A NEW LOOK AT STROMATOLITE FORM DIVERSITY

Plotted through time, stromatolite form diversity shows an increase through the Paleoproterozoic, reaches a maximum in the Mesoproterozoic, and steadily declines to several taxa by the Precambrian-Cambrian boundary. This pattern has been noted by several authors with similar databases (e.g., Awramik, 1971; Walter and Heys, 1985; Semikhatov and Raaben, 1994, 1996; Awramik and Sprinkle, 1999.). The “decline” of stromatolites would appear to initiate ca. 1 Ga, and thus would preclude the advent of grazing metazoa as a cause for this decline. Using Awramik’s database (which does not include oncoids), we refined the time categories to reflect new precision in chronostratigraphy of the Proterozoic and Paleozoic. Then, we reanalyzed stromatolite form diversity with respect to their geographic distribution, volume of carbonate rock, and authorship.

While regional differences are to be expected, reanalysis indicates that one region imposes patterns not present in other regions, patterns that cannot be explained solely by differing geological histories (also noted by Semikhatov and Raaban). For instance, a mid Paleoproterozoic maximum in stromatolite form diversity ca. 2 Ga appears to be a basinal artifact: nearly 50% of all stromatolite forms in this interval come from one basin and one author. However, when these ‘regional’ stromatolites are removed, the steepest decline in forms occurs in the Early Cambrian. Furthermore, when the number of stromatolite forms in each interval is normalized against the volume of preserved carbonate rock (using rock-volume data from Ronov, 1982, as a first approximation), the decline in form diversity is also pushed forward in time, even on the unculled dataset. Our reanalysis demonstrates that the steepest decline in stromatolite form diversity occurs in the late Neoproterozoic, not ca. 1 Ga, and culminates in the Early Cambrian, coincident with the widespread appearance of macroscopic metazoa and significant bioturbation.