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The oldest Phanerozoic reefs were built in the Early Cambrian (Sauk I) by a consortium of calcimicrobes and archaeocyaths, with the calcimicrobes being the critical reef-building organisms. Archaeocyath-Kena/c/s reefs first occur at the base of the Tommotian Stage on the Siberian Platform; these are cavernous, framework pinnacle reefs about two meters in diameter, with constructor, baffler, and dweller guilds already present. Throughout the rest of the Early Cambrian, archaeocyath-calcimicrobe reefs spread around the world into a variety of low-latitude, mostly shallow-water settings, including shelf margin-oolite shoal environments and inner-shelf settings, often with considerable siliciclastic sediment present. Many Early Cambrian reefs are meter-scale in size and lenticular in shape, often with individual lenses (kalyptrae) stacked one upon the other to form a compound buildup. Coralomorphs and radiocyaths sometimes occur with archaeocyaths in these reefs. According to the Kiessling-Flügel database that accompanies this book, about one-third of Early Cambrian reefs are less than ten meters thick, and two-thirds are thicker than ten meters. Some were quite massive, ecologically zoned reefs with a rigid framework of branched archaeocyaths and encrusting calcimicrobes. Archaeocyaths reached a diversity of about 170 genera in the Botomian Stage, decreased precipitously in the Toyonian Stage, and became virtually extinct at the end of the Early Cambrian, thus ending the first episode of reef-building by metazoans. Bioerosion was apparently not as important a process in Early Cambrian reefs as in Mesozoic and Cenozoic reefs, although evidence of microboring is common.

During the Sauk II interval (Middle Cambrian through early Late Cambrian), reefs were common in subtidal and intertidal facies on low-latitude continents. These are microbialite reefs, which occur in three basic mesostrucrural types: dendrolites, stromatolites, and thrombolites. Many of the calcimicrobe taxa that are found in Sauk I reefs also occur in Sauk II reefs, most conspicuously in dendrolitic reefs, a distinctive type of Sauk II reef that is not known to occur in Sauk III. Stromatolitic reefs are common in Sauk II rocks, occurring in a wide range of morphologies. Thrombolitic Sauk II reefs are rare, but non-reefal columnar thrombolites are fairly common. The only known Sauk II reef with a conspicuous metazoan component is in Iran; it contains a framework of anthaspidellid-grade sponges encrusted by filamentous microbes. Echinoderm plates occur fairly commonly with reefs in this interval, especially eocrinoid columnals in the latest Sauk II reefs.

During the Sauk III interval (mid-Late Cambrian through earliest Ordovician) sea level was high and thrombolitic and stromatolitic reefs became very widespread. In some areas (e.g., the Great Basin) thrombohtes are the dominant reef type, while in others stromato Utes are more common. On most continents thrombolites tend to dominate in the earliest Ordovician. Calcimicrobes belonging to the Renalcis, Epiphyton, and Girvanella groups can often be identified in Sauk III reefs. Some endemism occurs among Sauk III stromatolite morphologies, which indicates the presence of geographically restricted microbial floras and/or unique environments. As in the Sauk II interval, one case is known of metazoan-built Sauk III reefs. The Sauk III metazoan reefs are in the Wilberns Formation of central Texas, in which anthaspidellid sponges constitute up to 25% of the reef boundstone. The most common invertebrates found in association with Sauk III microbialite reefs are eocrinoids, trilobites, hyoliths, and grazing molluscs. Toward the end of Sauk III, the diversity of reef-dwelling metazoans began to increase, leading up to the diversity explosion at the end of the Early Ordovician.

We examine six factors that may have influenced reef development during the Cambrian and earliest Ordovician: (1) biotic evolution, (2) plate motions, (3) eustatic sea-level change, (4) nutrient availability, (5) paleoclimate, and (6) technically forced shifts in seawater chemistry. All of these factors probably influenced reef building to some extent.

The most conspicuous change in reef building during the Cambrian and Early Ordovician was the collapse of the archaeocyath-calcimicrobe reef ecosystem at the end of the Early Cambrian, and the corresponding resurgence of microbialite reefs from the Middle Cambrian through the Early Ordovician. This anomalous interval of microbialite-dominated reefs, which lasted approximately forty million years, has not been satisfactorily explained. We explore seven hypotheses: (1) the post-extinction lag hypothesis, (2) the photosymbiosis recovery hypothesis, (3) the reduced grazing hypothesis, (4) the nutrient deficiency hypotheses, (5) the high levels of atmospheric CO2 hypothesis, (6) the global warming hypothesis, and (7) the Mg/Ca seawater chemistry hypothesis. We reject the first three hypotheses because they do not account for the range of phenomena associated with the microbialite resurgence. Some or all of the remaining four hypotheses probably operated synergistically, possibly with other processes not yet discovered, to inhibit metazoan reef building and promote microbialite reef building from the Middle Cambrian through the Early Ordovician.

We address the question of photosymbiosis in Early Cambrian reefs. Although photosymbiosis is very important in modern reefs, and presumably in Mesozoic and Cenozoic reefs, during the 1990s several papers were published in which it was concluded or inferred that photosymbiosis was probably not present in Early Cambrian and other Paleozoic reefs. This implies that the energetics of Cambrian reef communities were fundamentally different than those of Mesozoic and younger reefs. We find the arguments in support of this conclusion to be unpersuasive. Sponges are well known to form symbioses quite easily, and many modern reef-dwelling sponges harbor photosymbiotic cyanobacteria. As a preliminary test of the hypothesis that Early Cambrian reefs did not have photosymbiosis and therefore could not live in nutrient-poor waters, we use the Kiessling-Flügelreef database to compare depositional environments of Early Cambrian reefs with reefs of the mid-Early Cretaceous. There are no significant differences between the environments of the early Paleozoic and the late Mesozoic reefs, which suggests that Paleozoic reefs are not fundamentally different from Mesozoic reefs. This simple comparison does not settle the question of photosymbiosis in Paleozoic reefs, but it suggests a new approach using the Kiessling-Flügel database.

Other than the occasional building-stone quarry, Cambrian and Lower Ordovician reefs are not particularly important economically. They are not known to be significant hydrocarbon reservoirs, which is probably due to one or more of the following factors: a small amount of reef-derived debris, small size (in the case of many Sauk I reefs), and an abundance of microbialite (in the case of Sauk II and Sauk III reefs).

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