MINERALOGY OF HYDROXYAPATITE IN ALLIGATOR GAR FISH SCALE

Alligator Gar (Atractosteus spatula) is a modern armored fish thought to be descended from Mesozoic aged ray-finned bony fish (palaeoniscoids). Its scales are dermal denticles consisting of a bony core and a ganoid enamel-like (ganoine) cap. SEM and µCT imaging revealed that the ganoine cap was a dense layer of apatite anchored to the underlying bone by circumferential ridges. The bone beneath the ganoine was, in places, a dense and highly textured composite of hydroxyapatite (HAp) and type I collagen (Col) similar to bone of other vertebrates. The ganoine HAp was enriched in Na and depleted in Mg relative to the bone HAp. Ca/P ratios from EPMA tended to be higher in the bone relative to the ganoine. FTIR analyses indicated the presence of CO₃^-2 in both the ganoine and bone HAp, but the spectra showed different absorption bands for Col in the amide region (1100 to 1800 cm^-1). At the nanoscale, TEM tomography of partially demineralized bone identified two types of HAp, one intracollagenous and roughly centered on what may be the “gap” zone of Col, and another extracollagenous and larger than the banding repeat of Col. Both HAp crystal types were thin and oriented at an angle to the Col long axis, which implies an interpenetration of HAp and Col at the nanoscale. HAp_(bone) separated from Col had the form of thin ~50 x 50 x 5 nm³ sheets that were interconnected at corners. TEM EDX analyses of bone separates had a lower Ca/P than EPMA of HAp_(bone)+Col, a result indicating a possible third component in gar scale bone. STEM HAADF/EELS analyses revealed high Ca bands, correlating with the spacing of collagen D-banding, with sharp boundaries that appeared not to be from HAp crystals. This material may be the source of the third high-Ca compnent and may possibly be amorphous Ca-carbonate. The physical properties of this new three-component HAp/Col interpenetration model would be greatly different from one predicted by the current parallel HAp/Col model.