Uintacrinus socialis Grinnell, 1876 fossil crinoids in chalk from the Cretaceous of Kansas, USA (public display, Nebraska State Museum of Natural History, Lincoln, Nebraska, USA).

Stratigraphy: Niobrara Formation, Upper Cretaceous

Locality: western Kansas, USA

Some of the most remarkable fossils of a stemless crinoid species are Uintacrinus socialis. Spectacular examples occur as multiple, intertwined, complete specimens on thin slabs of chalk. The chalks are part of the Niobrara Formation (upper Santonian, mid-Upper Cretaceous) in western America. Many slabs have nothing but Uintacrinus on them. Most Uintacrinus slabs come from western Kansas, but the genus was near-cosmopolitan in its distribution.

The subspherical calices (heads) of Uintacrinus crinoids were covered with thin calcite plates. Extremely long arms extended from the calyx. These arms are often seen tangled together in the chalk slabs. The largest slabs consistently show the arms of all specimens pointing toward each slab’s center. Preservation of crinoid soft parts has been identified in some Uintacrinus slabs. Black, organic-rich laminations in the slabs have been suggested to represent degraded microbial mats, the presence of which may have promoted preservation of the crinoid tangles. The presence of soft-part preservation permits the use of the term lagerstätte for this deposit.