Summary
The microstructure of parrotfish pharyngeal teeth was examined using scanning electron microscopy to infer possible mechanical properties of the dentition with respect to their function. Parrotfish tooth enameloid is formed from fluorapatite crystals grouped into bundles. In the upper and lower pharyngeal jaw, the majority of the crystal bundles are orientated either perpendicularly or vertically to the enameloid surface. The only exception is in the trailing apical enameloid in which the majority of bundles are orientated perpendicularly or horizontally to the trailing surface. A distinct transition occurs through the middle of the apex between the leading and trailing enameloid in teeth of the lower pharyngeal jaw. This transition appears less distinct in the teeth of the upper pharyngeal jaw. Enameloid microstructure indicates that shear forces predominate at the apex of the teeth. In the remainder of the enameloid, the microstructure indicates that wear is predominant, and the shear forces are of less importance.
Morphology, occlusal surface topography, macrowear, and microwear features of parrotfish pharyngeal teeth were investigated to relate microstructural characteristics to the function of the pharyngeal mill using scanning electron microscopy of whole and sectioned pharyngeal jaws and teeth. Pharyngeal tooth migration is anterior in the lower jaw (fifth ceratobranchial) and posterior in the upper jaw (paired third pharyngobranchials), making the interaction of occlusal surfaces and wear-generating forces complex. The extent of wear can be used to define three regions through which teeth migrate: a region containing newly erupted teeth showing little or no wear; a midregion in which the apical enameloid is swiftly worn; and a region containing teeth with only basal enameloid remaining, which shows low to moderate wear. The shape of the occlusal surface alters as the teeth progress along the pharyngeal jaw, generating conditions that appear suited to the reduction of coral particles. It is likely that the interaction between these particles and algal cells during the process of the rendering of the former is responsible for the rupture of the latter, with the consequent liberation of cell contents from which parrotfish obtain their nutrients.
Scales of the Australian lungfish, Neoceratodus forsteri, are secreted within the dermis by a capsule of scleroblasts, and enclosed in a pouch made of collagen fibers, in contact with the epidermis over the posterior third of the scale. Each scale grows from a focus, which represents the first formed part of the scale. On the internal surface of the scale is elasmodin, made of collagen fiber bundles arranged in layers. Elasmodin, unmineralized in N. forsteri, contains cells in the living animal, and the number of layers increases as the scales grow. Squamulin, on the thin external part of the scale, is also laid down in layers, and based on a matrix of fine collagen fibrils, mineralized with a poorly crystalline biogenic calcium hydroxylapatite. Squamulin is divided into separate sections called squamulae, and contains long tubules with cells applied to the wall of the tubule. The anterior and lateral surfaces of the squamulin are ornamented with pediculae, and the posterior surface has longitudinal ridges, from which collagen fibers extend to anchor the scale within the pouch. Elasmodin and squamulin are linked by unmineralized collagen fibrils. The layers, formed at irregular intervals, are connected around the margin of the scale, effectively converting the whole scale into a flat structure resembling a pearl, with the first formed tissues deeply embedded inside the scale, and the youngest on the outer surface. Incremental lines in the hard tissue, and the number of layers in the elasmodin, do not reflect the chronological age of the fish.
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