Histology of Onchopristis numidus rostral denticles

The rostral denticles of the Cretaceous sclerorhynchoid Onchopristis numidus are readily recognizable by their impressive size and robust posterior barb, yet little is known about the microstructural organization of the crown enameloid. Histological examination of specimens recovered from the Kem Kem Beds of southern Morocco reveals the presence of a complex enameloid. The apical cap possesses a thin single crystallite enameloid unit of randomly oriented individual fluorapatite crystallites that overlies a thicker unit comprising of distinct crystallite bundles. These bundles are categorized into parallel, tangled, and radial components based on their orientation relative to the crown surface. The confamiliar species Ischyrhiza mira has an apical cap with a similar enameloid architecture; however, O. numidus possesses an additional thin layer of circumferential bundles that encircles this region just below the surface. Beyond the apical cap, the single crystallite enameloid in I. mira directly covers the orthodentine core, whereas this unit overlies a uniform enameloid layer of highly compacted perpendicular crystallites in O. numidus. Remarkably, the distinctive barb of the latter species has a microstructure that mirrors the crown apical cap suggesting a shared developmental process. Therefore, the barb of O. numidus is a serial homolog of the apical cap.