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Ammonoid (R) Evolution at the Permian-Triassic Boundary
Biologic revolutions at major stratigraphic boundaries, such as the Permian-Triassic, have been given numerous explanations involving endogenous biological, exogenous ecological, physical and cosmic, as well as sedimentary or chemical factors. In order to attempt to comprehend the true nature of these faunal revolutions and the possible influence of biological and/or physical factors, ammonite evolution at the Permian-Triassic boundary is reviewed and compared with that of other Mesozoic System boundaries. It is believed that the more detailed data now available give a clearer impression of evolutionary events at these boundaries.
In the present case it can be demonstrated that there is neither an abrupt and world-wide extinction, nor a spontaneous appearance of new elements at the boundary level, as was generally supposed. Instead, one can recognize three distinct phases in the sequence of events: 1) continuous disappearance of the “antique” fauna, 2) a similarly continuous, gradual, and largely synchronous appearance of, or substitution by, qualitatively distinguishable “modern” elements in small populations, and in geographically isolated parallel lineages, and 3) a quite revolutionary, and quantitatively very sudden, diversification of those new elements previously installed.
Thus one can demonstrate both continuous evolution of the modern faunas (“pre-adaptation”), as well as discontinuous spontaneous revolution, which does not produce qualitatively new characters and must be explained by diversification or true adaptive radiation. The latter phenomenon needs no further explanation by internal factors or by higher mutation rates resulting from the impact of cosmic rays. It is believed that, preceded by high extinction rates, world-wide ecological factors, such as the cyclicity of trans- and regressions, which promote higher niche diversity, suffice to explain these adaptive radiations.
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