AbstractThe fossil conchs of ammonoids provide valuable information about the life habits of this extinct group. A new conch measurement, the apertural surface area (ASarea), is introduced here along with modeled sizes of the buccal mass and the hyponome, based on ratios of these organs in comparison with the aperture height from the RecentNautilus pompilius. A principal components analysis was performed using the three main characters: (1) apertural surface area index (i.e., the ratio of the apertural surface and the conch diameter), (2) buccal mass area index (i.e., the ratio between the buccal mass area and the ASarea), and (3) coiling rate of the conch. It revealed an ecomorphospace where life history traits can be tentatively assigned to species of the Ammonoidea. In this morphospace, RecentNautilushas a marginal position, being one of the ectocochleate cephalopods with best properties for active life (capacity for handling large food items, rather good mobility). In contrast, most ammonoids possessed, at comparable conch sizes, much smaller buccal apparatuses and hyponomes, suggesting a more passive life history with reduced mobility potential and reduced capacities for larger prey items.