Article: The jaw apparatus of Recent Nautilus and its palaeontological implications

The jaw apparatus of extant cephalopods consists of articulated beak-like mandibles which are composed of chitin and powered by strong buccal muscles. Upper and lower jaws of Nautilus possess conspicuous calcite coverings; the upper jaw is characterized by a robust, arrow-shaped calcite element, the rhyncholite; the anterior region of the lower jaw possesses a delicately denticulated calcite element, the conchorhynch. The rhyncholite functions as an incisor operating against the lower cutting edge of the conchorhynch, producing a powerful shearing action. Rhyncholites extracted from the extant species Nautilus pompilius, N. macromphalus, and N. cf. pompilius are indistinguishable, in contrast to the conchorhynchs, which are species-distinctive. Both rhyncholites and conchorhynchs range from Middle Triassic to Recent. Circumstantial evidence, including the geologic range, the analogy of Nautilus, and the contrasting form of ammonoid and dibranchiate mandibles, indicates that rhyncholites and conchorhynchs belong exclusively to the Nautilaceae. In view of the close similarity between fossil and modern cephalopod jaws, the use of parataxial form genera such as Rhyncolites Biguet, 1819 and Conchorhynchus de Blainville, 1827 is both practical and necessary for classifying isolated cephalopod jaw elements.