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Morphologic and taxonomic history of Paleozoic ammonoids in time and morphospace

¹ W. B. Saunders and E. Greenfest-Allen. Department of Geology, Bryn Mawr College, Bryn Mawr, Pennsylvania 19010. wsaunder{at}brynmawr.edu, egallen{at}brynmawr.edu
² David M. Work. Maine State Museum, Augusta, Maine 04333. david.work{at}maine.gov
³ S. V. Nikolaeva. Paleontological Institute, Russian Academy of Sciences, 117997 Moscow, Russia. snikol{at}mtu-net.ru

Principal components analysis (PCA) of 21 shell parameters (geometry, sculpture, aperture shape, and suture complexity) in 597 L. Devonian to L. Triassic ammonoid genera (spanning 166 Myr) shows that eight basic morphotypes appeared within 20 Myr of the first appearance of ammonoids. With one exception, these morphotypes persisted throughout the Paleozoic, occurring in 75% of the 5-Myr time bins used in this study. Morphotypes were not exclusive to particular lineages. Their persistence was not just a product of phylogenetic constraints or longevity, and multiple iterations of the same morphotypes occurred at different times and in different groups. Although mass extinction events severely condensed the range of morphologic variation and taxonomic diversity, the effects were short lived and most extinct morphotypes were usually iterated within 5 Myr. The most important effect of mass extinctions on ammonoid evolutionary history seems to have been their role in large scale taxonomic turnovers; they effectively eliminated previously dominant orders at the Frasnian/Famennian (F/F) (Agoniatitida), the Devonian/Mississippian (D/M) (Clymeniida), and the Permian/Triassic (P/T) (Goniatitida and Prolecanitida) extinctions. Survivors varied from two (P/T) to four (D/M) and five genera (F/F). These events generated sharp reductions in morphologic disparity at the D/M (58%) and at the P/T (59%), but there was a net increase at the F/F (38%). There was no obvious survival bias for particular morphotypes, but 64% are interpreted to have been Nautilus-like nektobenthic. The recurrence of particular combinations of morphology and their strong independence of phylogeny are strong arguments for functional constraint. Intervals between mass extinctions seem to have been relatively static in terms of morphotype numbers, in contrast to numbers of genera. Significant decreases in genus diversity (54%) and morphologic disparity (33%) commenced in the mid-Permian (Wordian/ Capitanian boundary), well before the final P/T event.