The jaw apparatuses of Cretaceous Phylloceratina (Ammonoidea)

Tanabe, K., Misaki, A., Landman, N.H. & Kato, T. 2013: The jaw apparatuses ofCretaceous Phylloceratina (Ammonoidea). Lethaia, Vol. 46, pp. 399–408.The jaw apparatuses of two species of Late Cretaceous Phylloceratina (Ammonoidea),Hypophylloceras subramosum and Phyllopachyceras ezoensis, are described on the basisof well-preserved in situ material from Hokkaido, Japan. Gross morphological andX-ray CT observations reveal that the upper and lower jaws of the two species areessentially similar in their overall structure. Their upper jaws consist of a shorter outerlamella and a pair of larger, wing-like inner lamellae that become narrower and jointogether in the anterior portion, as in those of other ammonoids. The upper jaws ofthe two phylloceratid species are, however, distinguishable from those of other knownammonoids by the presence of a thick, arrowhead-shaped calcified rostral tip. Thelower jaws of the two species consist of a short, reduced inner lamella and a large,gently convex outer lamella covered with a thin calcareous layer, the features of whichare common with the rhynchaptychus-type lower jaws of the Cretaceous Lytoceratina.In the presence of a sharply pointed, thick calcareous tip on upper and lower jaws, thejaw apparatuses of the Phylloceratina resemble those of modern and fossil nautilids,suggesting that they were developed to serve a scavenging predatory feeding habit indeeper marine environments. This and other studies demonstrate that at least someMesozoic rhyncholites and conchorhynchs are attributable to the Phylloceratina andLytoceratina.