MORPHOLOGY AND PALEOECOLOGY OF A PRIMITIVE MOUND-FORMING TUBICOLOUS POLYCHAETE FROM THE ORDOVICIAN OF THE OTTAWA VALLEY, CANADA

Build-ups of the calcareous tube, Tymbochoos (gen. nov.) sinclairi (Ok ulitch), occur in mid Ordovician limestones of the Ottawa Valley; the oldest previously known build-ups of calcareous tubes are Devonian. Th e Tymbochoos build-ups occurred as elongate dune-shaped structures in tidal channels on intertidal flats, and as small isolated pillow-shape d structures on near-shore subtidal shoals. Clustered tubes radiated h orizontally from small attachment areas and then grew vertically. Indi vidual tubes widened quickly to a diameter of about I mm and then grew cylindrically; irregularly spaced concentric constrictions of the tub e wall developed in places into thin anteriorly directed internal coll ars. Growth forms include (1) a framework of concentrated clumps of de nsely packed, long vertical tubes, found only in the dune-shaped struc tures, (2) a sparser concentration of clumps with more loosely packed shorter vertical tubes in the pillow-shaped structures, and (3) scatte red tube aggregates. T. sinclairi was probably a primitive suspension- feeding polychaete that fed with short tentacles and was supported by its setae-bearing parapodia against the irregularities of the inner tu be surface. Few other species were associated with these Ordovician tu be build-ups; exploitation of the intertube environment probably began at the end of the Cretaceous.