Uintacrinus socialis: Evidence for Mode of Life From Brachial Stereom

Jeffrey LaRock
Department of Geosciences
University of Massachusetts
Advisor: Emily A. CoBabe

Uintacrinus is a Cretaceous unstalked crinoid which once flourished in the epicontinental seas of North America. To date, no one has been able to determine how Uintacrinus lived in these seas. It has been suggested that it might be a benthic crawler, a pelagic swimmer, or perhaps it floated passively near the ocean surface.

Crinoid microstructure is a porous lattice work made up of magnesium calcite called stereom. The porosity and size of the lattice structure in modern crinoids varies depending on whether they are benthic or pelagic. A bottom dwelling crinoid often has a dense lattice work to provide support. A pelagic crinoid stereom is more porous in order to increase buoyancy. Stereom of Uintacrinus from the Mancos Shale( Late Santonian) of Grand Junction, Colorado has been revealed using a scanning electron microscope. This stereom structure will be used to interpret Uintacrinusí mode of life.

The Mancos Shale around Grand Junction has a low diversity fauna. The shale component of the deposit has only a sparse population of pelagic foraminifera. The most common macrofossil is Inoceramus, a Cretaceous bivalve, many of which are encrusted by an unidentified oyster. The limestone component consists almost entirely of Uintacrinus skeletal material. These Uintacrinus fossils are often associated with a fossil volcanic ash layer (or bentonite) found above this limestone. The relationship between the bentonites and the Uintacrinus layer may indicate volcanism played a role in their demise.