Theoretical restoration of scaphitid deth-assemblages, a Late Cretaceous heteromorph ammonite

Abstract

In order to clarify the adaptive strategies of Japanese scaphitids, Cretaceous heteromorph ammonites, we restored the population dynamics of the hypothetical scaphitid species by computer simulations, and concluded as follows:
(1) Various patterns of size distributions observed in the immature scaphitid shells are interpreted unified as the mixture of the random-death daily accumulated and death from mass starvation eventually occurred.
(2) The biased sex ratio actually observed in the mature scaphitids is caused probably by the difference of duration in which the dimorphs are exposed under the high risk of starving.
(3) These ammonites may have been opportunistic species that migrated to fertile sea grounds and rapidly increased their populations there. The thanatocoenosis actually observed is well restored by hypothetical scaphitids breeding under the conditions of more or less limited food supply and poor random-death factor.