Lioestheriid conchostracans are numerous in thin

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beds within correlative uppermost Devonian rocks along the Cordilleran miogeosyncline. They are present in the Exshaw Formation of southwestern Alberta, Sappington Member of Three Forks Formation throughout western Montana, lower part of Leatham Formation of northern Utah, and middle part of Pilot Shale of west-central Utah and southeastern Nevada.

These phyllopod bivalve crustaceans or clam shrimp are found most commonly in greenish-gray and grayish-black shales. The shales directly overlie extremely thin discontinuous fish- and conodont-bone beds and are overlain by carbonate beds that generally contain numerous algal nodules (oncolites). Conchostracans also are present in limestone and in channel siltstone.

The associated biota generally comprises inarticulate brachiopods, principally Lingula and Orbiculoidea, orthocone, nautiloid, and goniatite cephalopods, Tasmanites, and fish fragments, but locally includes abundant to rare ophiuroids, blastoids, and other pelmatozoans, articulate brachiopods, ostracods, conodonts, trilobites, horn corals, and sponge spicules. The conchostracans are small and thin valved in noncalcareous shale but their size and valve thickness increase relative to higher carbonate content of enclosing rocks. The bivalves are flattened in shale but undistorted in carbonate beds; open articulated valves are commonly preserved.

A brackish-water environment is suggested for these latest Devonian conchostracans. Optimum conditions apparently were a muddy bottom, restricted circulation, shallow and quiet water, and slow deposition.

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