SIPHUNCULAR STRUCTURE IN SOME FOSSIL COLEOIDS AND RECENT SPIRULA

Abstract

Abstract: In Jurassic Phragmoteuthis huxleyi Donovan (Order Phragmoteuthida) the siphuncular wall shows unique structural and morphological features. The septal neck is short, about one‐eighth of chamber length, but the connecting ring is extremely long, extending through 5–6 chambers. The permeable siphuncular wall in each chamber is, therefore, unusually thick and consists of 5–6 consecutive connecting rings. Each connecting ring is calcified and has a highly porous structure in being composed of bundles of spicular crystallites, orientated more or less at right angles to the siphuncular wall, and separated by smaller or larger interspaces. A restudy of the belemnoid Megateuthis gigantea (Schlotheim) and the aulacoceratid Mojsisovicsteuthis? shows that the connecting rings in these taxa are also calcified. Each ring has a length of two chambers and consists of several calcified lamellae that are traversed by minute pores. The permeable siphuncular wall in each chamber therefore consists of two consecutive connecting rings separated by a porous prismatic layer. In Recent Spirula the connecting ring is composed of two layers: an outer spherulitic‐prismatic layer and an inner glycoprotein layer, of which the latter is not preserved in dry shells. The connecting ring structure is here similar to that in Recent Nautilus. Our study shows that at least three different structural types of siphuncular wall occur in coleoids. The phragmoteuthid connecting ring has a primitive structure, unknown in other cephalopods. This indicates that this taxon has no closer relationship with other coleoid taxa. The belemnitid‐aulacoceratid connecting ring is calcified and traversed by numerous pore canals. It shows a certain structural similarity to that in fossil actinoceratid and orthoceratid nautiloids. The spirulid connecting ring is structurally similar to that in Recent Nautilus and fossil nautilitid and tarphyceratid nautiloids. Thus the connecting ring structure indicates that coleoids include several, phylogenetically clearly separated lineages.