Iterative evolution of Middle Jurassic ammonite faunas

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Abstract
The phenomenon of iterative evolution of homeomorphs in identical temporal sequences is exhibited by Aalenian and Bajocian ammonite faunas in the south German depositional basin. Within each evolutionary cycle, inflated, evolute and ornamented shells with complex suture lines are successively replaced with discoidal, involute, smooth shells with simpler sutures. Three distinct and identical cycles of morphological change occur and involve ammonites belonging to 4 different familial/subfamilial groups: the Leioceratinae, Graphoceratinae, Hammatoceratidae and Sonniniidae, Geometric and statistical analyses of morphological change within each of these higher taxa reveal remarkable iteration in both the pattern and proportion of change with time. The iterative morphological cycles were not driven by orthogenesis or anagenetic transformation within lineages. A clear correlation exists between the successive ammonite, faunas and repeated cyclical changes in the physical marine environment of the depositional basin.

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