RANGEOMORPHS, THECTARDIS, AND DISSOLVED ORGANIC CARBON IN THE EDIACARAN OCEAN

The mid-Ediacaran Mistaken Point biota of Newfoundland represents the first morphologically-complex organisms in the fossil record. At the classic Mistaken Point localities the biota is dominated by the enigmatic group of ‘fractally’ branching organisms called rangeomorphs. One of the few exceptions to the rangeomorph body plan is the fossil Thectardis avalonensis, which has been reconstructed as an upright, open cone with its apex in the sediment. No biological affinity has been suggested for this fossil, but here we show that its body plan is consistent with the hydrodynamics of the sponge water-canal system. Further, given the habitat of Thectardis beneath the photic zone, and the apparent absence of an archenteron, movement, or a fractally-designed body plan, we suggest that it is a sponge. Nonetheless, as Thectardis lacks spicules or other distinguishing features its place in sponge phylogeny cannot be determined with precision. The recognition of sponges in the Mistaken Point biota provides the earliest body fossil evidence for this group, which must have ranged through the Ediacaran based on biomarkers, molecular clocks, and their position on the metazoan tree of life, in spite of their sparse macroscopic fossil record. The paleoecology of the Mistaken Point biota was dominated by sponges and rangeomorphs, organisms that are either known or hypothesized to feed in large part on dissolved organic carbon (DOC). The biology of these two clades gives insight into the structure of the Ediacaran ocean, specifically the dynamics of the DOC pool, which must have differed considerably from that in the modern ocean. Modern deep-ocean DOC is extremely refractory and resistant to biological utilization. The presence of osmotrophic organisms in the Ediacaran deep ocean therefore indicates that a non-uniformitarian mechanism delivered labile DOC to the Mistaken Point seafloor.