A MORPHOMETRIC STUDY OF THE EFFECT OF DIVERSITY CRISES ON TRIASSIC AMMONOID EVOLUTION

The Triassic is a critical interval for the Ammonoidea. The period is bracketed by major extinction events with accompanying morphological bottlenecks, and the group suffers three serious diversity crises within the Triassic. The relationship between taxonomic and morphological changes during such events are poorly understood. I analyzed morphometric data on over 300 Triassic genera to investigate the relationship between taxic and morphological evolution.

The first set of analyses utilized the WDS theoretical morphospace of Raup (1966) to consider basic shell shape. A comparison of the Triassic genera with data from the Paleozoic (Saunders and Swan 1984) and Lower Jurassic (Dommergues et al. 1996) showed that the Triassic ammonoids do not gradually bridge the evolutionary gap between the Paleozoic and Lower Jurassic. Instead they exhibited a mosaic evolutionary pattern. The WD distribution returned to the Paleozoic distribution, while the DS distribution closely resembled that for the Lower Jurassic. The reoccupation of WD space is remarkable, as only one atypical Paleozoic lineage crosses the P-TR boundary.

A stage by stage analysis of the WDS data indicates that significant changes in mean morphology occurred between the Spathian and Anisian and the Carnian and Norian. In both cases the significant shifts in the mean morphology did not occur immediately after the extinction events, but took more time, stressing the importance of studying the recovery phase after biotic crises as well as the boundary interval.

For an expanded data set of 13 characters a Principal Components Analysis (PCA), where the first three PC axes explained ~ 45% of the variance, established that most of the morphotypes that were identified in the Paleozoic by Saunders and Swan (1984) re-evolved during the Triassic, but with shifts in the relative frequencies of occurrence. Temporal analysis of the PCA results indicated that a gradual occupation of the PC space took place, and that one of the major trends in Triassic ammonoid morphology, the appearance of strong ornamentation, occurred mostly in the Upper Triassic. This change cannot be related to a taxonomic diversity crisis.

These results indicate that the morphological evolution of the Triassic ammonoids is the result of more than simple selectivity driven by taxonomic diversity crises.