A symphony of insect songs rang through the Jurassic rainforest, and scientists have now found the traces of this ancient soundscape.
Based on an analysis of fossilized katydid ancestors, the notes these insects played by strumming their wings can be reproduced, researchers report August 25 in the Proceedings of the National Academy of Sciences.
“Most fossils tell us about anatomy — shape, size, structure,” says Jun-Jie Gu, an evolutionary biologist at Sichuan Agricultural University in Chengdu, China. “But these fossils preserve something far more ephemeral: sound.”
The rocks holding the remains of these ancient insects come from rural Daohugou, China. This landscape of hills and grasslands was a hot, humid forest with giant ferns and ancient conifers during the Jurassic Period, 200 million to 145 million years ago. It hosted a stunning array of insects, Gu says, including ensiferans, the group that includes katydids, crickets and grigs.
The fossils reveal the wing structures used to make mating calls. They’re like “nature’s violin,” says biophysicist Fernando Montealegre-Zapata, of the University of Lincoln in England. One wing sports a row of microscopic teeth. As the wings close, a scraper on the other wing hits the teeth, making vibrations that are amplified by the rest of the wing. This forms what the team calls a syllable, the basic unit of sound. The song emerges as the creatures rhythmically open and close their wings.
For 20 fossils representing nine extinct species, the team simulated wing acoustics based on the teeth spacing, wing size and wings’ vibrating area. The researchers also used an evolutionary family tree to home in on each species’ call frequency, or its pitch. They checked their process by reproducing the calls of living katydids, crickets and grigs based on photos of these insects’ wings. This confirmed that the measurements needed to figure out the animals’ sounds can be obtained from the kinds of 2-D images of wings made from photos of fossils.
The cadence, or repetition, of those syllables isn’t clear from the fossils, Montealegre-Zapata says. So the team looked to living ensiferans, whose repertoire is related to body shape and size and changes with temperature. The team trained a machine learning model with those characteristics and used the temperature of the Jurassic rainforest to simulate ancient insect calls. The result was a chorus of katydid ancestor calls.
The work is “very exciting,” says Roy Plotnick, a paleobiologist at the University of Illinois Chicago who was not involved with the study. The team is “able to show that each particular species had a distinctive sound,” or characteristic frequency, which could be used for mate recognition, he says.
Scans of these ancient insects’ ears, which are located on their forelegs, support the notion that these insects hear those of their own kind.
The Jurassic insects’ sounds are clear and musical, unlike buzzing or hissing sounds, which contain many frequencies. The source of musical tones can be difficult for mammalian predators to identify based on the physiology of their ears. So these single note tones could have helped the creatures stay somewhat inconspicuous to predators while sending communiques to potential mates, says Daniel Robert, a biophysicist at the University of Bristol in England.
One of the nine ancient species produced calls in the ultrasound, above the range of human hearing. This creature evolved nearly 100 million years before bats evolved with their ears attuned to ultrasonic frequencies.
Meanwhile, other groups have found fossils of early mammals from the same formation in China. Their work on a shrewlike critter’s preserved inner ears suggests that their hearing range could capture the calls of some katydid forerunners. These insects may have been evolving to escape the hearing range of hungry mammals, which in turn may have evolved to hear higher pitches, Montealegre-Zapata says.
“That’s a very stimulating thought — a very interesting hypothesis — but it needs to be tested,” says Zhe-Xi Luo, a paleobiologist at the University of Chicago who wasn’t involved with the study. Luo’s team is working to better understand the hearing capability of early mammals by comparing their hearing structures with those of modern mammals. But it’s likely, he says, that insects were just abundant in the Jurassic and early mammals evolved capabilities to exploit the glut of food.
There’s still much to learn about the soundscape of the Jurassic and that of even earlier periods, Plotnick says. Researchers could search other fossils for insect body parts that make sound by rubbing together.
“We often think of fossils as silent stones,” Gu says, “but our work shows that we can actually hear what 165-million-year-old animals were saying.”
