Intraspecific variation of hatchling size in late cretaceous ammonoids from hokkaido, japan: Implication for planktic duration at early ontogenetic stage

摘要

Intraspecific variations of the early shell dimensions (ammonitella and protoconch diameters) of two Late Cretaceous (earliest Campanian) ammonoid species (Gaudryceras tenuiliratum and Hypophylloceras subramosum) from the Haboro and Ikushumbetsu areas, Hokkaido, Japan, show no significant difference between these areas that are approximately 110km apart. The geographic distributions of G. tenuiliratum and H. subramosum are supposed to be mainly controlled by the flotation and transportation during the embryonic stage within floating egg masses and/or post-embryonic stage because of their small hatchling sizes (1.18-1.46mm in diameter for G. tenuiliratum, and 0.91-1.13mm in diameter for H. subramosum), suggesting these two ammonite species at the embryonic and/or post-embryonic stages were transported at least 110km. Postulating that the velocity of palaeocurrent around the Haboro and Ikushumbetsu areas during the Cretaceous Period was 0.25m/s, similar to those in the modern ocean current flowing off the eastern Pacific coast of Hokkaido, the egg masses and/or hatchlings of G. tenuiliratum and H. subramosum were buoyant and transported more than 5days. The preliminary comparison of hatchling size through time suggests that the hatching sizes of H. subramosum in Hokkaido increased slightly from the Middle Turonian until the earliest Campanian (during about 7Myr). □ammonoid, hatchling, paleoecology, variation, Cretaceous.