Canopy flow analysis reveals the advantage of size in the oldest communities of multicellular eukaryotes.
At Mistaken Point, Newfoundland, Canada, rangeomorph "fronds" dominate the earliest (579-565 million years ago) fossil communities of large (0.1 to 2 m height) multicellular benthic eukaryotes. They lived in low-flow environments, fueled by uptake [1-3] of dissolved reactants (osmotrophy). However, prokaryotes are effective osmotrophs, and the advantage of taller eukaryotic osmotrophs in this deep-water community context has not been addressed. We reconstructed flow-velocity profiles and vertical mixing using canopy flow models appropriate to the densities of the observed communities. Further modeling of processes at organismal surfaces documents increasing uptake with height in the community as a function of thinning of the diffusive boundary layer with increased velocity. The velocity profile, produced by canopy flow in the community, generates this advantage of upward growth. Alternative models of upward growth advantage based on redox/resource gradients fail, given the efficiency of vertical mixing. In benthic communities of osmotrophs of sufficient density, access to flow in low-flow settings provides an advantage to taller architecture, providing a selectional driver for communities of tall eukaryotes in contexts where phototropism cannot contribute to upward growth. These Ediacaran deep-sea fossils were preserved during the increasing oxygenation prior to the Cambrian radiation of animals and likely represent an important phase in the ecological and evolutionary transition to more complex eukaryotic forms.
School of Civil, Environmental and Mining Engineering, University of Western Australia, Crawley, WA 6009, Australia.
Marco
M
Ghisalberti
Department of Ecology and Evolutionary Biology, University of California, Los Angeles, Los Angeles, CA 90095, USA.
David A
DA
Gold
Department of Chemical and Physical Sciences, University of Toronto Mississauga, Mississauga, ON L5L 1C6, Canada.
Marc
M
Laflamme
Department of Earth and Planetary Sciences, University of California, Santa Cruz, Santa Cruz, CA 95064, USA.
Matthew E
ME
Clapham
Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, ON K7L 3N6, Canada.
Guy M
GM
Narbonne
Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA.
Roger E
RE
Summons
Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA.
David T
DT
Johnston
Department of Ecology and Evolutionary Biology, University of California, Los Angeles, Los Angeles, CA 90095, USA; Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90095, USA. Electronic address: djacobs@ucla.edu.
David K
DK
Jacobs
D005075:Biological Evolution; D056890:Eukaryota / Q000502:physiology*