Uintacrinus socialis Grinnell, 1876 - fossil crinoids in chalk from the Cretaceous of Kansas, USA. (public display, Sternberg Museum of Natural History, Hays, Kansas, USA)

Crinoids (sea lilies) are sessile, benthic, filter-feeding, stalked echinoderms that are relatively common in the marine fossil record. Crinoids are also a living group, but are relatively uncommon in modern oceans. A crinoid is essentially a starfish-on-a-stick. The stick, or stem, is composed of numerous stacked columnals, like small poker chips. Stems and individual columnals are the most commonly encountered crinoid fossils in the field. Intact, fossilized crinoid heads (crowns, calices, cups) are unusual. Why? Upon death, the crinoid body starts disintegrating very rapidly. The soft tissues holding the skeletal pieces together decay and the skeleton falls apart.

Shown here is the underside of a relatively thin plate of chalk with multiple, intertwined, complete specimens of Uintacrinus socialis stemless crinoids. The chalk is from the Upper Cretaceous Niobrara Formation of western America. Many plates 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; the arms are often seen tangled together. The largest slabs consistently show the arms of all specimens pointing toward the center of the plate. Preservation of crinoid soft parts (e.g., tegmen, anal tube, ambulacra; see Meyer & Milsom, 2001) has been identified in some Uintacrinus slabs, which permits use of the term "lagerstätte" for this occurrence. Black, organic-rich laminations are present and have been suggested to represent degraded microbial mats, which may have promoted preservation of the crinoid tangles.

Stemless, float-like calices of Uintacrinus suggest that these crinoids lived at the sea surface, unlike Paleozoic or modern crinoids, which are mostly sessile benthic. The Niobrara seafloor was not suitable for crinoids having encrusting holdfasts or root-like anchors. Inoceramid bivalves are also found in Niobrara Formation chalks - they had large, spread-out shells to prevent sinking in the originally-soupy, calcareous ooze sediments.

Classification: Animalia, Echinodermata, Crinoidea, Articulata, Uintacrinida, Uintacrinidae

Stratigraphy: Smoky Hill Member (a.k.a. Smoky Hill Chalk Member; Smoky Hills Member), Niobrara Formation, upper Santonian Stage, mid-Upper Cretaceous

Locality: unrecorded/undisclosed site in Logan County, western Kansas, USA

Reference cited:

Meyer & Milsom (2001) - Microbial sealing in the biostratinomy of Uintacrinus lagerstätten in the Upper Cretaceous of Kansas and Colorado, USA. Palaios 16: 535-546.

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