Functional interpretation of inner shell layers in Triassic ceratid ammonites

Abstract

The wrinkle layer the inner prismatic layer are described in three Triassic ceratid genera: Phyllocludiscites. Megaphyllites Proarcestes. Both layers have their counterparts in the shell wall of the recent Nautilus: the wrinkle layer corresponds to the mantle‐adhesive layer the inner prismatic layer to the myostracal layer in Nautilus. A detailed structural functional comparison between these layers is given. The wrinkle layer is also compared with the oblique prismatic layer in recent gastropods.

The Additional External Shell Layers Indicative of “Endocochleate Experiments” in Some Ammonoids

“…In phylloceratids and lytoceratids it covers a short posterior portion of the body chamber approximately equal to the length of the last two or three chambers. In ammonitids this layer covers a longer posterior portion of the body chamber equal to half of the body chamber length (Drushchits and Doguzhaeva 1974;Drushchits et al 1978;Doguzhaeva 1981;Doguzhaeva and Mutvei 1986). The wrinkle layer was secreted on the inner surface of the nacreous layer in the body chamber and also on the surface of the adjacent whorl outside the body chamber (Ruzhencev 1962;Walliser 1970;House 1971;Senior 1971;Tozer 1972;Kulicki 1979;Doguzhaeva 1981;Doguzhaeva and Mutvei 1986;Korn 2000;Kulicki et al 2001).…”

“…In ammonitids this layer covers a longer posterior portion of the body chamber equal to half of the body chamber length (Drushchits and Doguzhaeva 1974;Drushchits et al 1978;Doguzhaeva 1981;Doguzhaeva and Mutvei 1986). The wrinkle layer was secreted on the inner surface of the nacreous layer in the body chamber and also on the surface of the adjacent whorl outside the body chamber (Ruzhencev 1962;Walliser 1970;House 1971;Senior 1971;Tozer 1972;Kulicki 1979;Doguzhaeva 1981;Doguzhaeva and Mutvei 1986;Korn 2000;Kulicki et al 2001). The adjacent shell wall of the consecutive whorls shows a sequence of six layers: four of these layers (outer prismatic, nacreous, wrinkle, inner prismatic) belong to the ventral wall of previous whorl, and the additional two = 200 μm.…”

“…The shell wall layers of the hatchlings lie on the inner surface of the primary varix (Drushchits and Doguzhaeva 1974;Howarth 1975;Birkelund 1981). The secreting sequence of the shell wall layers in adult shells has been established by tracing the layers in fully preserved body chambers with aperture (Doguzhaeva 1981;Doguzhaeva and Mutvei 1986). In the ventral and lateral walls of the adult planispiral shells, three of four layers (outer prismatic, nacreous and wrinkle) were secreted at the shell aperture; the fourth layer, the inner prismatic layer, was secreted in the posterior portion of the body chamber where it covers the wrinkle layer.…”

Ammonoid Shell Microstructure

“…In specimens in the final stages of ontogeny, the inner prismatic layer comes closer to the apertural zone (Doguzhaeva and Mutvei 1986b). This layer results from secretory activity of the myoadhesive epithelium (Mutvei 1964).…”

“…These triangular elements have a prismatic structure with the long axes of the prisms perpendicular to the overall surface of the wall of the ammonitella rather than to the surfaces of the gentle adapical slopes of the triangles. Kulicki (1979) and Doguzhaeva and Mutvei (1986b) investigated the complex structure of the dorsal wall. Kulicki (1979) distinguished two components, outer and inner ones in the dorsal wall.…”