Surprise in 50 Million-Year-Old Bug Fossil: Its Genitals

Though not the first bug fossil with genitals discovered, they were unusually well preserved.

Recovered from the Green River Formation in present-day Colorado, this fossil represents a new genus and species of predatory insects known as assassin bugs. (Credit: Palaeontological Association)

(CN) — Paleontologists rediscovered a rare 50-million-year-old fossilized assassin bug from the Eocene period, surprisingly with its genitals still intact. 

Originally found in 2006 in the Green River Formation in present-day Colorado, the fossil had been split in two and sold off to separate collectors.

Scientists later tracked down these collectors to piece together the complete fossil and dubbed the previously unknown species “Aphelicophontes danjuddi” in honor of Dan Juddi — the generous collector who donated his half of the ancient insect’s remains.

This species of assassin bug, which looks somewhat like a striped spider, measures less than one-tenth of an inch in length and has bold alternating light and dark bands running down its legs. The researchers described the fossil in a new study published Tuesday in the journal Papers in Palaeontology.

But the real prize was the male assassin bug’s genitals — a fantastically rare find in a fossil this old. It was discovered when a fossil hunter/dealer split a slab of rock down the middle, revealing an ancient assassin bug fossil cleaved cleanly in two from head to abdomen. 

To the delight of the researchers, the inner structure of the bug’s pygophore, or genital capsule, which is about the size of a grain of rice, remained identifiable after all those years.

“Fossilization involves a lot of pressure over a long period of time and these forces are not kind to insect bodies in general, let alone the tiny and delicate structures of the genitalia,” explained Daniel Swanson, a graduate student in entomology at the University of Illinois Urbana-Champaign, and co-author of the study, in an email. 

“The genitalia are usually tucked up inside the body, so to have had a fossil that split through the middle of the body, revealing these internal structures, is an even rarer treat. And on top of that, the structures are preserved so well that we can actually identify them with structures present in the genitalia of living insects. It’s like winning a fossil Mega-Million jackpot.”

Swanson said it’s quite rare to see the inner structures and internal genitalia in a fossil this old, as such an incredible level of detail is typically only found in modern-day species. The visible structures of its genitalia include the basal plate, a hardened, stirrup-shaped structure supporting the phallus, as well as the phallotheca, which acts like a pouch the phallus can tuck into.

Assassin bugs are among the most successful predators on the planet. Of the 7,000 species of assassin bugs on record, only around 50 such fossils are known to exist. 

Swanson said this further speaks to the improbability of finding one this old so well preserved with identifiable internal structures. The Green River Formation where the fossil was discovered is a sedimentary deposit in the western U.S., bordering Colorado, Utah and Wyoming, and dating back 50 million years ago to the Eocene period. 

Formed by the convergence of three lakes, it’s among the most famous fossil hunting grounds in the United States. Due to its unique fine-grained composition it’s able to preserve soft parts of insects and even fallen leaves in remarkable detail.

“If there was a formation that was going to turn out such a beautifully preserved specimen, the Green River Formation would be at or near the top of the list,” Swanson said.

If a 50-million-year-old fossilized sex organ wasn’t impressive enough, the authors claim that’s far from the oldest example of fossilized genitalia on record. That distinction belongs to a mind boggling 400-million-year-old fossilized daddy longlegs.

“Fossils are important because they show us a window to the past, highlighting forms and organisms that we don’t see around today,” Swanson said. “However, fossils also can be used as time calibration points for understanding the phylogenetic trees (think family tree but of a group of organisms). 

“A fossil can provide a minimum age for a lineage, and when you use multiple calibration points, you can infer ages and hypothetical diversification periods over a larger group of organisms. This can inform our understanding of how and when different groups of organisms evolved.”

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