Factors That May Control Faunal Stasis Over Paleoecological Timescales

The fundamental differences seen in these two patterns may be related to a number of factors. For example, the Silurian-Devonian P3 EEU examined by Brett and Baird (1995) is characterized by a relatively stable level of familial diversity, whereas faunal diversity increased rapidly during the Jurassic-Cretaceous M2 EEU (Sheehan 1996). In addition, the Jurassic is a time of rapid escalation, infaunalization, and ecological change (Vermeij 1977). This background of great global diversification and global change may have affected patterns of stability, although it is unclear what this relationship may have been.

Another difference of the geological periods is that there is a significant statistical difference between faunal constituents of the Paleozoic Fauna and the Modern Fauna (Sepkoski 1981). The Silurian-Devonian faunal system is dominated by articulate brachiopods, whereas the Jurassic fauna is heavily dominated by bivalves. The differences in faunal components may play a role in affecting patterns and strength of stability. For example, bivalves have been shown to possess relatively long species durations (Stanley 1979). If one takes this into account, it may be expected that some of the intervals of stability exhibited by Jurassic western interior bivalve-dominated assemblages are longer than those seen in the Silurian-Devonian Appalachian Basin. In modern neoecological studies, it has been acknowledged that long-lived individuals must be accounted for in evaluating stability (Connell and Sousa 1983). At paleoecological scales, Tang and Bottjer (1996) proposed that it is just as important to account for long-lived taxa in explaining patterns of stability in the fossil record.

Related to this hypothesis regarding the importance of intrinsic species durations in determining stability, Westrop (1996) has suggested that Upper Cambrian trilobites do not exhibit coordinated stasis because they had inherently higher turnover (speciation and extinction) rates than the brachiopod-dominated assemblages described by Brett and Baird (1995). If inherent turnover rates control stasis within lineages, long-term stability would be more likely to occur in post-Paleozoic systems as the level of constraint increased through time. Tang and Bottjer's (1996) results do indicate that the overall levels of stasis in some Jurassic species and paleocommunities may be higher than in the Silurian-Devonian (i.e., longer intervals of stasis). However, the higher level of stability did not lead to punctuated turnover events associated with coordinated stasis. Thus, lower turnover rates in individual lineages does not necessarily lead to a pattern of coordinated stasis (sensu stricto).

Another factor that controls species durations and thus could potentially control patterns of faunal stasis is the ecological nature of the species themselves. For example, as discussed earlier, one would intuitively suspect that dynamically robust taxa could have longer species durations simply because they can withstand larger environmental fluctuations. Eldredge and Cracraft (1980:304) have hypothesized that eurytopic species (which have "relative breadth of tolerance to specifiable parameters of the physical and biotic environment") have low extinction rates, react to interspecific competition by mutual exclusion, and occur over wide geographic ranges. In studies of Ceno-zoic mammals (Vrba 1987) and Paleozoic crinoids (Kammer, Baumiller, and Ausich 1997), eurytopic taxa have been shown to have longer durations than stenotopes.

In the case of the Jurassic faunas, species and paleocommunities can often be found in a number of different paleoenvironments, which indicates that they are eurytopic (Tang 1996). In addition, paleocommunities have low levels of alpha and beta diversity, which suggests low levels of both habitat and niche and resource specialization (Tang 1996). The generalist nature of the Jurassic fauna supports the idea that this system may have been predisposed to exhibiting high levels of stasis simply by containing long-lived taxa. It would be worthwhile to examine whether generalist taxa also dominate the most stable Silurian-Devonian Appalachian biofacies.

Last, if one looks at the specific environmental context for stable paleocommunities in both systems, one can see that there in fact may not be major discrepancies between the Paleozoic and Mesozoic examples of long-term faunal stability. Brett and Baird (1995) have shown that there appear to be varying levels of stability exhibited in different environments within the Appalachian Basin; for example, nearshore siliciclastic systems with lower species diversities than carbonate shelf environments exhibited higher levels of stability. Bretsky (1968,1969) also documented a pattern of long-term stability in low-diversity, nearshore Upper Ordovician communities of the central Appalachians, whereas more rapid compositional changes occurred in offshore environments. Thus, it is possible that the pattern of long-term stasis without synchronous turnover seen in the Jurassic shallow epicontinental seaway are comparable to those seen in corresponding shallow-water environments of the Paleozoic and that the lack of deeper, offshore environments in the Jurassic North American seaway is responsible for the different patterns. Although more study is needed to confirm this observation, an environment-specific analysis could do much to explain the dynamics of faunal stasis and answer some of the fundamental ecological questions about community stability.

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