Uintacrinus
socialis
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.
The stemless, float-like calices of Uintacrinus
lead to the interpretation that it 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 the chalks of the Niobrara Formation. They
had large shells with a spread-out shape to prevent sinking in the original
soupy calcareous ooze sediments.
Uintacrinus socialis Grinnell,
1876 from the Niobrara Formation (Upper Cretaceous) of western Kansas, USA.
(public
display, Nebraska State Museum of Natural History, Lincoln, Nebraska, USA)
Uintacrinus
socialis Grinnell, 1876 from
the Niobrara Formation (Upper Cretaceous) of western Kansas, USA. Centimeter scale.
(YPM IP collection, invertebrate paleontology
collection, Yale University's Peabody Museum, New Haven, Connecticut, USA)
Most
info. provided by Dave Meyer.