Abstract
The siphuncle is a long and narrow segmented soft tissue that occurs in four extant cephalopods with a phragmocone. It contains blood vessels, connective tissue and surrounding epithelium. In ammonoids, this organ was rarely replaced by phosphate minerals. We report here fossilized siphuncular remains from specimens of four ammonoid species. In these species, the siphuncle is found as truncated segments within the siphuncular tube, whose outer surfaces carry longitudinal ridges and furrows in three species. In cross-section, the siphuncle of Akmilleria electraensis consists of a large central vein, possibly two pairs of arteries, porous connective tissue with reticulate hemocoele, and a thin epithelium toward the outside, thus rather resembling that of Nautilus pompilius than that of Spirula. N. pompilius possesses smaller and more numerous epithelial cells around the siphuncle than A. electraensis. These observations suggest that the siphuncular epithelium of ammonoids served as the salt-concentrating organ for buoyancy regulation.
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Acknowledgments
We thank Tamaz A. Lominadze (Institute of Paleobiology, Georgian Academy of Science, Tbilisi) for providing an interesting specimen of Paracanthoplites sp. with preserved siphuncular tissue remains for this study, and Cyprian Kulicki (Institute of Paleobiology, Polish Academy of Science; Warsaw), Neil H. Landman (American Museum of Natural History, New York City), Yuri D. Zakharov (Far Eastern Geological Institute, Russian Academy of Science, Vladivostok), and an anonymous reviewer for helpful comments and discussion.
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Tanabe, K., Sasaki, T., Mapes, R. (2015). Soft-Part Anatomy of the Siphuncle in Ammonoids. In: Klug, C., Korn, D., De Baets, K., Kruta, I., Mapes, R. (eds) Ammonoid Paleobiology: From anatomy to ecology. Topics in Geobiology, vol 43. Springer, Dordrecht. https://doi.org/10.1007/978-94-017-9630-9_13
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DOI: https://doi.org/10.1007/978-94-017-9630-9_13
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