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Keywords:

  • adaptive radiation;
  • macroevolution;
  • ammonoids;
  • Jurassic

Abstract

Evolutionary radiations have been extensively studied especially in the fossil record and in the context of postcrisis recoveries. The concept of adaptive radiation that emerges from this very broad topic explicitly involves the effect of adaptation driven by ecological opportunity and is considered to be of the foremost importance. It is essential to be able to detect adaptive radiation because it points up factors that predispose a clade to radiate. Adaptive radiation has received much attention in recent decades based mostly on studies dealing with recent clades, but data from the fossil record are still scarce. This study begins to fill this gap with the example of Lower Jurassic ammonoids (through c. 8 Myr of history). A survey of several clades, using both taxonomic and disparity-based approaches, shows that they diversified successively through time, but not systematically, in terms of species numbers and morphological variety. Some clades seem to have exhibited adaptive radiation and to have become rapidly extinct. One clade (which engendered nearly all post–Lower Jurassic ammonoids) has a fossil record that begins with low diversity and disparity but is superseded by a sustained radiation pattern. The results are discussed in the light of the Modern Synthesis and its continuation into an Extended Evolutionary Synthesis.