Microbial sealing in the biostratinomy of Uintacrinus Lagerstatten in the Upper Cretaceous of Kansas and Colorado, USA

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 aggrega tions with articulated calyces and arms only on the lower surface, in contr ast to disarticulated material on the upper surface. Calyces may be imbrica ted within a lens and are mostly compressed laterally. Specimens also may b e preserved oral side up or down. Some specimens displaying the oral side r etain soft-part preservation of the tegmen, anal tube, and ambulacra. A thi n, jet black organic lamination is visible beneath calyx plates. SEM analys is reveals the presence of possible microbial spherules and filaments on th is lamination. Dense aggregations reveal a number of new preservational fea tures, including marginal indentations and lacunae that suggest cohesive be havior of the layer prior to burial. SEM also reveals well-preserved stereo m in articulated crinoids on the lower surface, in contrast to the upper su rface in which the stereom is apparently infilled with calcium carbonate. S ome 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 articulat ion is confined to lower surfaces.