POSTHATCHING EARLY-LIFE HISTORY OF CRETACEOUS AMMONOIDEA

Post-hatching early life histories in Cretaceous Ammonoidea are discus sed on the basis of density calculations of the shells in 71 species b elonging to four separate suborders. The calculation was made under th e assumption that a newly hatched ammonoid had a gas-filled chamber an d a succeeding body-filled whorl terminating at the primary constricti on. The results show that the density of the species examined at the h atching stage is almost constant and is relatively smaller than that o f seawater, i.e. the animals are positively buoyant. This fact strongl y suggests a planktic mode of life. In all species, the density increa ses gradually with growth and attains neutral buoyancy at 2.0-2.5 mm i n shell diameter. Thus, most ammonoids probably changed their mode of life from planktic to nektoplanktic or nektobenthic at this critical p oint. The rare occurrence of newly hatched specimens (ammonitellas) in many ammonoid assemblages may also support this interpretation. Plank tic duration of a newly hatched ammonoid might be regulated by the ani mal's density at hatching, shell growth pattern, cameral volume (or ha tching size), and rate of cameral-liquid removal (or siphuncle diamete r). The latter two seem to be very important factors in determining th e biogeographical framework of species, as demonstrated in the Tetrago nitaceae.