CANOPY FLOW MODELS IDENTIFY A SCALING ADVANTAGE FOR LARGE SIZE IN THE MISTAKEN POINT RANGEOMORPHS, THE WORLDS OLDEST COMMUNITY OF MULTICELLULAR EUKARYOTES

Rangeomorph “fronds” at Mistaken Point lived in an aphotic low-flow environment fueled by “osmotrophic” uptake and oxidation of reduced reactants (such as organic carbon or sulfide). However, the advantage of rangeomorphs relative to competing osmotrophic prokaryotes has not been assessed. In a novel application, we use canopy flow models to reconstruct and parameterize flow in this community. Flow reconstructions are based on distribution and size of individuals on ash-fall preserved on event-bed surfaces as well as sedimentologic inferences of velocity. Three concepts of advantage provided by height off the substrate (conferred by multicellularity) are interrogated based on the flow modeling. The possible advantages of frond height include: 1) exposure to a mixed layer (composed of reductants from the bottom and oxidants from the water column) on the scale of the height of the community; 2) extension above a layer near the bottom depleted of a reactant-resource by the uptake activity of the community; 3) exposure to higher velocity which breaks down limits to uptake of reactants associated with the diffusive boundary layer at the surfaces of the organism. The modeling results strongly supports #3 to the exclusion of the other arguments. In this low-flow context access to “velocity” provides a critical advantage to longer forms extending into the water column. Thus the evolution of multicellular rangeomorphs appears to have been initiated as an adaptive response to diffusion limitation in a low-flow environment. Other groups that actively move fluid (such as sponges), are themselves motile, and/or ingest non-soluble resources then diversified forming Phanerozoic communities.