STOP CLINGING! –HOW THE ORDOVICIAN BRACHIOPOD (FKA PLATYSTROPHIA) VINLANDOSTROPHIA PONDEROSA OUTGREW ITS MID-LIFE ATTACHMENT CRISIS

The high-energy nearshore environment and muddy shifting shell gravels recorded in Maysvillian strata of the Cincinnati Ordovician might seem particularly inhospitable to brachiopods, which generally require solid surfaces for attachment. Nevertheless, Vinlandostrophia ponderosa thrived and even characterizes these facies.

A preliminary study of growth series suggests that, like the full-grown stages of related species, smaller V. ponderosa were attached by pedicle. Smaller specimens have a large pedicle opening, a nearly flat shape, thin shell, and a shallow sinus/sulcus, leaving the commissure nearly flat. These characteristics are consistent with strong, erect pedicle attachment, even stronger than found in related species, whose deep sinus/sulcus suggests the need to lift inhalant currents from sediments. Consistent with instability of surrounding gravels, full-grown specimens, those over about 2 cm in diameter, have relatively small pedicle openings that are obstructed by the curve of the beak. The pedicle was probably not enough to stabilize such a large shell even if suitable attachment sites were available. How then did V. ponderosa adapt to grow larger than most related species?

The secret to its success may be the aberrant characteristics that make large specimens easily distinguishable from related species: an almost spherical inflated form, large size (up to 4 cm diameter), extreme secondary thickening of the pedicle valve near the beak, and, the one characteristic that it shares with related forms, a deep sinus/sulcus. These shell characteristics support the hypothesis that full-grown V. ponderosa did not need solid substrates for attachment, but would be washed into a viable position in the sediment. The shell thickening would have served as ballast, allowing the spherical body to roll into position, perhaps as it was partly buried by scouring currents. This fits Seilacher’s “passive implanter” strategy. The consistent pedicle valve-beak-down position in which they are found in storm deposits and the typical restriction of epibionts to the area near the commissure on the brachial valve further supports this hypothesis. Thus V. ponderosa, in exceeding the size of its cogenerics, adapted to frequent disturbance by abandoning pedicle attachment in favor of a passive implanter strategy.