Aptychi microstructure in Late Cretaceous Ancyloceratina (Ammonoidea)

Authors

  • ISABELLE KRUTA,

  • ISABELLE ROUGET,

  • NEIL H. LANDMAN,

  • KAZUSHIGE TANABE,

  • FABRIZIO CECCA


Isabelle Kruta [kruta@mnhn.fr], Muséum National d’Histoire Naturelle (MNHN), Centre de recherche sur la Paéodiversité et les Paléoenvironnements CNRS UMR 7207, UPMC, Case 104 – 4 Place Jussieu, 75252 Paris Cedex 05, France; Isabelle Rouget [isabelle.rouget@upmc.fr] and Fabrizio Cecca [fabrizio.cecca@upmc.fr], Université Pierre et Marie Curie, Centre de recherche sur la Paéodiversité et les Paléoenvironnements CNRS UMR 7207, Case 104 – 4 Place Jussieu, 75252 Paris Cedex 05, France; Neil H. Landman [landman@amnh.org], Division of Paleontology (Invertebrates), American Museum of Natural History, New York, New York 10024, USA; Kazushige Tanabe [tanabe@eps.s.u-tokyo.ac.jp], Department of Earth and Planetary Science, University of Tokyo, Tokyo 113-0033, Japan; manuscript received on 18 April 2008; manuscript accepted on 19 August 2008.

Abstract

The microstructure of aptychi (bivalved calcareous coverings on lower jaws) of three genera of Late Cretaceous Ancyloceratina, Baculites, Polyptychoceras and Jeletzkytes is described for the first time on the basis of well-preserved and in situ material from the Western Interior of the USA and Hokkaido, Japan. Optical and scanning electron microscope observations of aptychi on polished median and cross-sections reveal some variation in their relative size, shape and microstructure among the three genera. The aptychus of Baculites is composed of two calcitic layers: one with tilted lamellae and the other one with horizontal lamellae, whereas those of Polyptychoceras and Jeletzkytes consist of a thin layer with horizontal lamellae. Comparison with aptychi (e.g. Laevaptychus) of Jurassic Ammonitina shows that the aptychi of Ancyloceratina differ from those of Jurassic Ammonitina in the smaller number of layers and the absence of a sponge-like structure. We propose for the first time growth models for a sponge-like aptychus of Jurassic Ammonitina and the lamellar aptychus of Cretaceous Ancyloceratina. The remarkable microstructural variation of aptychi observed in Mesozoic Ammonoidea is probably related to the diversity of their modes of feeding and the secondary function of the lower jaws as opercula.

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