Male ghost sharks, scientifically known as chimaeras, possess a remarkable and unusual anatomical feature that has puzzled biologists for decades: a mysterious rod-like structure protruding from their foreheads. Unlike any conventional teeth arrangement observed in vertebrates, this appendage is adorned with rows of tiny, sharp, retractable teeth that grow independently of the mouth. Recent groundbreaking research, combining cutting-edge imaging techniques, fossil analysis, and genetic profiling, reveals that these teeth are not mere ornamental structures but fully developed teeth with shared developmental origins to oral dentition. This discovery sheds new light on the plasticity and adaptability inherent in tooth development among cartilaginous fishes.
The “forehead rod,” known as a tenaculum, is exclusive to male chimaeras and stands out as their only source of distinct teeth since their oral regions lack comparable dental morphology. Anatomically intriguing, the tenaculum’s sharp teeth are hypothesized to serve a specialized reproductive function. Parallel to some shark species that use their formidable oral dentition to clasp females during mating, male ghost sharks employ their toothed forehead clasper to grasp females, facilitating successful copulation. This functional adaptation of a dental structure outside the mouth is unprecedented among vertebrates and highlights a fascinating case of evolutionary innovation.
Gareth Fraser, Ph.D., a professor of biology at the University of Florida and the senior author of the study, emphasized the paradigm-shifting implications of this discovery. The presence of a developed set of teeth external to the oral cavity challenges existing concepts about where and how teeth can form during vertebrate development. Fraser reflects on the broader impact of these findings, suggesting the possibility of other unconventional dental structures evolving in unexpected anatomical locations across various species, thereby expanding the known boundaries of dental morphogenesis.
The research team, including experts from the University of Washington and the University of Chicago, embarked on a comprehensive investigation of this enigma by studying both extant chimaera specimens and fossilized relics spanning hundreds of millions of years. One particularly illuminating fossil, approximately 315 million years old, revealed a tenaculum attached near the upper jaw, bearing teeth remarkably analogous in structure, pattern, and arrangement to those found in the oral cavities of modern sharks. These findings indicate that the developmental mechanisms underpinning tooth formation were adapted early in chimaera evolutionary history.
Advanced computed tomography (CT) scanning of both fossils and contemporary ghost sharks provided unprecedented, high-resolution images detailing the minute morphology and growth patterns of the tenaculum teeth. This technology allowed researchers to discern intricate developmental stages invisible to conventional imaging, confirming that the teeth on the forehead appendage mirror the structural complexity and layered organization characteristic of true oral teeth. The remarkable visual evidence from these scans was pivotal in affirming the homology between the tenaculum teeth and traditional shark dentition.
Complementing morphological studies, molecular genetic analyses delivered the most compelling evidence of the nature of tenaculum teeth. The teeth express a suite of genes traditionally associated exclusively with oral tooth development, including master regulators of dental tissue differentiation and mineralization. Crucially, these gene expression patterns are absent in skin denticles—small, tooth-like scales that cover shark skin and some ghost sharks—but are a hallmark of true teeth. This genetic signature convincingly establishes that the teeth on the tenaculum are not superficial, denticle-like structures but bona fide teeth originating from a conserved developmental pathway.
Michael Coates, Ph.D., a biology professor at the University of Chicago and a co-author of the study, highlighted the evolutionary significance of this biological innovation. He described the tenaculum as a compelling example of evolutionary bricolage, whereby organisms repurpose preexisting genetic and developmental programs to create novel anatomical features. The co-option of an oral tooth development program to generate a functional, external mating device underscores nature’s capacity for reinventing evolutionary tools in service of reproductive fitness.
Karly Cohen, Ph.D., a postdoctoral researcher at the University of Washington’s Friday Harbor Labs and lead author of the paper, noted the historic nature of this finding. Prior to this study, no vertebrate species was known to develop true teeth outside of the oral cavity, marking the tenaculum as the first clear example of an ectopic toothed structure with a defined physiological role. This discovery not only enriches our understanding of chondrichthyan biology but also opens new avenues for exploring dental evolution across vertebrates.
The evolutionary trajectory of the tenaculum suggests a fascinating narrative of functional repurposing, where ancient cartilaginous fish gradually shifted the site of tooth formation from the mouth to a novel cranial appendage. Over millions of years, this appendage shortened and specialized, yet retained a fully operational tooth generation capability, creating an effective grasping tool used specifically during mating. Such evolutionary flexibility demonstrates the remarkable plasticity of developmental genetic programs and their influence on phenotypic innovation.
This novel adaptation raises compelling questions about the constraints and potentials of tooth development. The fluidity with which vertebrates can adapt shared genetic pathways to new morphological contexts suggests undiscovered developmental plasticity throughout the animal kingdom. By exploring these unique biological instances, scientists can gain greater insight into the fundamental mechanisms of organogenesis, morphogenesis, and evolutionary developmental biology.
Moreover, the discovery underlines the importance of integrating fossil evidence with molecular biology and advanced imaging to unravel the complexities of evolutionary history. Such interdisciplinary approaches allow researchers to connect morphological features across vast evolutionary timeframes, enhancing our understanding of how anatomical novelties arise and persist in natural populations.
The implications of this research extend beyond marine biology and evolutionary theory, potentially influencing biomedical sciences through improved comprehension of tooth development and regeneration. Understanding how genetic pathways can be co-opted or reactivated in novel sites may inform future strategies in dental tissue engineering and regenerative medicine.
In conclusion, the enigmatic tenaculum of male ghost sharks stands as a testament to nature’s inventive capacity for evolutionary tinkering. By developing a functional set of teeth outside the oral cavity, these deep-sea chimaeras challenge long-held notions of vertebrate dental biology and underscore the intricate interplay between genetics, development, and evolution. As researchers continue to explore the hidden depths of aquatic life, more surprising discoveries await, constantly reshaping our views of biology’s potential.
Subject of Research: Animals
Article Title: Teeth outside the jaw: Evolution and development of the toothed head clasper in chimaeras
News Publication Date: 4-Sep-2025
Web References: https://www.pnas.org/doi/10.1073/pnas.2508054122
References: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.2508054122
Image Credits: Specimen scanned by Karly Cohen; rendering and image by Ella Nicklin
Keywords: Marine fishes, Evolutionary biology, Evolution, Evolutionary processes, Paleontology, Dentition, Teeth, Mating behavior
