Substrate specificity in the Devonian tabulate coral Pleurodictyum

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Abstract
The tabulate coral P. americanum Roemer has been cited as an example of a host-specific organism occurring exclusively on the shells of gastropods, particularly Palaeozygopleura hamiltoniae (Hall). Examination of over 1600 specimens of P. americanum from the Middle Devonian Hamilton Group of western New York [USA] reveals additional complexities which require reinterpretation. While substrate selectivity for Palaeozygopleura shells is evident in all 42 subsamples, a variety of other substrates were also used by Pleurodictyum including corals, brachiopods, other mollusks and pebbles. Recent scleractinian corals inhabiting soft bottoms show similar substrate preference, selecting for the tubes of live serpulids, or gastropod shells (invariably with a secondary sipunculid host), but also occasionally settling on unoccupied shells or pebbles. Shell surfaces of Palaeozygopleura hamiltoniae, preserved as external molds on the Pleurodictyum epitheca, exhibit encrustation by worm tubes and bryozoans as well as borings and mechanical shell damage, suggesting that these were not the shells of live gastropods. The invariant aperture-downward orientation and the high degree of selectivity of P. americanum strongly suggest that the shells were occupied by secondary hosts.