ANCIENT MARINERS AND THEIR ENCRUSTERS: IDENTITY, TAPHONOMY AND EPIZOA OF “BY-THE-WIND SAILORS” (CHONDROPHORA) IN THE GENUS PLECTODISCUS

Identification of fossils as chondrophoran cnidarians (the group including the modern by-the-wind sailor Velella) is replete with controversy. Some species are based on ambiguous or scanty fossil material. The “by-the-wind sailor” genus Plectodiscus Ruedemann, however, is represented by a number of well preserved specimens. Plectodiscus specimens have been described from the Silurian through Triassic of North America (Montana, Nevada, New York, and Oklahoma), the Devonian of Germany, and the Carboniferous of Malaya. Some "giant orbiculoid” specimens, including those in the North American species Orbiculoidea magnifica Clarke, should also be referred to Plectodiscus, as should a fossil from the Mississippian Cuyahoga Formation of Ohio originally described as an unidentified soft-bodied organism. Specimens from Ohio offer insights into the epizoa and taphonomy of the genus. An 11.5 cm long specimen, almost certainly from the Cuyahoga Formation, is preserved in three dimensions on the bottom of a siltstone slab. It has well delineated corrugations (pneumatocysts) and a distinct furrow that crosses the disc. Poorly preserved inarticulate brachiopods are attached to it. A smaller Cuyahoga Formation specimen (formerly described as a soft-bodied organism) is preserved in a concretion. It has less well-preserved corrugations; but well-preserved specimens of Orbiculoidea newberryi Hall are attached to it. The brachiopods are in two main size ranges, indicating two periods of colonization. It is possible that the brachiopods colonized the pneumatophore (float), but it is more likely that the brachiopods were attached to coenosarcal (soft) tissues covering the pneumatophore of the live Plectodiscus. Orbiculoidea newberryi must have been able to colonize pelagic organisms. Encrusting by brachiopods is consistent with that previously reported for Plectodiscus from the Hunsrück Slate of Germany.

Difference in preservation of the two Ohio Plectodiscus specimens and their epizoa is due to different taphonomic histories. Not all reported morphological features of Plectodiscus may be authentic. Crushing of simple paper-cone models indicates that polygonal patterns (“trapezohedral areas”), as well as other creases, preserved on fossils may be taphonomic. The presence of a sail is problematic.