Kosmoceras: Evolutionary Jumps and Sedimentary Breaks

Abstract

Brinkmann (1929) argued that phyletic evolution in the Jurassic ammonite Kosmoceras is sufficiently linear that the amount of time missing at diastems can be estimated by the magnitude of morphologic jumps at those horizons. Brinkmann's unpublished raw data have been re-analyzed in order to evaluate the association between evolutionary jumps and physical breaks. All horizons in Brinkmann's 1400 centimeter stratigraphic section have been tested for morphological jumps. Up to 40 statistically significant morphological jumps exist in the section, the actual number depending on the sample interval used. A few of these correspond to the jumps found by Brinkmann and a few coincide with diastems. However, the correspondence between morphological jumps and physical breaks is not as consistent as was thought by Brinkmann. Furthermore, it is argued that the association between physical and biological discontinuities is not dependent on linear evolution but rather is the expected condition wherever phyletic evolution occurs. Due to the irregularity and unpredictability of evolutionary change in Kosmoceras, morphologic jumps cannot be used to assess the amount of time missing at diastems.