Marion Nixon

Morphology of the Jaws and Radula in Ammonoids

Some ammonoids have been found with structures still in the living chamber that are now recognized as jaws (beaks or mandibles) because small teeth, characteristic of those of the radula of extant Cephalopoda, lie between upper and lower jaws. The radulae so far found in ammonoids have the same number of teeth in each transverse row as do those of living Coleoidea, and the teeth are of similar shape and form. The first discovery was of just a few teeth in Eoasianites (Closs and Gordon, 1966), a member of the taxon Goniatitina. A year later, eight specimens of the same species, in which the radula and jaws were preserved together, were reported (Gloss, 1967). The jaws and radulae of two adult microconchs of Eleganticeras elegantulum, a Liassic ammonite, were found by Lehmann (1967); other descriptions have followed, mostly of jaws but also of radulae (for reviews see Lehmann, 1971a, 1981a,b, 1988, 1990; Dagys et al.,1989).

The cells that secrete the beaks in octopods and squids (Mollusca, Cephalopoda)

SummaryA single layer of cells secretes the hard cephalopod beaks. The beccublasts are tall columnar cells that separate the beak from the surrounding buccal muscles, and must serve to attach these muscles to the beak. Within the cell layer there are three types of cells. The first, and most frequently found contain cell-long fibrils. These fibrils may have contractile and tensile properties. Complex trabeculae extend from the beccublasts into the matrix of the beak. The fibrils are attached to these trabeculae and at the other end of the cells they are anchored near to the beccublast-muscle cell interface, closely associated with the muscles that move the beak.The second group of cells contain masses of endoplasmic reticulum the cysternae of which are arranged along the long axis of the cell. These cells also contain dense granules and are probably the major source of beak hard tissue. It is probable that each cell secretes its own column of beak hard tissue. The third group of cells contains a mixture of fibrils and secretory tissue.In the beccublast layer there are changes in the proportion of the three types of cells depending upon the region sampled. In the region where growth is most active there are mostly secretory cells, whereas near the biting and wearing tip there are mainly anchoring type cells.