THE “PASSIVE IMPLANTER” STRATEGY OF THE ADULT ORDOVICIAN BRACHIOPOD, PLATYSTROPHIA PONDEROSA

Platystrophia ponderosa is found throughout the Maysvillian Strata of the Cincinnati Ordovician. This species thrived in a high energy environment with only muddy shell gravels, and no solid substrates for pedicle attachment.

Our growth-series studies show juveniles of this species had large pedicle openings, thin shell, small size, nearly flat shape, and shallow sinus/sulcus. In contrast, the adults had relatively small pedicle openings obstructed by a large beak, secondary thickening of the pedicle valve making it considerably thicker than the brachial valve, large size (up to 4cm in diameter), spherical shape, and deep sinus/sulcus.

The morphological characteristics of the adult P. ponderosa’s valves were key to its ability to survive the turbulent waters without a proportionally large pedicle and are characteristic of Seilacher’s “passive implanter” strategy. We hypothesize that an adult P. ponderosa did not need solid substrates for attachment, but would have been passively washed into a viable position in the sediment. The secondary thickening of the pedicle valve worked as the species ballast allowing the brachiopod to settle if it had been tipped by currents. The large size of the brachiopod helped prevent damage from the currents and the spherical shape allowed the shell to roll back into life position. Finally, the deep sinus/sulcus lifted the inhalant currents above the sediments.

Ongoing research will examine direct evidence of shell behavior, specifically, epibionts and geopetals. Epibionts would have encrusted the exposed areas of the shell more heavily than the portions hidden by burial in a muddy bottom. The distribution of epibionts on the valves will be mapped for a large sample of P. ponderosa. Geopetal sedimentary fillings should be consistent with inferred life position. Members of this sample will also be cut to observe the orientation of sediment fillings.