Spatial competition among clonal organisms in extant and selected Paleozoic reef communities

Occurrences of densely packed benthic organisms in extant reefs are of two types: 1) live-live interactions, where two living organisms interact, and 2) live-dead associations, where only one is alive and uses the other as a substrate. The latter are common in reef deposits due to biostratinomic fee dback, i.e. dense skeletal accumulations provide hard substrates for clonal recruitment, thus facilitating greater frequency of live-dead encounters t han in lower biomass level-bottom communities dominated by solitary organis ms. Differentiating between these two types in ancient reefs is difficult, often impossible. Most live-live interactions among clones in extant reef communities involve competition for space. Clonal spatial competition is divisible into four t ypes: 1) direct-aggressive: encrusting overgrowth; 2) indirect-passive: dep riving neighbors of resources, chiefly sunlight, by growth above them; 3) s tand-off: avoidance of competition by organisms adopting positions that avo id or minimize direct polyp/zooid contact; and 4) overwhelming: one clone/ species volumetrically or numerically overwhelms the other, meeting minimal resistance. Despite class-order level differences in taxa, our results ind icate that extant analogs, based on the arrangement and distortion of skele tons, are valuable for recognizing live-live interactions in Silurian and C arboniferous reefs and interpreting the types of spatial competition repres ented. Comparison of overhead (plan) views of live-live coral competition in Polyn esian reefs with vertical sections of Silurian and Carboniferous sponge-dom inated reefs and biostromes suggests that direct-aggressive competition is more common among extant than among Paleozoic reef-builders. Stand-offs sho wing clone margin distortion and overwhelming with minor skeletal distortio n are most common in our fossil examples and probably relate to the dominan ce of these reefs by sponges. Success by extant sponges in spatial competit ion is largely due to allelochemical deterrence which may explain the predo minance of stand-off and overwhelming confrontations in fossil sponges rath er than tentacle-mesentery based direct aggression among extant corals and bryozoans.