Article: New late Palaeozoic Hyolitha (Mollusca) from Oklahoma and Texas, and their palaeoenvironmental significance
John M. Malinky [sic], Royal H. Mapes and Thomas W. Broadhead
Recovery of over 900 hyoliths from fourteen localities in Oklahoma and Texas greatly increases the number of late Palaeozoic hyolith occurrences in North America; they include Lirotheca wilsoni Malinky and Mapes, 1983 and Darwinites grafordensis gen. et sp. nov. Many of the hyoliths lack opercula and taxonomically important features of the aperture, or they are crushed; these specimens are indeterminate. In Oklahoma the hyoliths are restricted to the dark grey to black, non-fissile, phosphatic shale members ( = 'core' shales) of Pennsylvanian cyclothems. These shales are thought to be the most offshore facies of the cyclothem. The occurrence of hyoliths in dark grey, locally phosphatic, shale members of cyclothems farther south in Texas further supports the assignment of these shales to offshore, slightly oxygen-poor marine environments rather than to shoreline lagoons.