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Following the collapse of the Early Cambrian archaeocyathan–calcimicrobial reef consortium, the Middle–Late Cambrian – Furongian was an interval dominated by purely microbial dendrolite and stromatolite reefs. However, among these latter, a few exceptional occurrences of metazoan reefs are known. One such reef complex occurs in the late Middle – early Late Cambrian – Furongian portion of the Mila Formation of northern Iran. In the otherwise low-energy interval of this formation, the anthaspidellid demosponge Rankenella hamdii sp. nov. is associated with encrusting Girvanella , eocrinoid plates, rhynchonelliformean brachiopod valves and subordinate hyoliths and trilobites in tempestite shell beds; these beds underwent synsedimentary cementation on the seafloor to form hardgrounds. In the succeeding, higher energy interval, a complex of metre-scale bioherms and (or) taphoherms incorporates toppled or transported Rankenella hamdii in association with brachiopods, echinoderm plates, trilobites and some red argillaceous lime mud. Among these, undoubted reefs were constructed from a framework of digitate Rankenella hamdii with thick Girvanella encrustations. These encrustations locally developed as subvertical columnar ministromatolites, which could also merge laterally to form more extensive masses. Subsequent pervasive cementation generated isopachous rinds that preserved the reef framework intact. Coeval and younger Cambrian anthaspidellid–calcimicrobial reefs are known from California–Nevada and Texas, USA. These were heralds of the Early Ordovician resurgence of metazoan reefs.

The transition from the terminal Ediacaran to early Cambrian (ca. 550−530 Ma) witnessed both the decline of Ediacaran-type soft-bodied and skeletal biota and the rapid diversification of Cambrian-type skeletal biota, which dominate the Terreneuvian (ca. 538.8−521 Ma) fossil record. This interval hosts globally widespread positive and negative δ13Ccarb excursions, including a negative δ13Ccarb excursion near the Ediacaran-Cambrian boundary termed the 1n/BACE. Efforts to produce a global composite chemostratigraphic and biostratigraphic correlation through this interval are complicated by stratigraphic incompleteness and a dearth of radiometric ages with which to constrain δ13Ccarb chemostratigraphy. Extensive and richly fossiliferous open-marine carbonates of the Siberian Platform were deposited from the terminal Ediacaran to beyond Cambrian Series 2, and they offer a unique archive to refine this chemostratigraphic and biostratigraphic framework. Here, we present new δ13Ccarb data from two sections of the southeastern Siberian Platform, and we synthesize these with published δ13Ccarb data from multiple sections throughout the Siberian Platform that record near-continuous carbonate deposition from the latest Ediacaran to Cambrian Series 2. This compilation allowed the construction of two possible chemostratigraphic age models that conform to a coherent framework of lithostratigraphic correlation and platformwide stratal stacking patterns. These age models were then used to test alternative calibrations of fossil first appearances and the spatiotemporal evolution of carbonate deposition on the Siberian Platform. Both models support a pre-1n/BACE appearance of anabaritids in the most distal open-marine sections, and they confirm a transitional Ediacaran-Cambrian biotic assemblage that consisted of co-occurring cloudinids and anabaritids. Sedimentologic and sequence stratigraphic analysis on the Siberian Platform also provides strong evidence to indicate that the 1n/BACE marks the onset of a gradual, pulsed rise in relative sea level that was sustained throughout the Terreneuvian and Series 2 of the Cambrian.