Inferences drawn from the biology, function, and behavior of closely related living forms facilitate interpretation of the mode of life of groups known only from the fossil record. The choice of phylogenetically relevant modern ‘model organisms’ can have critical bearing on the resulting interpretations. The biology and behavior of fossil ammonoids are often interpreted in the light of evidence derived from the study of modern Nautilus. However, examination of the fossil record and cladistic analyses both indicate that coleoids are much more closely related to ammonoids than is Nautilus. Coleoid biology and behavior differ dramatically from the biology and behavior of Nautilus. Thus, the inclusion of coleoids as examples, rather than reliance on Nautilus alone, produces a strikingly different vision of ammonoid biology and suggests that inferences of ammonoid biology and behavior that rely exclusively on Nautilus should be reviewed. Two features related to swimming ability in Nautilus, static stability and large retractor muscles, are much reduced in many ammonoids, leading to the interpretation that ammonoids were poorer swimmers than Nautilus. However, reexamination of the evidence indicates that static stability should not play a role in the swimming of ammonoids with long body chambers. In addition, functional arguments suggest that a coleoid-like swimming mechanism should have evolved prior to the loss of the body chamber in coleoids. Thus, a coleoid-like swimming mechanism is likely to have evolved prior to the separation of ammonoid and coleoid lineages. A mechanism is proposed by which a coleoid swimming mechanism, independent of retractor muscle size, could function in ammonoids with long body chambers.□Ammonoids, ammonites, evolution, functional morphology, Nautilus, phylogeny.