HOW 'COMPLEX' WERE EDIACARAN ECOSYSTEMS?

Ediacaran ecosystems are thought to have been relatively ‘simple’ compared to those of the Paleozoic, comprising few interspecific biotic interactions and few unique modes of nutrient acquisition. Establishing the feeding modes of Ediacaran organisms is thus critical to determining the complexity of Ediacaran ecosystems; however, inferring feeding strategies has long proven problematic because many Ediacarans are characterized by body plans that lack clear modern analogues. Here, we use computational fluid dynamics (CFD) to rigorously analyze the hydrodynamic behavior of the iconic Ediacaran organism Tribrachidium heraldicum, thereby testing between hypothesized feeding modes. We show that the external morphology of Tribrachidium passively directs water flow towards the apex of the organism, and generates low-velocity eddies above apical ‘pits’. These patterns of current flow support the interpretation of Tribrachidium as a passive suspension feeder, and thus provide evidence for suspension feeding ~10 million years before the Cambrian. We further test the hypothesis that Ediacaran communities were surprisingly complex by analyzing Ediacaran through early Cambrian benthic communities using rank abundance distributions (RADs), which reflect community complexity (i.e., the number and division of available resources, and the diversity of ecological guilds). We find that the proportion of ‘complex’ vs. ‘simple’ community RADs remains largely equivalent through the Ediacaran and Cambrian, with the exception of the latest Ediacaran (Nama assemblage), consistent with an end-Ediacaran extinction ('biotic replacement') scenario. Our combined results demonstrate that the Ediacara biota formed ecosystems in the latest Precambrian with equivalent complexity to those that characterized the Cambrian, and suggests hidden ecological diversity among Ediacaran problematica.