Links between the rise of the metazoa and the decline of stromatolites

Through a review of the literature we have documented three major events in the history of stromatolites. Those stromatolites which formed in quiet, subtidal environments began to decline in both abundance and diversity about 1 Ga ago. This was followed by a general decline of all stromatolites beginning 0.7–0.8 Ga ago. Then during the Cambrian there appeared the first thrombolites, stromatolites with a clotted, unlaminated fabric. All three events can be related to the evolutionary history of the Metazoa, and in fact shed new light on that history. We consider that the thrombolites owe their origin to and record the first macroscopic burrowing and possibly boring by animals, an interpretation consistent with the well known first abundant occurrence of vertical burrows in the earliest Cambrian. The 0.7–0.8 Ga decline can be attributed to the first widespread grazing by animals (an interpretation made earlier by other authors and supported by our review of new information). The decline of subtidal stromatolites which began 1 Ga ago records the early, subtle effects of grazing, presumably by small acoelomate animals. While this is a speculative hypothesis it can be tested and it explains many features of the temporal and palaeoenvironmental distribution of stromatolites. Furthermore, it is consistent with the observed and predicted history of the Metazoa.