Mode of Early Shell Growth in the Ammonite Promicroceras marstonense Spath

Abstract

SOME authors have suggested that the nature of the contiguity between the ammonoid proseptum and the shell wall indicates that there is no junction, but that the two are continuous resulting from contemporaneous formation^1–3. This contrasts with the relationship between “normal” septa (that is, all septa excluding the proseptum) and the shell wall to which they are attached. There is abundant evidence in the case of “normal” septa that they were secreted by the animal's adapical soft parts to the inner surface of previously formed shell. The time relationship between each septum and the shell to which it is attached will therefore be the amount of time the animal takes to secrete shell proper equivalent in length to its body chamber, as shown for Nautilus pompilius Linné by Eichler and Ristedt⁴. Other authors^5–8 maintain that the proseptum was inserted in an identical manner to normal septa. House⁶ (page 88) points out that the protoconch apparatus (prosiphon and caecum, Figs. 1A and 2E) is attached to the protoconch wall and must have been formed after this wall, but have preceded the proseptum.