Rhyncholites and conchorhynchs as calcified jaw elements in some late Cretaceous ammonites

Abstract

Conspicuous calcareous coverings are present in the anterior region of 17 fossil jaws from late Cretaceous rocks of Hokkaido (Japan) and Sakhalin (U.S.S.R.). The jaws were preserved in calcareous nodules either in situ in body chambers of ammonites or in close association with identifiable ammonite conch remains. From the morphologic similarity between in situ and isolated jaws, they may be attributed to Tetragonites glabrus, Gaudryceras tenuiliratum, G. denseplicatum, G. sp., and Neophylloceras subramosum. The jaw apparatus of these species is composed of two three‐dimensional black walls of carbonate apatite, which might be a diagenetic replacement of chitinous material. The calcareous coverings in both upper and lower jaws closely resemble those of upper (rhyncholite) and lower (conchorhynch) jaws of modern Nautilus as well as rhyncholite and conchorhynch fossils in their gross morphology, microstructure, and chemical composition. Calcified remains of cephalopod jaws known as rhyncholites and conchorhynchs have been reported from late Paleozoic to Recent. The present discovery of ammonoid rhyncholites and conchorhynchs suggests that at least some previously known late Paleozoic and Mesozoic counterparts belong to the Ammonoidea. The essential similarity of jaw elements of some Late Cretaceous ammonites and modern Nautilus gives reliable information on the feeding habits of the former. The sharp and thick ammonoid rhyncholites and conchorhynchs may have had a special function for cutting up food, similar to those of Nautilus.

Non-destructive, safe removal of conductive metal coatings from fossils: a new solution

“…They have been found from the Late Paleozoic and younger marine deposits, and some of them from the Jurassic and Cretaceous are interpreted to be the calcified deposits of the upper and lower jaws of either lytoceratid or phylloceratid ammonoids (see review by Tanabe et al, 2015). The overall morphology of the calcareous covering of the lower jaw of the examined P. ezoensis could be comparable to other known conchorhynchs (e.g., Teichert et al, 1964;Saunders et al, 1978) and conchorhynch-like structures of Lytoceratina (Tanabe et al, 1980;Kanie, 1982) and Phylloceratina (Tanabe and Landman, 2002;Tanabe et al, 2013). However, that of the upper jaw of P. ezoensis is quite different from other known rhyncholites.…”

Jaws of Triassic ammonoids from New Zealand

“…These jaws were presumably originally composed of chitin without any calcareous covering (Dagys et al 1989), although, as noted, the lower jaws of some Cretaceous phylloceratids possess a calcareous deposit at the apical tip (Tanabe et al 1980;Tanabe & Landman 2002).…”

“…5E,F), and Early Jurassic (Liassic) ammonitids (Lehmann 1970). It is similar to the rhynchaptychus type jaw of the Lytoceratina, although these forms bear a calcareous deposit at the apex (Tanabe et al 1980). The anaptychus type jaw differs from the aptychus type jaw of the Aptychophora Engeser and Keupp, 2002, which includes the Ancyloceratina and most of the Ammonitina.…”

“…However, they are very similar to the jaws of ceratitids and phylloceratids, both of which occur in the same strata. Ceratitid jaws have been described from the Triassic of Siberia, Russia, and Spitzbergen (Dagys & Dagys 1975;Zakharov 1979;Dagys & Weitschat 1988;Dagys et al 1989) and phylloceratid jaws have been described from the Jurassic and Cretaceous of Germany and Japan (Tanabe et al 1980;Riegraf et al 1984;Lehmann 1990;Keupp & Veit 1996;Tanabe & Landman 2002).…”

“…The ridges run straight across or develop a slight anterior flexure along the middle. The lower jaws of some Cretaceous phylloceratids also bear a calcareous deposit on the anterior end (Tanabe et al 1980;Tanabe & Landman 2002). This feature is not evident in any of the specimens from New Zealand, although poor preservation may account for this difference.…”