Connecting Ring in Nautiloids

The connecting ring in living Nautilus is composed of an inner glycoprotein (con-chiolin, horny) layer and an outer, calcified spherulitic-prismatic layer (Mutvei, 1972, 2002a; Grégoire, 1984). The inner layer is an uncalcified and structurally modified continuation of the nacreous layer of the septal neck. It is composed of a thin lamella of glycoprotein fibers similar to those in the connecting ring of ammo-noids (Obata et al., 1980; Westermann, 1982; Grégoire, 1984). In contrast to that in ammonoids, this layer is elastic and lacks pore canals. It can stand against hydrostatic pressures of 50-80 atm, but has a low permeability for cameral liquid. The outer, calcified, spherulitic-prismatic layer is a continuation of a structurally similar layer on the outer surface of the septal neck. The latter layer is porous without mechanical strength.

As reported by Mutvei (2002a), the fossil nautiloid taxa Nautilida and Tarphycerida have a similar connecting ring structure to that seen in Nautilus and, hence fundamentally different from that in ammonoids. The fibrous, glycoprotein layer of the connecting ring in the Nautilida and Tarphycerida is practically always destroyed during fossilization. However, the direction of the growth lines in the distal end of the septal neck suggests that an inner glycoprotein layer of the connecting ring was present during the animal's lifetime.

In Paleozoic orthoceratid and actinoceratid nautiloids the connecting ring has a different structure. The outer spherulitic-prismatic layer is present but is usually thinner than that in other nautiloids. The inner layer of the connecting ring is calcified and perforated by numerous pore canals, usually about 0.1-0.5 mm in diameter (Fig. 12.5A, B; Mutvei, 1997, 1998, 2002a, b). Thus, the pore canals occur in the connecting rings of actinocerid and orthocerid nautiloids, but they have a considerably larger diameter than those in ammonoids. It is highly probable that the pore canals in orthocerids and actinocerids also housed cellular extensions from the

Fig. 12.5 Connecting rings in Ordovician nautiloids. A - Orthoceras scabridum. Longitudinal section ofconnecting ring to show the calcified layer with pore canals. B - Lituites sp. Paramedian section of connecting ring to show the calcified layer with pore canals. Scale bar - 1 mm.

epithelium of the siphuncular cord, and that these extensions considerably increased the surface of the physiologically active epithelium, thereby increasing the capacity and speed of the "osmotic pumping" function of this epithelium.

Was this article helpful?

0 0

Post a comment