Are those teeth on your forehead or are you just happy to see me? In the curious case of chimaeras, also called ghost sharks, the answer is ‘both.’
These forehead-teeth are attached to a retractable stalk called a tenaculum, and they help chimaeras have sex. Because of course.
Whether these nodules are true teeth in every sense of the word, or some kind of dermal protrusion, has never been clear. So biologists Karly Cohena and Gareth Fraser from the University of Florida and Michael Coates from the University of Chicago took a closer look at this most unusual marine creature.
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Chimaeras are strange, sometimes venomous deep-sea fish that sport an evolutionarily unique body part, the tenaculum, an appendage that extends from the males’ forehead like a retractable Rolls-Royce hood ornament.
This appendage is covered in many rows of hooked, flexing teeth that males use to grasp onto females’ pectoral fins during copulation, to avoid drifting away.
Despite their mystic moniker, these animals aren’t actual sharks, having diverged from the shark lineage around 400 million years ago.
Though chimaeras also have cartilage skeletons, they lack the ferocious rows of regenerating teeth that sharks have. Instead, chimaeras have toothy, grinding dental plates adapted for crushing shelled creatures, like crabs and molluscs.
Chimaeras are also nude. Unlike sharks, they generally don’t have protective, hydrodynamic scales. Called dermal denticles, these scales are a modified version of teeth, rather than the skin-based scales that adorn reptiles, for example.
On chimaeras, dermal denticles are limited to the claspers on their pelvis, which they use alongside their tenaculum to grasp onto females during copulation.
Yet here’s the scientific mystery: since vertebrates usually have all their teeth in their mouth, chimaeras are an excellent, ancient opportunity to explore the evolution of extra-oral dentition, or teeth occurring outside the mouth.
To find out whether these anatomical oddities qualify as teeth, the researchers turned to Puget Sound to study the unfortunately labeled spotted ratfish (Hydrolagus colliei), so named because its tail comprises half its body length. The researchers studied fossil data, performed CT scans of the ratfish, and conducted a genetic analysis on the forehead-teeth themselves.
First, going back in time, fossil evidence from 315-million-year-old chimaeras (Helodus simplex) showed that the tenaculum grew on the upper jaw, before migrating to the top of the head.
Intriguingly, this tooth-growing process is similar to the tooth-growing process seen in the jaws of modern sharks. In fact, it grows from a tissue known as the dental laminate, which has never before been documented outside the jaw.
It’s a “beautiful example” of evolutionary bricolage, or taking what’s available to make something new.
“We have a combination of experimental data with paleontological evidence to show how these fishes co-opted a preexisting program for manufacturing teeth to make a new device that is essential for reproduction,” explains Coates.
So, there you have it, the forehead-teeth used for sex today are genetically transferred versions of the mouth-teeth used for consuming sustenance, though given the choice, many creatures may choose the former over the latter.
In addition to making for great headlines, this research shows one of nature’s core principles of creativity; evolution frequently reuses designs or repurposes existing structures in strange, unexpected ways.
Accordingly, scientists previously wondered whether teeth evolved inwardly, turning from dental denticles on the skin to oral teeth within the mouth, or if they evolved outwardly, turning from pharyngeal (throat) spikes that migrated outwards to occupy the mouth.
The findings of this study suggest that both scenarios may be true for different species across evolutionary time, showing that teeth deployment is incredibly and surprisingly flexible in the development of early vertebrates.
Finally, this study opens a door to a more tooth-ful future, overturning the assumption that teeth are solely dental features.
“I think the more we look at spiky structures on vertebrates, the more teeth we are going to find outside the jaw,” concludes Cohen.
This research is published in Proceedings of the National Academy of Sciences.