Iterative evolution of Middle Jurassic ammonite faunas

Authors

  • ULF BAYER,

    1. Institut für Geologie und Paläontologie der Universitat Tubingen, Sigwartstraβe 10, D-7400 Tübingen, F. R. Germany
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  • GEORGE R. McGHEE JR.

    1. Institut für Geologie und Paläontologie der Universität Tübingen, Sigwartstraβe 10, D-7400 Tübingen, F. R. Germany; present address: Department of Geological Sciences, Wright Geological Laboratory, Rutgers University, New Brunswick, New Jersey 08^03 U.S. A.; 4th February, 1983.
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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 four 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, as has been previously suggested. A clear correlation exists between the successive ammonite faunas and repeated cyclical changes in the physical marine environment of the depositional basin. D Ammonoidea, Aalenian, Bajocian, south Germany, iterative evolution.

Ancillary