LONG-TERM TRENDS IN NAUTILID MORPHOSPACE AND THE ROLE OF MASS EXTINCTIONS

The morphology of the order Nautilida is conservative compared to that of Ammonoidea; however, long term trends and the effects of mass extinction events are poorly constrained. In particular, the Cretaceous-Paleogene extinction of ammonoids may have been enabled nautilids to occupy niches previously filled by ammonoids. To assess the role of extinctions on nautilid shape evolution, we quantified shell geometry, aperture shape, and suture complexity of Carboniferous-Recent nautilids and Devonian-Cretaceous ammonoids. We used principal components analysis of traditional measurements (whorl width, height, and umbilical diameter) to quantify shell geometry and elliptical fourier analysis to describe aperture shape. Suture complexity was measured with the suture index, the ratio of suture perimeter to linear distance. Results indicate that nautilid shell geometry, aperture shape, and suture complexity were not affected by the Permian-Triassic extinction, yet the Triassic-Jurassic extinction had a pronounced effect on these parameters. Average suture complexity increased immediately after the Cretaceous-Paleogene extinction, but there was only a subtle shift towards shells with more compressed whorls. Furthermore, Paleogene and latest Cretaceous nautilid species occupied the same total range of morphospace. Species that have both aperture shape and suture complexity data show a clear trend toward more compressed whorl shape in species with higher suture complexity.