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Recurrence of Morphologic Types and Evolutionary Cycles in Mesozoic Ammonites
Journal of Paleontology
Vol. 16, No. 5 (Sep., 1942), pp. 643-650
Published by: SEPM Society for Sedimentary Geology
Stable URL: http://www.jstor.org/stable/1298786
Page Count: 10
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Examples of striking resemblances, both in shape and in ornamentation, between forms, or even groups, of ammonites of quite different geological age have long been known. They are not, as Steinmann (1908, 1909) erroneously asserted, due to persistence of races, but are merely due to recurrence of morphologic types. In some cases even the same differentiation of certain types as to their whorl sections, degree of involution, character of costation, etc., recurs within groups separated by wide time intervals. However, not only types and groups of types reiterate themselves in the history of ammonites, but also certain evolutionary cycles, each proceeding along definite anagenetic trends, e.g. from evolute, sturdy forms with coarse and stiff costation to more involute and discoidal ones with a finer and more sigmoidal ribbing; also there is the same increase of the elaborateness of the suture lines in each of these cycles. Similar thoughts have been expressed, in a more general sense, or with regard to other groups, in recent papers of Schindewolf (1940: "iterative repetition of the same evolutionary course") and of Zoch (1940: "repetition of trends of forming forces").
Journal of Paleontology © 1942 SEPM Society for Sedimentary Geology