Shallow-water late middle Eocene crinoids from Tierra del Fuego: a new southern record of a retrograde community structure

One of the very few crinoid records in Patagonia is that of the abundant columnals of the genus Isselicrinus found in several localities in shallow marine, glauconitic sandstones of the Leticia Formation (upper Middle Eocene of Tierra del Fuego Island). Some of them, up to 10 cm long, are preserved in a position almost perpendicular to the stratification, which is attributed to episodes of high sedimentation rate. The Isselicrinus remains occur either almost alone or associated with solitary corals, gastropods, bivalves, rare nautilids, shark teeth, penguin bones and well-preserved specimens of Ophiura elegantoides. The associated microfossils, mainly Foraminifera, are characterised by the anomalous abundance and dominance of large Nodosariacea. These records reflect a peculiar success of a retrograde, dense, suspension-feeding crinoid population as a regional extension of previously described distribution in the Antarctic upper Eocene.Original Abstract: Uno de los pocos registros de crinoideos en Patagonia esta representado por abundantes columnas del genero Isselicrinus, en areniscas glauconiticas someras de la Formacion Leticia (Eoceno Medio superior de Tierra del Fuego). Algunas columnas, de hasta 10 cm de largo, estan preservadas en forma casi perpendicular a la estratificacion, posicion que se atribuye a episodios de alta tasa de sedimentation. Los restos de Isselicrinus se presentan en forma aislada o estan asociados a corales solitarios, gastropodos, bivalvos, escasos nautiloideos, dientes de tiburones, huesos de pingueinos, y ejemplares bien preservados de Ophiura elegantoides. Los microfosiles asociados, principalmente foraminiferos, se caracterizan por la dominancia y abundancia anomala de grandes conchillas de Nodosariacea. Estos registros reflejan un peculiar exito de poblaciones suspensivoras, retrogradas y densas, de crinoideos y demuestran una extension regional de poblaciones similares descriptas para el Eoceno superior de Antartida.