — in the fossil record, which is significant,” said Pérez-de la Fuente, who was not involved in the research.
“The work is also an important step towards formally describing the immature stages of insects, especially egg stages,” said Pérez-de la Fuente. This branch of science, called ootaxonomy, “can provide invaluable data on the evolution, behavior and ecology of insects over time, but which are usually neglected in paleontological studies.” In addition, he said, the pod and eggs may provide clues about the environment in which they fossilized.
The eggs had an unusual curvature
Christopher Schierup, a collection manager for the National Park 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 chunk of rock that yes. rolling down a hill, Famoso recalled.
“It didn’t require any tool work to get it out of the ground,” he said. Schierup swallowed the object in toilet paper, “and carefully returned it to the visitor center where our lab 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, as their curve differed from the curves of ant eggs and pupae. His suspicions were supported 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 Knights Campus in Eugene, where study co-author Angela Lin, director of the Central X-ray Imaging Research Facility, performed a micro-CT scan.
“That’s when we discovered that this protein layer was holding everything together,” Famoso said. This wasn’t just a cluster of eggs – it was a type of underground egg pod called an ootheca, with the eggs covered in a protective layer that had mineralized into a stony rind.
“Only two groups of insects currently produce underground pods,” Lee said. These are grasshoppers (Orthoptera, suborder Caelifera) and heel walkers (order Mantophasmatodea).
Radial arrangement
On the surface, 28 ellipsoid eggs were visible, each no more than 0.18 inches (4.65 millimeters) long and 0.07 inches (1.84 millimeters) wide (this is comparable to modern grasshopper eggs, although it is possible the size of the eggs varies depending on the species. ). The scans revealed more than two dozen additional eggs buried in the matrix in four or five rows, arranged in a radial pattern. Some of the eggs were hollow, while others were filled with sediment, the study authors reported.
“It was clear from the mineralization that we could see in each of the eggs that this was a fossil structure,” Famoso said.
Because fossil insect eggs are so rare, few specimens were available for comparison. So Lee consulted a global database of insect eggs, which contained more than 6,700 living species, to identify the eggs in the fossil pod.
“I compared the defining features of eggs including size, length to width ratio, and curvature of individual eggs with those of living people,” he said. “Such large, curved, elliptical eggs in a large clutch size (~50 eggs in total) are not known from any other living group of insects except grasshoppers and locusts.”
This unusual discovery provides unprecedented insight into reproduction in ancient relatives of the modern reed. The nearly pristine specimen also speaks to the level of preservation in the national park site’s fossil beds, Famoso added.
“Just being able to see that internal structure and describe properly what these things look like – that was really exciting for us,” Famoso said. “There is nothing else like this in the fossil record anywhere that we know of.”
Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American and How It Works magazine.
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