In the depths of the abyss, an evolutionary mystery has just been solved by biologists. The spotted ratfish, a deep-sea chimera, possesses a unique dental structure... on its forehead!
This anatomical peculiarity, observed only in males, has long intrigued the scientific community. An international research team has shed light on the origin and function of these unusually placed teeth, revolutionizing our understanding of dental development in vertebrates.
Basic anatomy of H. colliei.
(A and B) Photograph of an adult male H. colliei in Puget Sound. (Photograph used with permission from Tiare Boyes)
(C) Micro-CT scan of a male H. colliei.
(D) Volume rendering of the dorsal spine in ventral view (micro-CT scan), highlighting serrations along the length of the spine.
(E) Volume rendering of the modified beak, hypermineralized regions visible as pearly columns.
(F) Volume rendering of the claspers and prepelvic anatomy. The pelvis is studded with six large denticles, and the claspers are covered with hundreds of small rhomboidal denticles.
(G and H) Segmentation of the adult tenaculum, with teeth colored to highlight their arrangement. An unusual reproductive tool
The tenaculum is a cartilaginous appendage located between the eyes of male chimaeras. It usually remains retracted, resembling a small white bump. This organ deploys during courtship displays or mating.
Males use this hook lined with teeth to firmly grasp the female's pectoral fin. This hold is essential for ensuring the stability of the pair during reproduction in an aquatic environment. The manipulation sometimes leaves distinctive marks on the partners.
This reproductive use explains the exclusive presence of the tenaculum in male individuals. Females retain only an undeveloped precursor of this structure. Its appearance is synchronized with the maturation of other secondary sexual characteristics.
A dental origin confirmed
The study combines the analysis of ancient fossils and the examination of contemporary specimens. A 315-million-year-old chimaera fossil shows a tenaculum attached to the upper jaw. This ancestral position indicates an evolutionary migration towards the top of the skull.
The researchers identified the presence of a dental lamina, a tissue structure identical to the one that, in the jaw, allows for tooth growth. This structure had never been observed outside the oral cavity. Its existence here confirms the dental nature of the tenaculum's spikes.
Genetic analysis of modern tissues corroborated these observations. The genes expressed are those associated with tooth development in all vertebrates. They clearly differ from those responsible for dermal denticles, the modified scales covering the skin of sharks.
Article author: Cédric DEPOND