The bizarre bulbous bucklers of skates: Denticles, dermal bone or something else?

The morphology and arrangement of skin denticles in sharks is often related to hydrodynamic function, whereas denticles of batoids (rays) are often sparser and considerably larger. In particular, some skates (Raja) bear modified ‘bucklers’, denticles with greatly swollen bases, often far larger than the denticle’s cusp. Our imaging/materials/histology techniques reveal bucklers have a curiously bipartite structure: the cusp is a standard placoid scale jutting from the skin, whereas the massive spheroidal base envelops it from beneath, like an inverted mushroom cap. The base is tethered to the cusp by dense arrays of Sharpey’s fibers and anchored into underlying muscle by connective tissue seams. Contrary to previous interpretations, we show bucklers are not bone, but involve multiple types of dentine. Like other placoid scales, the cusp is mostly a dense orthodentine, perforated by microtubular passages and pulp cavity. The base, however, is more similar to a bone-like osteodentine, forming concentric, irregular layers indicating accretive growth with no remodeling, with some specimens suggesting the cusp can be shed independently from the base. Whereas superficial layers of the base are rich in vascular passages, the tissue beneath the cusp appears to be a third type of dentine, peppered with lobulate voids, often associated with regions of higher mineralization. We compare bucklers to other elasmobranch skin ornaments and discuss the functional implications for their bizarre structure.