BECOMING A WINNER IN JUST A MILLION YEARS: EXAMINING THE FUNCTIONAL CONSEQUENCES OF AMMONOIDS ACROSS THE END-TRIASSIC
We consider the ammonoid response to end-Triassic mass extinction as a case study. Across this transition, ammonoids show a dramatic shift in conch morphospace occupancy. A Late Triassic wealth of shell shapes was abandoned by the paltry repertoire of the extinction-era ammonoids. For millions of years after the extinction, ammonoid clades produced great diversity and abundance, yet produced few conch shape varieties. Earliest Jurassic (Hettangian; Sinemurian) ammonoids chiefly grew serpenticonic (strongly coiled) or moderately oxyconic (wedge-like) morphs with low conch width. Moreover, many serpenticonic species reached large sizes. The apparent success of serpenticonic ammonites during the Early Jurassic has previously been troublesome to interpret; the simple conch shape should limit these animals to be poor swimmers relative to their oxyconic contemporaries. We present refined hydrodynamics simulations and trajectory analyses for ammonoids with conchs ranging from serpenticonic to oxyconic. Oxycones present distinct advantages in specific conditions – rapid acceleration at moderate sizes. Serpenticones appear to allow adequate performance across a much broader scope of sizes, musculature capacities, and jet propulsion tactics. Overall, we interpret the success of serpenticone ammonites in the Early Jurassic was fostered by this generalist capacity rather than a specific response or adaptation to extinction-related forces. This provides a straightforward explanation for the morphological shift as a consequence of the extinction without being a direct response.