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The effect of the Permo-Triassic bottleneck on Triassic ammonoid morphological evolution

¹ Alistair J. McGowan. Department of the Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637
² Present address: Department of Earth Sciences, University College, London, Gower Street, London WC1E 6BT, United Kingdom. alistair.mcgowan{at}ucl.ac.uk

Ammonoid taxonomic history is a well-documented series of diversity "booms and busts", but the effect of this taxonomic pattern on morphological evolution has not received as much attention. We know particularly little about the effects of the Permo-Triassic mass extinction on the subsequent morphological evolution of the ammonoids.

Morphological data from 322 Triassic ammonoid genera (322 species) were combined with previously published data for Late Paleozoic ammonoids. This data set of 601 specimens was subjected to PCA to assess (1) the effects of the Permo-Triassic taxonomic bottleneck on ammonoid morphological evolution, by comparing the strength and sign of correlations in the Paleozoic and in the Triassic, and (2) whether the Triassic ammonoids immediately shifted to Jurassic morphologies, retained Paleozoic morphologies, or evolved in a more "mosaic" fashion.

The Triassic ammonoids recapitulate the late Paleozoic W-D distribution, but in S-D space their distribution closely foreshadows that of the Lower Jurassic ammonites. Given these findings it is clear that at even the level of shell geometry the Triassic ammonoids evolved in a mosaic fashion

The Triassic ammonoids reoccupy, and extend, the volume of morphospace occupied by the Paleozoic ammonoids. Overall, Triassic correlations between pairs of characters are weaker, but not significantly so.

All seven of the generalized morphotypes were reinvented in less than 10 Myr. However, the relative frequencies of the morphotypes change, with a decline in benthic forms. Among Triassic ammonoids interpreted as benthic, much coarser ornamentation becomes prevalent.

The Permo-Triassic mass extinction may have left Ceratidida as the sole ammonoid lineage, but it did little to remove the constraints that were responsible for the limited range of basic ammonoid coiling geometry throughout their history.

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