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Septal necks in Mesozoic Ammonoidea: structure, ontogenetic development, and evolution
Tanabe, K.; Landman, N.H.; Weitschat, W. (1993). Septal necks in Mesozoic Ammonoidea: structure, ontogenetic development, and evolution, in: House, M.R. (Ed.) The Ammonoidea: environment, ecology, and evolutionary change. pp. 57-84
In: House, M.R. (Ed.) (1993). The Ammonoidea: environment, ecology, and evolutionary change. The Systematics Association Special Volume, 47. Clarendon Press: Oxford. ISBN 0-19-857765-6. 353 pp., more
In: The Systematics Association Special Volume, more

Available in Authors 
    VLIZ: Geology and Geophysics [8739]

Authors  Top 
  • Tanabe, K.
  • Landman, N.H.
  • Weitschat, W.

Abstract
    Ontogenetic and phylogenetic changes in the structure of the septal neck were studied in Mesozoic ammonoids based on well-preserved specimens from Spitsbergen, Siberia, England, Germany, Italy, Japan, Alaska, California, and the US western interior. In every specimen the septal neck is continuous with the rest of the septum, and consists primarily of a nacreous layer. Based on their three-dimensional architecture, septal necks may be classified into four major types: (1) retrochoanitic (entirely projected adapically); (2) modified retrochoanitic (projected both ado rally and adapically on the dorsal side and adapically on the ventral side (Type A) or adorally on the dorsal side and adapically on the ventral side (Type B) ); (3) amphichoanitic (projected both ado rally and adapically); and (4) prochoanitic (entirely projected adorally). A spherulitic-prismatic deposit called the auxiliary deposit commonly appears on the adoral side of the septal neck and covers the inner surface of the nacreous layer. Another spheruliticprismatic deposit (cuff) may also occur on the adapical side of prochoanitic necks. The type of septal neck and its ontogenetic development are distinctive features that help characterize the Mesozoic suborders. The Ceratitina have retrochoanitic necks that persist throughout ontogeny or develop into modified retrochoanitic necks, first type A, then type B. The Phylloceratina have retrochoanitic necks in early to middle ontogeny, which later develop into amphichoanitic necks. The other three suborders the Lytoceratina, Ammonitina, and Ancyloceratina have prochoanitic necks throughout most of ontogeny. Differences in the ontogenetic timing of the transformation from retrochoanitic to other kinds of septal necks may reflect heterochronic processes. Despite the variation in the ontogenetic development of septal necks among suborders, some similarities appear in unrelated species adapted to the same mode of life. For example, most Cretaceous tetragonitids and gaudryceratids (Lytoceratina), phylloceratids (Phylloceratina), and desmoceratids (Ammonitina) that occur abundantly in offshore mudstone facies possess unusually long septal necks with well-developed cuffs and/or auxiliary deposits in later ontogeny. These ammonoids also have a relatively narrow but thick-walled siphuncular tube. Such features of the septal neck-siphuncular complex may have served to enhance the mechanical strength of the septa and siphuncle against high hydrostatic pressure.

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