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\u201cThis absolutely spectacular feature flips the long-standing assumption in evolutionary biology that teeth are strictly oral structures,\u201d said lead author Karly Cohen, a postdoctoral researcher at the University of Washington<\/a>.<\/p>\n \u201cThe tenaculum is a developmental relic, not a bizarre one-off, and the first clear example of a toothed structure outside the jaw.\u201d<\/p>\n Shark\u2019s cousin has tenaculum<\/p>\n Spotted ratfish<\/a> are chimaeras, a branch of cartilaginous fishes that split from sharks long ago. <\/p>\n They\u2019re compact \u2013 about two feet long \u2013 with a whip-like tail. In males, the unmistakable forehead \u201cpeanut\u201d can extend into a spiny hook.<\/p>\n Like sharks and rays<\/a>, many chondrichthyan fishes are armored in tooth-like skin structures called denticles.<\/p>\n Ratfish are oddballs: aside from denticles on their pelvic claspers (another mating aid), their skin is mostly bare. <\/p>\n That contrast made scientists wonder what, exactly, the tenaculum spines are. Are they denticles in disguise \u2013 or true teeth transplanted to the head?<\/p>\n \u201cSharks don\u2019t have arms, but they need to mate underwater,\u201d Cohen said. \u201cSo a lot of them have developed grasping structures to connect themselves to a mate during reproduction.\u201d <\/p>\n Ratfish have two: the forehead tenaculum and pelvic claspers.<\/p>\n Competing theories on tenaculum<\/p>\n The researchers laid out two competing hypotheses. If the tenaculum spines were denticles<\/a>, they\u2019d be skin-derived armor that just happens to look toothy. <\/p>\n If they were genuine teeth, they would develop and regenerate using the same cellular machinery that makes jaw teeth.<\/p>\n To tease this apart, the team collected hundreds of ratfish in the shallows around the University of Washington\u2019s Friday Harbor Labs. <\/p>\n The specimens were scanned with micro-CT, sampled tissues for gene expression, and compared the living fish to fossil relatives.<\/p>\n The researchers watched the tenaculum form in both sexes. <\/p>\n In males, a tiny cell cluster between the eyes swelled into a white bump that anchored to jaw muscles, broke through the skin, and sprouted multiple rows of sharp elements. <\/p>\n In females, the early cluster never mineralized, leaving only a developmental trace.<\/p>\n The clearest evidence came from histology. Each row of tenaculum spines sits atop a ribbon of tissue called the dental lamina \u2013 an epithelial stem-cell<\/a> niche that seeds new teeth in a conveyor belt inside the jaws of many vertebrates.<\/p>\n \u201cWhen we saw the dental lamina for the first time, our eyes popped,\u201d Cohen said. \u201cIt was so exciting to see this crucial structure outside the jaw.\u201d<\/p>\n Dermal denticles don\u2019t have a dental lamina. The team also found the classic \u201ctooth toolbox\u201d of genes active in the tenaculum but absent in skin denticles. They spotted similar head-mounted teeth in the fossil record of related chimaeras.<\/p>\n \u201cWe 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,\u201d said co-author Michael Coates of the University of Chicago<\/a>.<\/p>\n Flexible teeth for reproduction<\/p>\n An adult male ratfish can stack seven or eight rows of these forehead teeth. Unlike our rigid canines, they retract and flex \u2013 handy when you\u2019re trying to latch onto a mate while swimming.<\/p>\n The tenaculum\u2019s growth schedule follows the maturation of the pelvic claspers, not the animal\u2019s overall length. <\/p>\n This suggests that the \u201cmigrant\u201d tooth-forming tissue is now controlled by reproductive hormonal cues and local gene networks. <\/p>\n Functionally, the system behaves like mouth dentitions in sharks: a renewable lamina constantly seeds replacements.<\/p>\n Sharks are often the go-to model for tooth evolution because they churn out endless oral teeth and wear a coat of denticles.<\/a> This study widens the lens. <\/p>\n The research shows that vertebrate \u201ctooth programs\u201d are modular and mobile. Under the right developmental signals, genuine teeth can be built well beyond the jaw margins.<\/p>\n Tissue samples from the tenaculum expressed the same core genetic toolkit seen across vertebrate teeth, demonstrating that these aren\u2019t just spiky skin ornaments. \u201cVertebrate teeth are extremely well united by a genetic toolbox,\u201d Cohen noted. <\/p>\n \u201cIf these strange chimaeras are sticking teeth on the front of their head, it makes you think about the dynamism of tooth development more generally,\u201d said study senior author Gareth Fraser of the University of Florida<\/a>.<\/p>\n Ancient clues from ratfish tenaculum<\/p>\n Finding a working dental lamina outside the mouth suggests ancient flexibility in where and why teeth evolve \u2013 whether for feeding, defense, or, in this case, reproductive grip. <\/p>\n The discovery also hints that evolution has repeatedly redeployed the same genetic circuitry to build similar hard parts in new places.<\/p>\n For paleontologists, the forehead teeth help reconcile decades-old fossils that hinted at non-oral dentitions in early jawed fishes. <\/p>\n For developmental biologists, they invite a new question: What other \u201cspiky structures\u201d might secretly be teeth?<\/p>\n \u201cChimaeras offer a rare glimpse into the past,\u201d Cohen said. \u201cI think the more we look at spiky structures on vertebrates, the more teeth we are going to find outside the jaw.\u201d<\/p>\n In other words, the next chapter in the story of teeth may not be written in the mouth at all, but on an unassuming little hook between a ratfish\u2019s eyes \u2013 proof that evolution<\/a>\u2019s dental toolkit travels farther than anyone imagined.<\/p>\n The study is published in the journal Proceedings of the National Academy of Sciences<\/a>.<\/p>\n \u2014\u2013<\/p>\n Like what you read? Subscribe to our newsletter<\/a> for engaging articles, exclusive content, and the latest updates.\u00a0<\/p>\n Check us out on EarthSnap<\/a>, a free app brought to you by Eric Ralls<\/a> and Earth.com.<\/p>\n \u2014\u2013<\/p>\n","protected":false},"excerpt":{"rendered":"For more than a century, biologists have treated teeth as strictly oral equipment \u2013 no matter the animal.…\n","protected":false},"author":2,"featured_media":126808,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[26],"tags":[49,48,66,323],"class_list":{"0":"post-126807","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-wildlife","8":"tag-ca","9":"tag-canada","10":"tag-science","11":"tag-wildlife"},"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/ca\/wp-json\/wp\/v2\/posts\/126807","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/ca\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/ca\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/ca\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/ca\/wp-json\/wp\/v2\/comments?post=126807"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/ca\/wp-json\/wp\/v2\/posts\/126807\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/ca\/wp-json\/wp\/v2\/media\/126808"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/ca\/wp-json\/wp\/v2\/media?parent=126807"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/ca\/wp-json\/wp\/v2\/categories?post=126807"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/ca\/wp-json\/wp\/v2\/tags?post=126807"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}![\"CT](\"https:\/\/www.newsbeep.com\/ca\/wp-content\/uploads\/2025\/09\/tenaculum_ratfish_ct-scan_credit-Cohen-Nicklin_1s.webp.webp\")
<\/a>CT (computed tomography) scan of the adult male Spotted Ratfish frontal clasper (Tenaculum) covered in rows of teeth (rainbow colors). Click image to enlarge. Credit: Karly Cohen and Ella NicklinStem cells prove tooth identity<\/p>\n