Abstract

New information pertaining to the biostratinomy of Uintacrinus assemblages has been derived from re-examination of museum collections and analysis of in situ material. Individuals are preserved as thin lenses in dense aggregations with articulated calyces and arms only on the lower surface, in contrast to disarticulated material on the upper surface. Calyces may be imbricated within a lens and are mostly compressed laterally. Specimens also may be preserved oral side up or down. Some specimens displaying the oral side retain soft-part preservation of the tegmen, anal tube, and ambulacra. A thin, jet black organic lamination is visible beneath calyx plates. SEM analysis reveals the presence of possible microbial spherules and filaments on this lamination. Dense aggregations reveal a number of new preservational features, including marginal indentations and lacunae that suggest cohesive behavior of the layer prior to burial. SEM also reveals well-preserved stereom in articulated crinoids on the lower surface, in contrast to the upper surface in which the stereom is apparently infilled with calcium carbonate. Some specimens retain a black, organic lamination within the crinoidal layer itself that may represent remnants of a microbial mat. It is proposed that a microbial mat of necrolytic origin provided cohesion and that microbial sealing during decay may help to explain other instances of similar crinoid preservation, including both benthic and pelagic forms, in which articulation is confined to lower surfaces.

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