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Post-hatching early life histories in Cretaceous Ammonoidea are discussed on the basis of density calculations of the shells in 71 species belonging to four separate suborders. The calculation was made under the assumption that a newly hatched ammonoid had a gas-filled chamber and a succeeding body-filled whorl terminating at the primary constriction. The results show that the density of the species examined at the hatching stage is almost constant and is relatively smaller than that of seawater, i.e. the animals are positively buoyant. This fact strongly suggests a planktic mode of life. In all species, the density increases gradually with growth and attains neutral buoyancy at 2.C2.5 mm in shell diameter. Thus, most ammonoids probably changed their mode of life from planktic to nektoplanktic or nektobenthic at this critical point. The rare occurrence of newly hatched specimens (ammonitellas) in many ammonoid assemblages may also support this interpretation. Planktic duration of a newly hatched ammonoid might be regulated by the animal's density at hatching, shell growth pattern, cameral volume (or hatching size), and rate of cameralliquid removal (or siphuncle diameter). The latter two seem to be very important factors in determining the biogeographical framework of species, as demonstrated in the Tetragonitaceae.□Cretaceous, Ammonoidea, density calculation, early life history.