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

Much less is known about the lifestyle of assassin bugs of yore, of which only 50 or so species have been unearthed in fossil form. The newcomer to the bunch, named Aphelicophontes danjuddi, is one of the most intact to date.

The fossil was first pried from a wreath of rock in Colorado’s Green River Formation, a treasure trove of fossil fish and insects. The extraction process split the fossil into two mirror images, each stretching the length of the bug’s body, that ended up in the hands of different fossil collectors. One of them, Yinan Wang, contacted Sam Heads, Mr. Swanson’s adviser at the University of Illinois, on a hunch that it was “new to science and paperworthy.” It was — so Mr. Wang donated the fossil to the team’s cause. Dan Judd, the owner of the piece’s partner fossil, soon followed suit, earning the insect its species name.

Once the fossil halves were reunited, the researchers began the tough task of placing the bug in its family tree. The fossil’s impeccable quality appears to have eased this process immensely, said Mercedes Burns, an evolutionary biologist at the University of Maryland, Baltimore County, who wasn’t involved in the study.

Particularly well preserved was the bug’s genital capsule, or pygophore, a cuplike shield that cradles the fragile phallus and other jiggly reproductive accouterment until it’s time for copulation. It’s a hard shell that protects the penis, Mr. Swanson said, not unlike the exoskeletal structures that swaddle the rest of the bug’s body.

What was left of the pygophore was cracked in two when the fossil first split. But careful scrutiny of the two imprints revealed that some of the capsule’s contents had persisted. Among them were the insect’s basal plate, a stirrup-like structure, and hints of the pouch-like phallotheca, which supports the penis. In living assassin bugs, the entire package looks not unlike a Darth Vader mask, or a translucent athletic cup.

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