ECOLOGICAL COMPLEXITY AND ENVIRONMENTAL PREFERENCES IN EDIACARAN SKELETAL METAZOANS

Little is known of the paleoecology and environmental preferences of terminal Ediacaran skeletal metazoans, even though they grew in variable carbonate ramp settings with dynamic redox regimes.

Quantitative analysis of skeletal biota from the Nama Group, Namibia (~550-541 Ma) reveals considerable variability and community complexity. In-situ Namacalathus occupied inner, mid and outer ramp settings, but shows variation in maximum size and aggregation style. In outer ramp environments, Namacalathus formed small, likely short-lived aggregations of small individuals. But in mid-ramp settings with persistent oxygenation Namacalathus formed many associations including large reef aggregations, intergrowth with Cloudina and cryptic forms. Individuals in transiently oxic inner ramp settings grew to large sizes, but did not form complex aggregations but rather inferred short-lived communities. We conclude that Namacalathus was an ecological generalist, attaining a wide range of sizes and aggregation styles in persistently oxic environments, but also opportunistically colonising transiently oxic inner ramp and outer ramp settings.

Similarly, Cloudina is found in both persistently oxic and transiently oxic environments, though it has only been found to build reefs in oxic mid-ramp settings. However, not all Ediacaran skeletal metazoans could colonise opportunistically. Namapoikia, a relatively persistent, meter-scale metazoan appears to be restricted to cryptic niches within long-lived mid-ramp reefs.

The environmental preferences of Ediacaran skeletal metazoans varied considerably, with environmental redox conditions, substrate type and energetic regime controlling distribution. Oxygenation was a significant control on ecological development, with persistently oxic conditions being required for the development of more complex and persistent metazoan ecologies.