A NEW LOOK AT STROMATOLITE FORM DIVERSITY
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.