Ancient sawfish help illuminate the scaly origins of our teeth

You probably don’t think of yourself as a scaly creature, but the hardened structures behind your lips might say otherwise. A new analysis of one of the world’s strangest animals – the sawfish – supports the idea that teeth first appeared when the body scales of ancient fish migrated into their mouths about 400 million years ago.

Early teeth offered a major evolutionary boost for jawfish. “If you’re eating, unless you’re sucking up really small plankton-like stuff, there are definitely advantages to being able to grab objects with your mouth,” says Per Ahlberg, a paleontologist at Uppsala University in Sweden who wasn’t involved in the new study. Biting was a big leap forward; Chewing, Ahlberg notes, came later. And all toothed animals alive today — from trout to humans to crocodiles — appear to have descended from a single group of jawfish, says University of Chicago paleontologist Yara Haridy, who was also not involved with the research.

However, scientists have conflicting ideas about the origin of the earliest teeth. Hard, protective body scales of mineralized tissue such as dentin or enamel may have migrated into the mouth – a hypothesis known as outside-in. Or teeth could have evolved internally from the same tissue as gills, a hypothesis known as inside-out. The new sawfish study, published in the Journal of Anatomy, provides new evidence for an outside-in origin.

The study authors collected fossils of the extinct sawfish species Ischyrhiza mira, which lived about 70 million years ago. They analyzed the fossils’ rostral denticles, the spines that protrude from the sides of a sawfish’s snout to aid in foraging and self-defense. Rostral denticles look like teeth but are actually specialized body scales.

Unlike previous studies on extinct and current sawfish, this one looked at the internal structure of the hard outer layer of scales, called tooth enamel. “It’s basically the primitive form of [tooth] Melt,” says Haridy.

When the researchers scraped away the outer layers of these scales with sandpaper and acid, and then viewed them under a scanning electron microscope, they were amazed at the complexity they found. Scientists would have expected a homogeneous structure like many other body scales, says Pennsylvania State University paleontologist Todd Cook, lead author of the new study. Instead, they saw specific areas of microcrystals that resist mechanical stress. “In fact, the entire organization of tooth enamel resembled that of modern shark teeth,” says Cook.

Although the rostral denticles did not themselves become teeth—the ancestors of sawfish had teeth—this discovery indicates that scales like those on a fish’s body surface have the ability to evolve a tooth-like internal structure, and they would have at least once can do before. Cook says it’s less likely that such a similar structure would have evolved independently of the vastly different internal neck tissues.

“This finding supports outside-in,” says Ann Huysseune, an evolutionary developmental biologist at Ghent University in Belgium, who was not involved in this study. “But that doesn’t surprise me — it’s one of many arguments for it.”

Ahlberg notes that an animal’s scale-forming outer body tissue meets its inner tissue somewhere around the mouth. But the exact boundary between the two is difficult to pinpoint in old fish — and the location of that boundary holds the key to understanding what type of tissue first produced teeth. Fossils do not preserve most soft tissue, so researchers can only make inferences about the properties of such tissue or study modern-day equivalents. For example, Huysseune studies the mouths of modern zebrafish to better understand tooth formation and origin.

For Ahlberg, it is clear that external tissue can produce complex, tooth-like scales. The sawfish study is a vivid example of this mechanism in action. But can inner throat tissue or mixed inner and outer tissue near the mouth do the same? That is still uncertain, says Ahlberg.

Proponents of the inside-out hypothesis say the answer is yes. For a time, Haridy says, the group’s main point of evidence was a series of eel-like creatures that appeared to have evolved mineralized tooth-like structures in the pharynx and mouth — but nowhere else on the body. Several articles published in the 1990s and 2000s used these eels as the cornerstone of the inside-out idea, but later research suggested the structures were lineage-unique and unrelated to vertebrate teeth. At the moment, scientists continue to search for modern analogues and fossilized examples of early proto-teeth.

Pinning down the origin of teeth is unlikely to improve our lives in any tangible way right away, Ahlberg says — though he does mention a hypothetical far-future scenario in which humans learn to continuously regrow teeth like most non-mammalian animals can. “But I find it inherently interesting to understand how our bodies came about,” he says. “It is this strange thing that we inhabit, and it has evidence of a very long and strange history.”