Evolution and extinction in a supercontinental world: did the breakup of Rodinia provide metazoans with evolutionary salvation?

A time discrepancy of at least 200–300 million years exists between the generally accepted onset of metazoan evolution as currently evidenced from fossils compared with that from studies at the molecular level. That temporal disparity coincides with the existence and subsequent breakup of the Rodinia Supercontinent when the earliest evolutionary crucibles were isolated intracratonic basins rather than the globally connected shallow seaways of subsequent eras. However, the discovery of fossil evidence to complement the extrapolations from molecular data has been, and continues to be, hampered, by both the geographical limitations of these isolated intracratonic basins and the fact that any such discovery would conflict with the global geological mindset that macrofossils of such age do not exist. Yet recently identified within the intracratonic Amadeus Basin of central Australia is a suite of macroscopic metazoan fossils that dates to ca 850–840 Ma, exemplifying an early attempt at animal evolution within the restrictions of the Rodinia Supercontinent. Such early metazoan evolution within such isolated crucibles, however, was extremely vulnerable to variations in climatic conditions, which threatened total extinction because the isolation of these basins limited metazoan distribution by denying them the possibility of migrating to safer havens. The Amadeus Basin evidence suggests that early metazoan evolution perhaps comprised cycles of about 10 million years when evolution flourished followed by climate change induced extinction and a subsequent evolutionary void of between 50 and 100 million years. Such cycles possibly characterised metazoan evolution until the Rodinia Supercontinent broke-up, after which connections between smaller oceans, shallow seas and narrow waterways provided metazoan life passageway to sanctuary during times of environmental stress. As the Amadeus Basin was just one of a number of isolated Rodinian-aged intracratonic basins, it is likely that similar fossils of the earliest metazoans are preserved elsewhere awaiting discovery. KEY POINTS: Metazoan evolution is widely believed to have followed the Gaskiers Glaciation at ca 580 Ma, yet studies at the molecular level suggest evolution began 200–300 million years earlier. Fossils discovered in the Amadeus Basin of central Australia suggest that a diverse metazoan biota had evolved by 850 Ma. Before the Rodinia Supercontinent fragmented during this temporal disparity, intracratonic basins acted as crucibles for early metazoan evolution. Being isolated, intracratonic basins were vulnerable to periodic environmental change causing metazoans, with no means of escape, to periodically become extinct. Amadeus Basin fossils suggest that pre-Ediacaran metazoan evolution possibly comprised cycles of ca 10 million years when evolution flourished before extinction and a following evolutionary void of between 50 and 100 million years. [ABSTRACT FROM AUTHOR]