Ediacaran skeletal Metazoans: affinities, ecology and the role of oxygenation

Abstract

The evolution of the Metazoa is among the greatest success stories in Earth history. From modest origins, probably in the Cryogenian (~720 - 635 Ma), metazoans had acquired hard parts, and a vast range of life strategies and body plans by the middle Cambrian (around 520 Ma). This leaves a long delay between the origin of the Metazoa and their rise to ecological dominance.

A popular explanatory hypothesis for this delay is that atmospheric oxygen levels, low in the Proterozoic (< 0.001 % PAL), began to rise towards modern levels towards the end of the Neoproterozoic. Among the earliest known putative metazoans are Namacalathus, Namapoikia and Cloudina, calcified marine invertebrates abundant in the latest Ediacaran (~ 548-541 Ma) Nama Group, Namibia.

Although they were pioneers of metazoan biomineralisation, little is known of their affinities or palaeocology. The Nama Group, a well-characterised, relatively undeformed mixed carbonate and siliciclastic succession, provides a rare opportunity to investigate the palaeoecology of these important organisms in their environmental context.

New geochemical data from the Nama Group confirm the heterogeneity of Ediacaran redox conditions. These contextualise in situ fossil assemblages which reveal diverse ecological strategies among the calcified metazoans of the Nama Group, and offer constraints on their affinities. Based on its large size (< 1 m), modular body plan and internal structure of interlinked tubules, Namapoikia was a long-lived specialist and possible Poriferan. I show that Namapoikia colonised both lithified and living microbial substrates in oxic, mid-ramp reef crypts. By contrast, size and occurrence data show that Namacalathus was an environmental generalist, forming large, thick aggregations in persistently oxic, mid-ramp reef environments but opportunistically exploiting the transiently oxic, inner ramp setting.

Bilaterally symmetrical, asexual budding and a microlamellar skeletal ultrastructure suggest that Namacalathus may have been an early lophophorate, and had flexible growth depending on environmental setting, showing a cup diameter of 2 – 35 mm, and size distributions varying with substrate type, redox and water depth. In oxic mid-ramp reefs, Cloudina constructed large (> 20 m) reefs showing mutual attachment and consistent orientation in life position, making it the earliest known reef-building metazoan and suggesting that it was a passive suspension feeder. I further present food webs based on fossil assemblages from Ediacaran to Cambrian Stage 4 carbonate successions and evaluate their usefulness in tracking metazoan trophic diversification in the early Cambrian.

Ediacaran redox conditions were a major control on the ecologies of the earliest metazoans. A requirement for oxygen made persistently oxic conditions a prerequisite for complex and long-lived ecologies, while highly flexible life strategies were used to exploit changeable environments. Ediacaran metazoans represent a phylogenetic and ecological foreshadowing of the complexity of the Phanerozoic, but it was not until much later that the Metazoa would attain their evolutionary potential.International Centre for Carbonate ReservoirsDaniel Pidgeon Fund of the Geological Society of London