Article: Observations on the shell structure of Triassic ammonoids

Despite mineralogical alteration Triassic ammonoids provide significant data the layers forming the shell wall and umbilical plug. Two layers (outer and inner test) are recognized in the outer wall. Outer test incorporates growth lines, ornament and colour markings, defines the fundamental architecture, was evidently secreted only on the flanks and venter, probably only at the mantle edge. Inner test deposits are secondary, modifying in various ways the chamber interior, and were secreted both dorsally and ventrally, probably at the mantle surface. Discotropites has a dorsal secondary layer within the phragmocone described as dorsal shield and interpreted as a manifestation of the inner test. Nathorstites, in contrast, has no dorsal deposit in this position. Secondary deposits secreted within the flanks and venter (preseptal layer of Guex) occur in both phragmocone and in part, but not all, of the body chamber of many Triassic ammonoids. The position of this layer may have exercised buoyancy control. In Ceratitida wrinkle-layer (Runzelschicht) with fingerprint pattern was deposited only on the dorsum and above the umbilical plug and is thus comparable with the Nautilus black layer in position, although different in composition and texture. Like the outer test it was probably a secretion of the mantle edge. This kind of wrinkle-layer evidently characterizes Ceratitida and Palaeozoic Ammonoidea but not Phylloceratida, Lytoceratida and Ammonitida. Internal moulds of the flanks of Nathorstites have markings (ritzstreifen) with a pattern unlike that of the wrinkle-layer. Accordingly, with Mojsisovics, they are interpreted as impressions of the inner surface of the shell wall, not of wrinkle-layer. Maclearnoceras enode sp. nov. is described.