A 400-million-year-old penislike piece of anatomy strengthens the argument that ancient armored fish were engaging in surprisingly modern internal fertilization.

Earlier this year researchers discovered that a group of extinct fish—known as placoderms—had been giving birth to live offspring an astonishing 380 million years ago. And live birth can only mean one thing: eggs must have been fertilized within the female’s body. But how? The team was stumped by a lack hard evidence of a crucial piece of male anatomy.

But a paper, published today in Nature, reports the discovery of a 400-million-year-old bony protrusion that could have made the live birth possible (Scientific American is part of the Nature Publishing Group).

The authors identified a “pelvic clasper” on a male Incisoscutum ritchiei, which was a “slender rod” near the pelvic region that “most probably corresponds to the core of an erectile element, as in exant sharks,” they wrote.

“It provides a pedigree of nearly 400 million years for the ‘advanced’ and seemingly specialized reproductive biology of modern sharks,” lead author Per Ahlberg, a professor in evolutionary biology at Sweden’s Uppsala University, said in a statement. “It was lying in plain view but had been misinterpreted as part of the pelvis and was overlooked,” he said. 

It’s also the first confirmed piece of external anatomy that differentiates males and females of that order (Arthrodires), the paper’s authors note.

The live-birthing fish (Materpiscis attenboroughi) was listed as among 2008’s top 10 newly discovered species. The placoderm group also included fierce members, such as the Dunkleosteus terrelli, which reportedly had a bite force of about 8,000 pounds per square inch. The group died out mysteriously about 364 million years ago in the late Devonian period.

Artist rendering of a
Dunkleosteus in the arthrodires order courtesy of Wikimedia Commons