Internal structures in the early whorls of Mesozoic ammonites. American Museum novitates ; no. 2823

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New York, N.Y. : American Museum of Natural History
"The first few septa and associated structures in the early whorls of Mesozoic ammonites were studied in a number of genera including Quenstedtoceras, Kosmoceras, Euhoplites, Hypacanthoplites, Baculites, and Scaphites and its related genera. Exceptionally well-preserved specimens with little obscuring matrix inside permitted observations of the spatial arrangement of the first few septa and were supplemented by sections polished parallel to the median plane. Our observations indicate that: 1. The proseptum is a single structure and does not consist of two septa. Prismatic attachment deposits of the caecum and siphuncle occur around the proseptal opening. 2. In all genera except Quenstedtoceras, the second septum is moderately distant from the proseptum and, in median section, is slightly convex, not concave, toward the aperture. In Quenstedtoceras, however, the second septum grows dorsally into the proseptum and is only conspicuous on the venter. These relationships are also expressed in the shape and spacing of the corresponding sutures on steinkerns of the initial whorls. 3. In all genera in which the original shell structure was preserved, the second septum is nacreous, not prismatic. Therefore, in agreement with Drushchits and Khiami (1970), we prefer the simpler terms second septum and third septum for primary septum and nacroseptum, respectively. 4. The development of a prismatic attachment ridge at the base of the proseptum, dorsal muscle scars just adoral of each septum, and wrinkles in the proseptum and prosiphonal attachment sheets support the model of early ammonite ontogeny proposed by Bandel (1982)"--P. [1].
21 p. : ill. ; 26 cm.
Includes bibliographical references (p. 21).