The fossil record is a lot more than just giant dinosaurs. In fact, some of the most impressive fossils are not hulking, pillar-like bones but the remains of small, delicate organisms that were buried in just the right conditions to reveal lost worlds to us millions of years after their deaths. A fossil bug recovered from the ancient strata of Colorado offers one such window into the past.
The fossil assassin bug, described today in Papers in Palaeontology, has perked paleontological attention because the rare specimen has its genitals intact. Chipped out of 50 million-year-old rock, the bug is so exquisitely-preserved that it almost looks ready to crawl out of the stone. Even the colored bands along the insect’s legs and body are visible. And just like exceptional circumstances were needed for the fossil to form, a lucky break was also needed to piece together this Eocene insect’s story.
The insect comes from a time when vast lakes covered swaths of what would become the western United States. Surrounded by warm, semitropical forests that hosted lemur relatives, early bats and other creatures, these lakes quickly buried organisms that settled to the mucky, oxygen-poor bottom and pressed many of them paper thin over 50 million years. Known as the Green River Formation, this rock unit has yielded some of the most exquisite and detailed fossils ever found.
Paleontologists aren’t the only people interested in Green River fossils. Privately owned quarries specialize in extracting, cleaning and selling fossils from the Green River Formation, especially rare species. That’s how half of the fossil assassin bug featured in the study came into the possession of private fossil collector and study co-author Yinan Wang. Word had it that the other half was owned by another private collector, Dan Judd, who donated the second piece to the researchers behind the new study.
The fossil assassin bug is named Aphelicophontes danjuddi in honor of Judd’s donation. What makes it truly special is that the genitals of this fossil bug can be seen in detail—a critical anatomical aspect that entomologists often use to distinguish assassin bugs from each other.
In technical terms, says University of Illinois at Urbana-Champaign entomologist and study co-author Daniel Swanson, the genital organ of the assassin bug is called a pygophore. “The word comes from two Ancient Greek roots that literally mean ‘rump’ and ‘something that carries,” Swanson says. The organ is a hardened anatomical cup that surrounds the genitals just like the bug’s exosekeleton surrounds the body. And despite being about 50 million years old, Aphelicophontes danjuddi has genitals similar to assassin bugs today—the basic setup has remained the same since the Eocene, even as subtle differences distinguish one species from another.
“This is definitely an example of exceptional preservation,” says University of Colorado Boulder paleontologist Dena Smith, who was not involved in the new study. To start with, assassin bugs are rare in the fossil record. Despite there being over 7,000 species of these familiar bugs alive today, only about 50 have ever been found as fossils.
More than that, Aphelicophontes danjuddi isn’t merely a fragment or an impression. The entire animal down to its reproductive anatomy has been preserved. “Genitalia are important characteristics of insects that are often used to describe and define species,” Smith says, especially because they are so seldom seen in fossil assassin bugs. For these arthropods, genital anatomy is as distinctive as a fingerprint in determining who’s who.
Finding direct evidence of fossil genitals is relatively rare. Even in cases where mating animals have been found—such as courting sharks or prehistoric turtles caught en flagrante—the actual soft tissue anatomy is usually missing. The same goes for insects. Fossil arthropods have been found in mating positions before, but actually being able to clearly see their genital anatomy is practically unheard of.
The preserved pygophore isn’t the only reason the find of Aphelicophontes danjuddi is important. While the Green River Formation is famous for the exceptional vertebrates that have been found there—from crocodiles to early horses and birds preserved with feathers—insects can often reveal much more about the prehistoric habitat.
“Many insect groups have fairly specific environmental needs for their growth and development,” Smith says, “which can be used to provide information about past environmental conditions.” At Green River Formation sites in Wyoming, for instance, paleontologists have found that particular species of prehistoric plants grew in small pockets—rather than spreading everywhere. These plants supported unique insect communities, which researchers detected by looking at patterns of leaf damage. Assassin bugs, for their part, likely preyed on these herbivorous insects, and so now paleontologists can see how insects like Aphelicophontes danjuddi fit into the broader patterns of who lived where.
Insects like the new assassin bug also formed an important part of ancient food webs, too, especially because they often served as meals for many of the charismatic vertebrates that often nab the spotlight. In 2019, researchers named a finch-like bird called Psittacopes from the same formation. This bird’s beak was specifically adapted to plucking insects off bark, and Aphelicophontes danjuddi could have certainly been on the menu. “Studying fossil insects not only allows us to understand past ecosystems,” Smith says, “but also helps us to understand the evolutionary history and ecology of this important group.”