AMMONITES WERE SUCCESSFUL SLACKERS AFTER THE TRIASSIC/JURASSIC MASS EXTINCTION

Ammonites bounced back from the end-Triassic mass extinction thanks largely to the extremely abundant and cosmopolitan species that produced distinctive serpenticonic shells throughout the Early Jurassic. New flume experiments and synthetic computational results evaluate the hydrodynamic efficiency of a range of ammonoid shell shapes and ornament styles. We test 3D printed models of synthetic, ideal, and replica ammonite shells in a 1-meter flume at speeds of 5-20 cm/s, with lower speeds tested in simulations using ANSYS Fluent. Unsurprisingly, the results show that the simple coiled-snake shape of serpenticonic shells is not optimized for swimming efficiency. Rather than swimming forcefully against ocean currents, these widespread cephalopod species probably used jet propulsion for punctuated activity such as dodging predators. Synthetic models of shell growth across ammonoid shell shapes show that the serpenticones did a decent job of protecting the body within. For a given amount of biomineralized growth, the shells gained greater overall length than more inflated forms, and greater volume than more compressed forms. Early Jurassic serpenticones also frequently feature ribs, which have dynamic influence on drag and lift. We suggest these ribs actually aided ammonites in prey capture by providing greater stability during an abrupt extension of the soft body. Combining experimental and synthetic methods, using real and idealized fossil shells, allows first order interpretations of the ecological complexity within post-extinction Mesozoic oceans. These experiments also provide context for comparison of ammonoid shells in a hydrodynamically-relevant morphospace (Ritterbush and Bottjer, 2012). Despite their high taxonomic diversity, it is possible that the behavior repertoire and ecological tiering of Early Jurassic ammonites remained limited.