An exceptionally well preserved brittle star bed from the Miocene of Patagonia, Argentina

MARTINEZ SERGIO,; DEL RÍO CLAUDIA,; PEREZ DAMIAN

Granada, España

Congreso; 5ª Reunión sobre Tafonomía y Fosilización y 3rd Meeting on Taphonomy and Fossilization (; 2008

<!-- /* Font Definitions */ @font-face {font-family:Calibri; panose-1:0 0 0 0 0 0 0 0 0 0; mso-font-alt:"Opus Metronome"; mso-font-charset:0; mso-generic-font-family:swiss; mso-font-format:other; mso-font-pitch:variable; mso-font-signature:3 0 0 0 1 0;} /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; text-autospace:none; font-size:10.0pt; font-family:Calibri; mso-fareast-font-family:"Times New Roman"; mso-bidi-font-family:Calibri; mso-ansi-language:ES-UY;} @page Section1 {size:612.0pt 792.0pt; margin:70.85pt 3.0cm 70.85pt 3.0cm; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> An exceptionally well preserved brittle star bed from the Miocene of Patagonia, Argentina   Sergio Martínez1, Claudia J. del Río2 & Damián Pérez2 1 Dpto. Evolución de Cuencas, Facultad de Ciencias, UDELAR, Iguá 4225, 11400 Montevideo, Uruguay.smart@fcien.edu.uy 2 Museo Argentino de Ciencias Naturales B. Rivadavia-CONICET. Av. Angel Gallardo 470, C1405DJR Buenos Aires, Argentina. cdelrio@macn.gov.ar     Brittle star beds, or more or less dense aggregations of shallow-water brittle stars, are presently known throughout the world, although only the populations of a few species are well known. In the fossil record and due to their low potential of fossilization, brittle star beds are very scarce. Aronson (1989, 1992) listed up to 28 assemblages, seven of which are from the Cenozoic, being this low representation the reason for which each new fossil record of brittle star beds is substantially important.  The Miocene brittle star bed studied herein was first mentioned by Caviglia (1978), and the species involved, Ophiocrossota kollenbergorum, has been recently described by Caviglia, Martínez and del Río (2007). Although Caviglia (1978) provided some basic data, new field exploration was done, and results are here discussed. The ophiuroids were found in the neighborhood of Comodoro Rivadavia City (Chubut Province, Argentina) in sedimentites of the Sequence 1 of the Chenque Formation, composed by gray very fine silty sandstones, deposited in a distal lower shoreface environment, highly bioturbated  and dominated by the Cruziana Ichnofacies (Buatois et al., 2003). There were found three discrete thin monospecific patches of Ophiocrossota kollenbergorum (of 2 m2 on average), and outside these patches there are no specimens. The disks – always in horizontal position- are very delicate (ca. 6 mm wide on average) and their tiny plates are entire and very well preserved (recrystallized calcite); moreover, specimens in each one of the patches have complete arms, not preferentially oriented, and sometimes with their very delicate tips preserved (not wider than 0.2  mm). Only in a few cases arms are broken, but it can be attributed to the present day very erosive environment (abrasion platform). Isolated plates in the sedimentary matrix have not been recorded. Moreover, at the same horizon of the brittle star beds, there are patches of other fossils, including among them delicate well preserved articulated shells of Atrina. All available evidence indicates no transport; on the contrary, a sudden burial should have occurred.