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

The exclusive club of fossilized phalluses has a new member.

The latest addition is the sexual organ of a 50-million-year-old assassin bug. Some well-placed sediments and the protective powers of a prehistoric jock strap preserved his penis, according to a study published Tuesday in the journal Papers in Paleontology.

The exquisite preservation of the fossil, which represents an undescribed species, is “extraordinary,” said Daniel Swanson, an entomologist at the University of Illinois at Urbana-Champaign and the paper’s lead author. Running about the length of an aspirin tablet from head to bum, the bug would have been full of soft innards and “easy to squish,” he said. And yet, it persevered, delicate genital tissues and all.

The specimens described in the paper “blew me away,” said Katy Estes-Smargiassi, the collections manager of invertebrate paleontology at the Academy of Natural Sciences of Drexel University, who was not involved in the study. “They’re just incredible.”

Assassin bugs, which belong to an insect family called Reduviidae, date at least as far back as the Jurassic. In the eons since, the group has splintered substantially, with more than 7,000 known species still around. Most are ambush predators, and a number of the family’s modern members are vectors of Chagas disease, a tropical parasitic illness that spreads when the bugs sip the blood of sleeping humans, then defecate near the wound.

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.

“Besides just being cool to look at, these structures also allowed us to home in better on the classification,” Mr. Swanson said.

Genitalia isn’t always the go-to structure for differentiating insect species. But scoping out these delicate structures can be a tiebreaker among ambiguous specimens.

Combined with some of the insect’s other anatomical features, certain shapes within the pygophore helped make the case that the team’s new assassin bug was in its own league. The specimen’s stripy, spindly legs also made it a likely candidate for a group called banded assassin bugs, some of which are still alive today.

The researchers can’t be sure what females of this species looked like. But if they follow the patterns still present in modern assassin bugs, the long-gone ladies would most likely have been bigger than their mates. And in place of penises, they would have had a vent-like structure, guarded by plates that would part when a worthy male came near.

Despite some subtle differences, the team’s new assassin bug doesn’t look all that different from many of its contemporary kin — a hint that this group of insects hit upon evolutionary success early, and stuck with it.

Then again, “50 million years in the invertebrate world is really not that long,” Dr. Estes-Smargiassi said. Studying how ancient animals lived and died and changed with the planet “can tell us a great deal about the species that are alive today.”

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