Reinterpretation of brunonia annulata yokoyama as an early cretaceous carinariid mesogastropod mollusca
Kase, T.
Journal of Paleontology 62(5): 766-771
1988
ISSN/ISBN: 0022-3360
DOI: 10.2307/1305398
Accession: 006292885
Brunonia annulata Yokoyama), a large and previously thought to be shell-less, limpet-shaped fossil from the Lower Cretaceous (Barremian) Ishido Formation of the Sanchu area, central Japan, the Aptian of Tunisia, and the Barremian to Aptian of the Crimea, has previously been interpreted as belonging to the Archaeogastropoda, Mesogastropoda, or Pulmonata, or as the pneumatophores of a chondrophorine hydrozoan. Discovery of shell-bearing specimens has revealed that B. annulata has an exceedingly thin calcuareous shell. Athough the shell is recrystallized to calcite, its outer surface has faint growth lines while its inner surface is smooth, suggesting that it is best interpreted as a molluscan shell rather than as a chondrophorine pneumatophore. The presence of a paper-thin shell, the overall shell morphology, and the taphonomic implication suggest that B. annulata can best be interpreted as a member of the carinariid mesogastropods, fossils of which are poorly represented and were hitherto unknown prior to the Eocene.