Much less is known about the lifestyle of previous assassins, of which only about 50 species have been discovered in fossil form. The newcomer to the group, Aphelicophontes danjuddi, is one of the most intact to date.

The fossil was first extracted from a 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 beetle’s body and ending up in the hands of different fossil collectors. One of them, Yinan Wang, turned to Sam Heads, Mr. Swanson’s advisor at the University of Illinois, with the suggestion that this was “new to science and paper worthy”. 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 and was given the name of its species for the insect.

After the fossil halves were reunited, the researchers began the difficult task of adding the beetle to its family tree. The flawless quality of the fossil appears to have made that process a lot easier, said Mercedes Burns, an evolutionary biologist at the University of Maryland, Baltimore County who was not involved in the study.

The beetle’s genital capsule, or pygophore, was particularly well preserved, a cup-like shield that weighs the fragile phallus and other wobbly reproductive objects until it is time for copulation. It’s a hard shell that protects the penis, Mr. Swanson said, much like the exoskeletal structures that coat the rest of the beetle’s body.

What was left of the pygophore was broken in two when the fossil was first split. However, a careful examination of the two impressions revealed that part of the capsule contents had been preserved. Among them were the insect’s base plate, a bow-like structure, and references to the pouch-like phallotheca that supports the penis. For live assassin bugs, the entire package resembles a Darth Vader mask or a translucent sports trophy.