555 million-year-old fossils reveal early feeding strategies

An international team of researchers has shed light on the feeding mode of Pentaradial Arkarua, a strange, alien-like creature.

This creature inhabited tropical, shallow waters covering the Flinders Ranges of South Australia 555 million years ago according to the collaborative team involving South Australian Museum and Environment Institute’s Associate Professor Diego C. García-Bellido, several European institutions. Their discoveries were recently published in Nature's Scientific Reports.

The multi-celled organisms of the Ediacara Biota are the oldest complex life forms on our planet. In most cases they are very different from anything around today, or for that matter, around for the last 500 million years of animal evolution, making it difficult to put them in the appropriate branch of the tree of life.

[caption id="attachment_15505" align="alignnone" width="925"] Image: Fossil moulds (top) and 3D models (middle) of small and large Arkarua from the Flinders Ranges, and Cambrian echinoderm models used for comparison in the study (bottom)[/caption]

A/Prof. García-Bellido said "Similarly, their bizarre shapes do not easily lend themselves to comparison with the modes of life of modern marine organisms, but in this study, researchers used three-dimensional modelling and computational fluid dynamics to establish the feeding mode of Arkarua. The modelling measured the water flow over the body of Arkarua, compared with the slightly younger, Cambrian seafloor dwelling echinoderms (cousins of living sea urchins and sea stars)."

'Their analyses reveal that Arkarua was capable of affecting the flow patterns over the seafloor in a way that would allow organic particles to be directed towards the central depression and the associated grooves on its dorsal surface, supporting its interpretation as a passive suspension feeder. This strategy has been reported in a variety of modern invertebrates fixed to the seafloor, like bivalves, corals and crinoids."

Suspension feeding is an important ecological strategy in our oceans today, as it provides a transfer of energy from organisms swimming in the water column to those living on the seafloor. Establishing when suspension feeding first evolved has crucial implications in researchers understanding of the origins of the complex, modern biosphere in general, and marine food webs in particular.

[caption id="attachment_15504" align="alignnone" width="987"] Image: Computational fluid dynamics surface plots (horizontal and vertical cross-sections) at four different speeds over the small and large Arkarua models (top) and the 2 echinoderms (bottom). Arrows indicate direction of water flow and colours indicate speed.”[/caption]

Applying this type of modelling and computational fluid dynamics to other Ediacaran organisms, the group expects to reveal insights into the enigmatic multicellular organisms that first populated our planet. This new technique and further fossil discoveries in the Flinders Ranges will help visitors to the soon-to-be declared Ediacara-Nilpena National Park envision the diversity of forms and complexity of lifestyles thriving over half a billion years ago.

These items on display and can be visited at the South Australian Museum's Ediacara Gallery.

Article written byAssociate Professor Diego C. García-Bellido.