EVOLUTION OR REVOLUTION OF AMMONOIDS AT MESOZOIC SYSTEM BOUNDARIES

Abstract

Summary1. Biological revolutions at major stratigraphical boundaries have been given numerous explanations involving endogenous biological, exogenous ecological, physical, and cosmic, as well as sedimentary or chemical factors. In an attempt to elucidate the true nature of these faunal revolutions and to assess the possible influence of biological and/or physical factors, the evolution of ammonites at the boundaries of Mesozoic stratigraphical Systems is reviewed. It is believed that the more detailed data now available can give a clearer impression of evolutionary events at these boundaries.2. It can be demonstrated that there is neither an abrupt and world‐wide extinction, nor a spontaneous replacement by new elements at these caesuras as had been generally supposed to have occurred at the Triassic‐Jurassic boundary, for example. Instead, one can recognize three distinct phases in the sequence of events: (1) a continuous disappearance of the ‘antique’ faunal elements; (2) a similarly continuous, gradual, and largely synchronous appearance of, or substitution by, qualitatively distinguishable ‘modern’ elements in small populations, yet in various parallel lineages (mosaic evolution); (3) a quite revolutionary, and quantitatively very sudden, diversification of these new elements, occurring at or with some delay above the boundary.3. Thus one can demonstrate both continuous evolution of the modern faunas (‘preadaptational phase’), as well as ‘discontinuous’ spontaneous revolution, which does not produce qualitatively new characters and must be explained by diversification or adaptive radiation. This means that no further explanation by internal factors or by higher mutation rates resulting from the impact of cosmic rays becomes necessary. It is believed that, preceded by high extinction rates, world‐wide ecological factors promoting higher niche diversity suffice to explain these adaptive radiations. The high degree of provincialism, endemism and specialization of the ‘antique’ faunas and the constant survival of smooth oxycones — regarded as inhabitants of a deep‐sea environment — demonstrate that marine regressions and transgressions were the most effective ecological factors.4. If there is not too much time involved between the two events, the caesura (Faunenschnitt) between final extinction of the old faunas and the radiation of the new is the most appropriate point by which to define System boundaries.