2010 Annual Science Report
Massachusetts Institute of Technology Reporting | SEP 2009 – AUG 2010
Paleoecology of the Mistaken Point Biota
The Ediacara biota represent an enigmatic group of soft-bodied organisms that flourished in the late Precambrian oceans some 578-542 million years ago. These exquisite fossils have a worldwide distribution, however some of the most remarkable specimens are found all along the southeastern coast of Newfoundland, Canada, known worldwide as the Mistaken Point Ecological Reserve. Here, Ediacaran fossils are preserved as complete census populations allowing scientists to investigate these ancestral organisms using modern ecological analyses. Our work used ecological and morphometric studies to investigate the likelihood that some of these enigmatic organisms actually represent some of the oldest examples of sponges. Furthermore, our work highlights the necessity of a non-uniformitarian mechanism to deliver bio-accessible (labile) dissolved organic carbon to the Mistaken Point seafloor, which formed the primary food source for these deep-water organisms.
The Ediacara biota from the Mistaken Point Ecological Reserve along the southeastern coast of the Avalon Peninsula of Newfoundland, Canada, represents the oldest examples of large and morphologically complex organisms in the fossil record. In addition, Mistaken Point is important in Ediacaran studies because it represents a deep-water slope/basin environment well below storm wave base and the photic zone. As such, the Ediacara biota at Mistaken Point could not have relied on sunlight as a source of energy, and therefore must have gathered nutrients through either filter-feeding or osmosis of dissolved organic nutrients. The biological affinities of the Ediacara biota at Mistaken Point is a topic of considerable debate, due in part to the unique modular and fractal (self-repeating) morphologies exhibited by the overwhelming majority of the fossils at Mistaken Point. This architectural design is unknown from any organisms following the Cambrian explosion of animal life. In contrast, a series of locally-dominant cone-shaped fossils called Thectardis (Fig. 1) have been demonstrated by our group to represent early sponges, thus representing some of the oldest true metazoans known. Our assignment is based on two independent lines of evidence. First, the dominance of Thectardis in a deep-water setting requires that it was heterotrophic and therefore feeding on particulate or dissolved organic matter. Second, in order for a sponge water canal system to work properly, a conical sponge such as Thectardis would require a length to width ratio greater than ~1.6 to ensure that the inhalant surface area is equal to or greater than that of the osculum, thus causing the exhaled water to be forced away from the sponge in order to avoid recycling. This requirement is effectively, and often comfortably, met by Thectardis. As sponge fossils from the Ediacaran are astonishingly rare, this find helps explain the conundrum in which molecular clocks and the organic geochemical record independently predict an abundant and relatively derived population of sponges in the Cryogenian and Ediacaran despite an almost complete absence of such fossils from the rock record.