Changes in Diversity

In the Famennian, the number and diversity of ammonoids (both goniatitids and clymeniids) in all sections studied in the Uralian-Kazakhstanian Region first increased during the transition from the pseudogoniatites to the delphinus to the annulata Zones, but then gradually decreased until clymeniids, the dominant group, totally disappeared in the Late Famennian. All the Middle-Late Famennian ammonoid assemblages in this region are dominated by clymeniids. The diversities of the goniatitids and clymeniids evolved differently in the Middle Famennian, but became more similar to each other toward the end of the Famennian (Fig. 15.6).

The total number of goniatitid genera in the Uralian Ocean sections halved from the end of the Middle Famennian to the end of the Famennian, while the number of clymeniid genera decreased by a third. However, the proportions of clymeniids and goniatitids in ammonoid assemblages in the Uralian Ocean did not change much during the transition from the Middle to the Late Famennian. Both groups showed a considerable decline. No indications of sudden extinction events are apparent.

At the same time, in the Rhenish Massif in the latest Devonian all clymeniid lineages but one became extinct immediately at the base of the Hangenberg Black Shale; "the extinction event appears to have been very sudden and did not only affect species but also entire evolutionary lineages" (Korn et al., 2004: 311). At the end of the Famennian, all clymeniid families (except Cymaclymeniidae) experienced an extinction crisis.

Clymeniids

Goniatitids

Early Famennian Middle Famennian Late Famennian

Clymeniids

Goniatitids

Early Famennian Middle Famennian Late Famennian

Fig. 15.6 Diagrams showing changes in diversity of goniatitids and clymeniids. (A). Decrease in diversity of both groups from the Middle to Late Famennian in the sequences of the Uralian-Kazakhstanian Region. Note the sharp rise in the diversity of clymeniids shortly after their appearance in the Middle Famennian. (B). Decrease in diversity of goniatitids and increase in diversity of clymeniids in the Famennian in the sequences worldwide. Note the rise in the diversity of clymeniids just prior to the extinction of most clymeniid taxa in the Late Famennian.

Fig. 15.6 Diagrams showing changes in diversity of goniatitids and clymeniids. (A). Decrease in diversity of both groups from the Middle to Late Famennian in the sequences of the Uralian-Kazakhstanian Region. Note the sharp rise in the diversity of clymeniids shortly after their appearance in the Middle Famennian. (B). Decrease in diversity of goniatitids and increase in diversity of clymeniids in the Famennian in the sequences worldwide. Note the rise in the diversity of clymeniids just prior to the extinction of most clymeniid taxa in the Late Famennian.

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