2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 1
Presentation Time: 1:30 PM

Is There Variation In the Microstructure of Ammonite Aptychi?

KRUTA, Isabelle, Histoire de la Terre, UMR 5143, Muséum National d'Histoire Naturelle (& American Museum of Natural History, NY), 8 rue Buffon, Paris, 75005, France, ROUGET, Isabelle, Université Paris VI, CNRS – UMR 5143 ‘Paléobiodiversité et paléoenvironnements’, Case 104 – 4, Place Jussieu, Paris, 75005, France, LANDMAN, Neil H., Division of Paleontology (Invertebrates), American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024-5192, TANABE, Kazushige, Department of Earth and Planetary Science, Univ of Tokyo, Sci. Buid. No.5, University of Tokyo, 7-3-1, Hongo, Bunkyo, Tokyo, 113-0033, Japan and CECCA, Fabrizio, Université Paris VI, CNRS-UMR 5143, case 104, 4 place Jussieu, Paris, 75252, kruta@mnhn.fr

Like modern cephalopods, ammonites are equipped with a large buccal apparatus (Meek and Hayden, 1865; Lehmann, 1971). Many studies have speculated about the mode of feeding of ammonites based on the size and shape of the jaws, in comparison with modern species (Hewitt et al. 1993; Michalik, 1996). Other studies have documented the microstructure of the calcitic valves (aptychi) that cover the lower jaws (Farinacci et al. 1976). However, most of these analyses have been restricted to Jurassic Ammonitina, and the microstructure of these aptychi is usually assumed to apply to all forms. We describe for the first time the microstructure of the lower jaws of the Ancycloceratina, as represented by Baculites, Polyptychoceras, and Jeletzkytes, based on well preserved material from the Campanian and Maastrichtian of the U.S. Western Interior and Hokkaido, Japan. The lower jaws are in situ, that is, they occur inside the living chambers of the ammonites. The lower jaws of these species consist of an inner chitinous layer with a ridge along the midline. The outer layer consists of a pair of calcareous valves (the aptychus). All of these aptychi are thinner than those in Jurassic forms, such as Laevaptychus and Punctaptychus. In addition, the calcite in the Cretaceous aptychi occurs in regular layers whereas in Jurassic aptychi, most of the calcite occurs in a sponge like structure. These differences in microstructure may reflect an evolutionary trend. Moreover, these features may provide additional characters to use in reconstructing the phylogeny of the Ammonoidea.