MORPHOGENESIS OF PHRAGMOCONE CHAMBERS IN MODERN AND FOSSIL NAUTILOIDS, AND SOME CRETACEOUS AMMONOIDS RECONSTRUCTED BY HIGH-RESOLUTION COMPUTED- AND GRINDING TOMOGRAPHY

Ectocochleate cephalopod (ammonoid and nautiloid) conchs provide important information on their development because the entire life history is recorded within the conch. Recent advancements of various tomographic techniques allow for highly resolved three-dimensional reconstruction of the cephalopod conchs and, in turn, research on the development of ectocochleate cephalopods in detail. We applied X-ray computed tomography to modern Nautilus conchs with differing degrees of pathology to investigate effects of pathology on the formation of chambers. In fossil specimens, X-ray computed tomography did not yield good results due to similar densities of sediment infills and calcium carbonate shells. Thus, a destructive method (grinding tomography) was applied to fossil nautiloids and ammonoids. The 2D images obtained were used to produce 3D models on the basis of which chamber volume was measured through ontogeny. The volumes were plotted against chamber number through ontogeny to determine if there are differences in the pattern of chamber volume increase in the specimens of Nautilus with various degrees of pathology. Results reveal that only fatal pathology strongly affects the pattern of chamber volume increase. Similarly, the Cretaceous nautiloid Eutrephoceras also shows the same pattern of volume increase. A difference appears to exist between nautiloids and ammonoids. Although there was no trace of pathology on the surface of the ammonoid conchs studied, some ammonoids show abrupt decreases of chamber volume, which is not visible in nautiloids without pathology. Previous research documented such abrupt decreases in the modern coleoid Spirula spirula, which may indicate that ammonoids and coleoids share either a similar developmental program or physiology, which differs from that of Nautilus.