50-million-year-old assassin bug fossil features near-perfectly preserved genitalia

Paleontologists have recovered a 50-million-year-old assassin bug fossil featuring near-perfectly preserved genitalia, a rarity.

Though the ancient insect's pygophore, a genital capsule, is no bigger than a grain of rice, researchers were able to clearly see the genitalia's internal features.

The prehistoric assassin bug's unique features, its pygophore and its banded legs, required scientists to name a new genus and species for the bug's classification.

Researchers described the novel insect in a new paper, published this week in the journal Papers in Palaeontology.

The assassin bug was originally found inside a slab of rock split by an amateur fossil-hunter. Sourced from Colorado's Green River Formation, the split rock divided the insect almost perfectly in half. The head-to-toe split also sliced the pygophore right down the middle.

After one of the rocks made it into the hands of paleontologists, researchers had to work to track down the other half, as the fossil dealer sold the two halves to different buyers.

Scientists named the new species, Aphelicophontes danjuddi, for the collector, Dan Judd, who originally donated his half of the fossil to researchers.

Sexual anatomy is an important source of diversification, as small changes in genitalia can lead to incompatibility among would-be mates, triggering the split of new species. As such, finely preserved genitalia are quite useful for species classification.

"To see these fine structures in the internal genitalia is a rare treat," lead researcher Daniel Swanson said in a news release.

"Normally, we only get this level of detail in species that are living today," said Swanson, a graduate student in entomology at the University of Illinois Urbana-Champaign.

Inside the split pygophore, scientists observed what's called a basal plate, a stirrup-shaped structure that anchored phallus. Researchers were also able to make out the contours of the phallotheca, the sack into which the phallus could be withdrawn.

The fossil suggests banded assassin bugs arrived on the scene at least 25 million years earlier than previously thought.

"There are about 7,000 species of assassin bug described, but only about 50 fossils of these bugs are known," Swanson said. "This just speaks to the improbability of even having a fossil, let alone one of this age, that offers this much information."

The assassin bug's pygophore isn't the oldest insect genitalia known to science.

Researchers in Scotland previously characterized the fossilized genitalia of a harvestman arthropod dated to between 400 and 412 million years old.

"And there are also numerous fossil insects in amber as old as the Cretaceous Period with genitalia preserved," said co-author Sam Heads, paleontologist at the Illinois Natural History Survey. "However, it is almost unheard of for internal male genitalia to be preserved in carbonaceous compressions like ours."

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