“LAZING ALONG AND FRAZZLING OUT”: TESTING TWAIN’S VISION OF AMMONITE ECOLOGY

Dueling visions of ammonites compete for our imagination: sluggish hordes, as teased by Mark Twain; or dashing competitors, fleeing ichthyosaurs across children’s picture books. Marine ecological reconstructions from the Mesozoic must guess ammonoids’ first-order metabolic demands, trophic structure, and population-level stability, all of which hang on their radically under-resolved swimming ability. We analyze the hydrodynamic and hydrostatic consequences of ammonoid shell shape, and synthesize an emerging picture of ammonoid ecological breadth. We create computer-based replicas of specific fossils, and idealized shell coil morphotypes. We developed detailed computational fluid dynamics simulations of water flow around these 3D models to illustrate how ammonoid shell shapes contribute to, or unload, hydrodynamic drag. Size, shape, and speed combine: as in living squid, a single ammonoid would experience a drastic range of physics regimes in its lifetime. We suggest the range of swimming speeds suited to key species through their ontogeny. Generally, ammonoid shells may have offered modest opportunities to escape predators for most shell shapes in the most common size classes. Critical thresholds depend on size and speed: compared to inflated shells, slim shells likely allowed fast speeds at small sizes. But at larger sizes, or higher speeds, exposed whorls of previous shell growth along the evolute umbilicus of a serpenticonic ammonite would host extra turbulence and added mass, slowing the shell relative to a streamlined oxycone of the same size. Shell ornament may mitigate these effects: by reducing friction drag; reducing added mass; and by adding passive lift. Each impact, or their combination, may reduce the duty cycle required to travel at a target speed. Finally, we present measurements of hydrostatic properties of key morphotypes, based on physical experiments in water tanks. Models are printed in a variety of materials and configurations to balance mass and buoyancy, and poised to test several features of stability. As we combine these methods to specific fossil puzzles, we generally agree with Mark Twain’s cheeky assessment: for all their abundance, many ammonites were “disappointments... there would be no end of failures again, no end of extinctions.”