Coordinated stasis or coordinated turnover? Exploring intrinsic vs. extrinsic controls on pattern

A number of potential mechanisms have been suggested to explain patterns of coordinated stasis in the fossil record. The major dichotomy of explanation lies between mechanisms emphasizing the maintainance of stability and those emphasizing the patterning role of turnover events. A classification scheme of causes is presented here that breaks the pattern into its two aspects: (1) long term evolutionary and ecological stability and (2) rapid synchronous faunal turnover. Within each, causal mechanisms arising from extrinsic (physical, environmental) and intrinsic (evolutionary or ecological) sources are discussed and their ability to produce different aspects of the coordinated stasis pattern are examined. While exploration of potential causal forces has only just begun, two potential scenarios emerge from this analysis as most promising. Both acknowledge the primacy of the physical environment in driving turnover in some way (extrinsic control), but differ in their explanations for stasis. In one alternative, environmental selection of taxa with similar tolerances leads to persistent associations that track preferred habitats in a 'stable' environment until a large disturbance initiates collapse and re-establishment; this mechanism emphasizes extrinsic causes for stasis (or, more aptly put, the lack of extrinsic impetus for change). In the second scenario, internal organization of ecosystems provides resilience to disturbance, allowing persistence of faunal associations and tracking despite a fluctuating environment until a threshold of environmental change is reached, causing collapse and re- establishment; this mechanism emphasizes intrinsic causes for stability. Any explanation, however, must incorporate some intrinsic concept of stability (e.g., incumbency, ecological locking) if the apparent non-invasability of communities is to be accounted for. The concentration of most ecological and evolutionary change (at least on a regional scale) into short discrete bursts suggests either (1) that the extrinsic mechanisms driving turnover operate episodically, or (2) that the environment is changing gradually and continually, but the response of faunas to such change is non-linear. Episodicity is a (necessary?) correlate of the first explanation for stability presented above, but is consistent with both scenarios; gradual change and non-linearity of response suggests that the second explanation (intrinsic stability) is more plausible. Future hypothesis-testing strategies to discriminate between categories of cause for both stasis and turnover should therefore not only include careful examinations of the ecological and evolutionary properties of faunas through time, but also a detailed and quantitative assessment of environmental change in relation to faunal change during the interval in question.