Devonian bryozoan diversity, extinctions, and originations

Observed diversity of bryozoans within Devonian stages is not significantly different from range-through values for species because very few species have ranges longer than a single stage. Generic diversity differs significantly between observed stadial and stadial range-through values except for the diverse Givetian faunas. Among families, observed stadial and range-through comparisons differ significantly in the lower diversity Early and Late Devonian but are not significant in the taxonomically diverse Middle Devonian. The patterns of Devonian generic and familial diversity are apparently robust proxies for specific diversity. Givetian specific and generic extinctions, based on generated bootstrap distributions, are significantly higher than the other six Devonian stages even when modifications for different stage durations are considered. Based on present data, we conclude that a major change in Devonian diversity occurred between the Givetian and the Frasnian stages. Givetian specific and generic extinctions are not "smeared" with respect to adjacent stages and should qualify as a mass extinction among bryozoans pending more accurate data on bryozoan ranges and more precision in radiometric dating of Devonian stadial boundaries. Devonian bryozoan familial extinctions are not numerous and do not exhibit a significant number of extinctions for any stage. The Givetian extinction, which for the time being ranks as the largest bryozoan extinction in the Phanerozoic, is a global event with a marked local effect in the Hamilton-Tully fauna of New York, Pennsylvania, and Ontario. Givetian specific originations are significantly higher than bootstrap distributions for both raw data and data modified for stadial durations. A Givetian generic high in originations for raw data is not significant when modified for stadial durations and only Eifelian familial originations are significantly higher than bootstrapped distributions. THE PURPOSE of this report is threefold: 1) to characterize the differences between observed and range-through stadial diversity of the presently known worldwide Devonian bryozoan fauna; 2) to utilize a bootstrap procedure to examine the sig- nificance of stadial differences in specific, generic, and familial Devonian bryozoan diversity, extinctions, and originations; and 3) to discuss Givetian bryozoan extinctions relative to other Paleozoic bryozoan extinctions. Since the preparation of the manuscript of Horowitz and Pa- chut (1993), Hubbard and Gilinsky (1992), following earlier work by Gilinsky and Bambach (1986), have presented an ex- plicit bootstrap procedure for nonparametric testing of the sig- nificance of changes in stadial extinctions, and Fordham (1992) has presented a chronometric calibration of some mid-Ordo- vician to Early Mississippian stages. Consequently, we have applied these proposals, which were implicitly or explicitly ig- nored in Horowitz and Pachut (1993), to our updated Devonian bryozoan data in order to evaluate the significance of changes in stadial diversity, extinctions, and originations. Because higher taxa are commonly used as a proxy for species diversity, the Devonian bryozoan data provide a partial test of this assump- tion.