Biomechanical analyses provide unique insights on state shifts in the ecology of extinct communities. Ammonoids present a compelling case study for coupling biomechanical analysis with ecology given their robust fossil record of external conchs. We present a trajectory model to evaluate hydrodynamical advantages and challenges associated with ammonoid conch form. The model is a one-dimensional calculation estimating the dynamic feedbacks between different components of an ammonite’s motion including thrust, drag, acceleration, and distance traveled. Computational fluid dynamics simulations were performed on eleven different ammonoid conch morphotypes and integrated into a mathematical model to analyze the dynamics of swimming across a combination of conch diameters (5, 10, and 20 cm) and jet rhythms (a single jet or series of three pulses). We compared the efficacy of short-term bursts of motion to that of long-term cruising and found: inflated shapes (i.e., spherocones) offer the fastest short-term motion, but at the greatest costs; heavily streamlined shapes (i.e., platycones) offer long cruise distances, but with ineffective short-term motion; and visibly-coiled shapes (i.e., serpenticones) appear to offer intermediate performance in both locomotion styles. Size is critical in ranking the performance of different conch shapes in both locomotion styles because ranking is determined predominantly by the amount of thrust an animal is capable of generating. With increasing size, Reynolds number increases and the effects of second-order morphological characters become more pronounced and alter the performance ranking of conchs. Finally, we present a visual analysis of the flow regimes and shape details that may drive these hydrodynamic consequences. We speculate that serpenticone morphologies capitalized on these subtleties with a morphology that provided reasonably high-speed swimming at small sizes relevant to juveniles while maintaining relatively efficient coasting locomotion at the larger sizes relevant to adult animals. We highlight the ubiquitous serpenticones of the Early Jurassic as a case study for applying biomechanical data to a paleoecological context. The broad range of morphotypes expressed by ammonoids in the Late Triassic is dramatically pared down during the End Triassic extinction. In the few million years following the extinction, ammonoids diversify into a suite of shapes with a more restricted range of locomotor performance, and only much later is the full range of morphology recovered.