You may not think of yourself as a scaly creature. However, the hardened structures behind you lips could suggest otherwise. New analysis of one of the world’s weirdest animals–the sawfish–supports the idea that teeth first appeared when ancient fishes’ body scales migrated into their mouths about 400 million years ago.
Early Teeth gave jawed fish an evolutionary boost. “Early teeth are important for feeding, except when you’re sucking in small plankton-type things, so it’s definitely advantageous to be able to grab objects in your mouth,” Per Ahlberg, a paleontologist from Uppsala University, Sweden, said. Ahlberg was not part of the new study. Biting was a big leap forward; chewing, Ahlberg notes, came later. All toothed animals, from humans to trout, appear to have come from one group of jawed fish, according to Yara Haridy (University of Chicago paleontologist), who was not involved in the research.
But scientists disagree on the origin of the earliest teeth. Hard, protective body skins made of mineralized tissue like enamel or dentine could have migrated into your mouth. This is known as the outside-in hypothesis. Or teeth could have formed internally from the same tissue that gills are made. This hypothesis is called inside-out. The new sawfish study, published in the Journal of Anatomy, offers fresh evidence in favor of an outside-in origin.
The study authors gathered fossils from the extinct sawfish species Ischyrhiza mira, which lived some 70 million years ago. The fossils’ rostral teeth, which are the spikes that protrude from the side of a sawfish’s snout to aid foraging and self-defense, were examined by the researchers. Although they look like teeth, rostral denticles are actually body scales.
This study, unlike previous ones of extinct and current sawfishes, probed the inner structure of the scales’ hard outer coating, called enameloid. Haridy explains that it is basically the primitive form [tooth] enamel.
When the researchers removed the outer layers of the scales using acid and sandpaper, and then examined them under a scanning electron microscope they were stunned at the complexity of the results. According to Todd Cook, a Pennsylvania State University paleontologist and lead author of the new study, scientists expected a homogeneous structure similar to many other body scales. Instead, they found distinct areas of microcrystals which are resistant to mechanical stress. Cook claims that the enameloid’s overall structure was similar to modern shark teeth.
Although rostral denticles weren’t made into teeth, sawfish ancestors had teeth, this discovery suggests that scales similar to those found on fish’s bodies can develop a tooth-like internal structure. Cook claims it is less likely that a similar structure would have evolved independently from the very different inner throat tissue.
” This finding is in support outside-in,” said Ann Huysseune (a Belgian evolutionary developmental biologist at Ghent University), who was not part of this study. “But I’m not shocked–it’s just one of many arguments for .”
Ahlberg points out that the animal’s outer tissue, which forms scales, meets its inner tissue around the mouth. The exact boundary between them is not easy to determine in ancient fish. This boundary is crucial for understanding the origin of teeth. Researchers can only draw inferences about the properties of soft tissue from fossils, and examine current-day equivalents. Huysseune, for example, studies the mouths modern zebra fish in order to better understand tooth formations and origins.
To Ahlberg, it is obvious that complex, toothlike scales can be produced by external tissue. This mechanism is illustrated by the sawfish study. But what about internal throat tissue? Or mixed internal-external tissue near the mouth? Ahlberg states that this is still uncertain.
Supporters of the inside out hypothesis believe the answer is yes. Haridy states that the main evidence for the inside-out hypothesis was for a while a group of eel-like creatures. They had developed toothlike structures in their throats and mouths, but not anywhere else on the body. Several papers published in the 1990s and 2000s used these eels as a cornerstone of the inside-out idea, but later research suggested the structures were unique to the lineage and were unrelated to vertebrate teeth. Scientists continue to search for modern analogues as well as fossilized examples of prototeeth.
Although Ahlberg admits that tracing the origins of teeth won’t make our lives easier, he does mention a future scenario in which humans could learn to regrow their teeth in a way that is similar to nonmammals. He says, “But I think it is intrinsically interesting to understand how our bodies came into being.” It’s this strange thing we live in, and it has evidence of a very long history
This article was originally published with the title “Scaling Up” in Scientific American 327, 6, 12-14 (December 2022)
doi: 10. 1038/scientificamerican1222-12
ABOUT THE AUTHOR(S)
Daniel Leonard is a freelance science journalist and current Scientific American editorial intern whose work focuses on space, tech and natural history. Follow Leonard on Twitter@dalorleon