Researchers studying deep-drilling cores have long noticed odd flecks of material in their samples, possibly from insects. They generally treated these as a distraction from their real work, and focused instead on pollen and spores as a continuous record for understanding past ecosystems. But a surprising abundance of those flecks in a recent sample from northern Germany has now led a team of scientists to pay closer attention.
Writing in Science Advances, evolutionary biologist Timo van Eldijk and his co-authors described their find as the earliest fossil record of Lepidoptera, the insect order that includes butterflies and moths. They date the sample to about 201 million years ago, the boundary between the Triassic and Jurassic periods. The new find fits the time line for Lepidoptera evolution suggested by molecular evidence—and helps correct a puzzling gap in the fossil record.
The study looks at 70 specimens found in a drill core from more than 300 meters below Earth’s surface, and identifies them as the wing scales that give butterflies and moths their spectacularly varied colors and patterns. A light microscope, and later a scanning electron microscope, revealed the scales to be petal-like structures. Some were beautifully preserved and had neat ridges interspersed with herringbone webbing as well as “micro-ribs” and, in some cases, perforated surfaces.
Those perforations turned out to be a critical detail. They indicate, according to the co-authors, a moth of that period had the hollow wing scales characteristic of Glossata, the taxonomic group that includes all modern moths and butterflies equipped with a sucking proboscis. The oldest previously known such fossil was from 129 million years ago—just as flowering plants were making their spectacular emergence around the planet. The accepted theory up to now has been that the sucking proboscis only emerged at that point as a product of coevolution between flowers and the insects that pollinate them.
That coevolution, and the often exquisitely precise matchup between flower and pollinator, have been a subject of perennial fascination for naturalists. In one of the most celebrated stories in botany, for instance, Charles Darwin was examining a shipment of orchids from Madagascar that included one flower with its nectar hidden at the bottom of a foot-long tube. “Good heavens, what insect can suck it?” Darwin wrote to a friend. The next day he conjectured there must be a moth with a proboscis roughly that long to do the job. Just such a moth, with an 11-inch-long coiled proboscis, finally turned up 45 years later.
But the new study pushes fossil evidence for the origin of the sucking proboscis back 70 million years. That “challenges the underlying notion,” the authors wrote, that the emergence of flowering plants roughly 130 million years ago drove the evolution of the Lepidoptera proboscis. They argue instead that the transition among these insects “to exclusively feeding on liquids was most likely an evolutionary response to widespread heat and aridity” during the late Triassic.
Whereas some moths—including species represented in the same core sample—continued to have chewing mouthparts, the authors wrote, others evolved sucking mechanisms for drinking water droplets or sap from damaged leaves. Although the paper does not make this point, van Eldijk, a graduate student at Utrecht University, suggests the early emergence of the sucking proboscis may even have helped drive the emergence of flowering plants, rather than vice versa.
Other scientists greeted the find with excitement, as it begins to fill what University of Connecticut lepidopterist David Wagner calls “this huge gap in the fossil record.” But Wagner, who was not involved in the new study, also describes its interpretation of this new evidence as “widely speculative and likely wrong.” He questions the idea that the proboscis evolved in response to aridity: “There are 24 other orders of flying insects” from the same period, he says, “that did just fine without having a [sucking] proboscis.” They got water, he adds, “the way other animals do—they drink it, lap it, use capillary action, whatever.” The short, simple proboscis in early Lepidoptera also has little to do, he says, “with the coiled proboscis that later evolved to get nectar from deep within a flower. There are just millions and millions of years of evolution” between the emergence of the proboscis in these primitive Glossata and the later appearance of “long, strawlike proboscis for feeding in flowers. Not all tongues are created equal.”
William Friedman, an evolutionary biologist and director of the Arnold Arboretum of Harvard University who was not involved in the work, agrees: “I don’t think this necessarily changes the story of the coevolution of lepidopterans and flowering plants,” he says. “There are lots of biological structures that hang around with one function—and at a later point that function gets changed, or all of a sudden that function has a new context. Even if Lepidoptera are significantly more ancient, which is wonderful, that doesn’t mean that they diversify, that they do a million and one things. They could be like ancient mammals, hanging around, being small and not doing much till the asteroid hits”—or in the case of the Lepidopteranproboscis, until the emergence of flowers.
It will take other samples from other drill cores to fill out the fossil record and enable researchers to begin making more sense of the Lepidopteranevolutionary story, says Maria Heikkilä, a lepidopterist at the Finnish Museum of Natural History who did not take part in the research. But she thinks the specimens in the new study—from less than a third of an ounce (about 10 grams) of material taken from one drill core—at least “show that there is potential.”
Past researchers have not looked at the insect material from drill cores in a systematic way, says Bas van de Schootbrugge, a pollen specialist at Utrecht University and senior author of the new study, “because you really need to pick them out. You have to imagine 70 scales amid millions and millions of pollens and spores. If you want to do this on a larger time scale, it’s going to be a lot of work. But we are hoping that other people are going to follow.”