Imagine a fish with rows of teeth sprouting from its forehead! Sounds like something out of a sci-fi movie, right? But scientists studying the spotted ratfish, a little-known species from the northeastern Pacific, have uncovered this jaw-dropping feature that challenges everything we thought we knew about teeth evolution.

Traditionally, vertebrates share a common design when it comes to teeth, with most of them confined to the mouth. However, this discovery flips that assumption on its head, quite literally! The spotted ratfish features a strange structure called the tenaculum, which resembles Squidward's nose and hosts rows of teeth. This revelation has ignited a fiery debate among biologists about the origins and evolution of teeth.

Karly Cohen, a postdoctoral researcher at the University of Washington, describes this finding as "insane and absolutely spectacular." She emphasizes that the tenaculum is not just an oddity, but the first documented example of teeth located outside the jaw. This opens a Pandora's box of questions about how widespread this trait might be among vertebrates and what it tells us about their evolutionary history.

Published in the Proceedings of the National Academy of Sciences, this groundbreaking research highlights that spotted ratfish are quite common in Puget Sound. These cartilaginous fish, which diverged from sharks millions of years ago, can grow up to two feet long. While males showcase the tenaculum prominently, it appears as a small white nub between their eyes when at rest. During mating, it becomes a remarkable hooked organ covered in teeth that helps them grip onto females.

Cohen explains that, just like many sharks use specialized structures for mating underwater, the tenaculum serves a dual purpose for spotted ratfish—both as a display feature and as a tool for reproduction.

Interestingly, while sharks and other cartilaginous fish are usually covered in tooth-like scales called denticles, spotted ratfish boast a largely smooth-skinned body, raising questions about the evolutionary path of these teeth. Were they modified denticles or entirely new teeth?

The researchers caught and analyzed hundreds of these fish, using advanced techniques like micro-CT scans and tissue samples to understand the tenaculum's development. What they found was astonishing—both male and female ratfish begin forming a tenaculum early on, but only males' develop into the fully formed structure covered with teeth. Evidence of an early structure remains in females, though it doesn’t mineralize.

What makes these teeth so special? They are rooted in a band of tissue called the dental lamina, traditionally associated with teeth in the jaw. Observing this structure outside the jaw for the first time made Cohen and her team ecstatic, proving the teeth on the tenaculum aren't just remnants of ancient features but legitimate teeth that share a genetic connection with the oral teeth of other vertebrates.

The combination of experimental data and fossil evidence suggests that these fishes have adapted an existing genetic program for tooth development into a new feature essential for mating. The modern male spotted ratfish can grow up to eight rows of teeth on its tenaculum, designed to grip their partners while swimming—a fascinating adaptation that showcases the fluidity of evolutionary traits.

As Cohen notes, while sharks have been the traditional models for studying teeth, the discovery of external teeth in chimaeras like the spotted ratfish reveals a much broader picture of vertebrate dental diversity. The more we investigate the bizarre and spiky adaptations in nature, the more we'll uncover about the evolution of teeth.