Abstract Extensive subsurface sampling of the Huqf Supergroup in the Sultanate of Oman has yielded microfossil assemblages of Cryogenian, Ediacaran and Early Cambrian age. Microfossils have been recovered from most stratigraphic units in the Huqf, including Marinoan-equivalent horizons of the Ghadir Manqil Formation (Cryogenian Abu Mahara Group), the Masirah Bay, Shuram and Buah formations of the Ediacaran Nafun Group, and the A3 (latest Ediacaran) and A6 (Early Cambrian) cycles of the Ara Group. Despite the extensive recovery of leiosphaerid acritarchs from the Shuram Formation, there is no indication of the large acanthomorphs typical of other early–middle Ediacaran assemblages. This absence suggests a relatively young (post-extinction) depositional age for the Shuram; however, the signal is complicated by local deep-water conditions and the facies-specific distribution of Proterozoic microfossils. A shallower-water sequence of undivided Nafun Group sediments preserves sphaeromorphic acritarchs in association with filamentous microfossils, fragmentary vendotaenids and possible vaucheriacean algae.

Abstract Ecological and evolutionary principles are often context-dependent, particularly where the context is biologically defined. Organ-grade animals (eumetazoans) are particularly powerful contextual agents, with a unique capacity to drive escalatory co-evolution and build multi-tiered food-webs. The evolution of eumetazoans through the Ediacaran and early Cambrian fundamentally altered macroecological and macroevolutionary dynamics, including the structure and function of the marine carbon cycle. Pelagic eumetazoans can be held responsible for driving the evolution of relatively large eukaryotic phytoplankton, thereby shifting the system from a turbid, stratified, cyanobacteria-dominated stable state to the clear-water, well-oxygenated, algae-dominated condition typical of the Phanerozoic. Intermittent return to the pre-Ediacaran state during Phanerozoic extinctions and oceanic anoxic events suggests that the widespread anoxia detected in pre-Ediacaran deep-marine sequences may be a consequence of this alternate biological pump rather than a reflection of fundamentally lower levels of atmospheric oxygen. The transition between the pre- and post-Ediacaran states is also associated with the oldest commercially exploitable hydrocarbons, a possible by-product of invading animals and their top-down impact on the biological pump.