Early Miocene sea pens (Cnidaria: Anthozoa) and the taphonomic history of an unconventional hard substrate community

CASADIO, SILVIO; PASTORINO, G.; GRIFFIN, MIGUEL

Buenos Aires

Encuentro; Reunión Anual de la Asociación Paleontológica Argentina; 2009

Museo de Argentino de Ciencias Naturales y APA

Sea pens are anthozoans belonging to the order Pennatulacea with a wide geographic distribution from the tropics to polar regions. They are the only octocorals adapted to live in soft sediments. Unlike other octocorals, they have an axial polyp that has a rigid, calcareous axis and a peduncle or stalk without secondary polyps at its base. Secondary polyps are originated from the axial polyp. They use the peduncle to anchor in sandy or muddy substrates. The exposed portion of sea pens may rise up to one meter above the bottom depending on the species. Rarely found in shallow water, sea pens are more common in low-energy deep water environments. The fossil record of the Pennatulacea is scarce but reaches back into the Paleozoic. However the first known sea pen with a calcareous axial rod is from the Late Oligocene of Germany. The recent discovery of a bedding plane within the lower part of the Chenque Formation (Early Miocene) containing hundreds of sea pen rods opens a geographic and temporal window allowing back-tracking of the history of this poorly studied group. The Patagonian specimens were collected from very fine-grained silty sandstones interpreted as low shoreface deposits, exposed in the lower part of the cliff at Rada Tilly (Chubut Province). Most of the rods were found in the infill of small channels and are encrusted by bryozoans and serpulids. Reciprocal overgrowth of serpulid worm tubes and bryozoans is common, suggesting that after coral death the rods were incorporated to the channels where they provided a hard substrate for encrusting organisms.