Evolution of ammonoid morphospace during the Early Jurassic radiation

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Abstract
The morphologic radiation of Early Jurassic ammonites following the near extinction at the end of the Triassic is analyzed from 436 species of 156 genera that form a representative sample of morphs occurring worldwide in the first three stages of the Jurassic (Hettangian, Sinemurian, Pliensbachian: 36 subzones, 24 m.y.). Morphologic diversity is analyzed independently of taxonomy by processing 18 shape parameters using multivariate analysis and clustering techniques. The morphospace thus defined indicates that morphs fall readily into two groups made up of four and five adjacent morpho-subsets. The temporal pattern of morphospace occupation in the 36 Lower Jurassic subzones displays diversification, depletion (sometimes total), and displacement of successive parts of the morphospace, reflecting a complex history in which morphologic radiation appears to be more than a process of diffusion. The history of the morphologic evolution is tentatively related to sea-level changes and there is a suggestion that morphologic diversity increases during second-order transgressive periods.