COORDINATED STASIS OR COORDINATED TURNOVER - EXPLORING INTRINSIC VS EXTRINSIC CONTROLS ON PATTERN

A number of potential mechanisms have been suggested to explain patter ns of coordinated stasis in the fossil record. The major dichotomy of explanation lies between mechanisms emphasizing the maintainance of st ability and those emphasizing the patterning role of turnover events. A classification scheme of causes is presented here that breaks the pa ttern into its two aspects: (1) long term evolutionary and ecological stability and (2) rapid synchronous faunal turnover. Within each, caus al mechanisms arising from extrinsic (physical, environmental) and int rinsic (evolutionary or ecological) sources are discussed and their ab ility to produce different aspects of the coordinated stasis pattern a re examined. While exploration of potential causal forces has only jus t begun, two potential scenarios emerge from this analysis as most pro mising. Both acknowledge the primacy of the physical environment in dr iving turnover in some way (extrinsic control), but differ in their ex planations for stasis. In one alternative, environmental selection of taxa with similar tolerances leads to persistent associations that tra ck preferred habitats in a ''stable'' environment until a large distur bance initiates collapse and re-establishment; this mechanism emphasiz es extrinsic causes for stasis (or, more aptly put, the lack of extrin sic impetus for change). In the second scenario, internal organization of ecosystems provides resilience to disturbance, allowing persistenc e of faunal associations and tracking despite a fluctuating environmen t until a threshold of environmental change is reached, causing collap se and re-establishment; this mechanism emphasizes intrinsic causes fo r stability. Any explanation, however, must incorporate some intrinsic concept of stability (e.g., incumbency, ecological locking) if the ap parent non-invasability of communities is to be accounted for. The con centration of most ecological and evolutionary change (at least on a r egional scale) into short discrete bursts suggests either (1) that the extrinsic mechanisms driving turnover operate episodically, or (2) th at the environment is changing gradually and continually, but the resp onse of faunas to such change is non-linear. Episodicity is a (necessa ry?) correlate of the first explanation for stability presented above, but is consistent with both scenarios; gradual change and non-lineari ty of response suggests that the second explanation (intrinsic stabili ty) is more plausible. Future hypothesis-testing strategies to discrim inate 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 an d quantitative assessment of environmental change in relation to fauna l change during the interval in question.