Origin of the Grayson Micromorph Fauna (Upper Cretaceous) of North-Central Texas

The macrofauna of the Grayson Formation (Upper Cretaceous) of Texas has been described as a micromorph fauna. Sedimentologic and paleontologic studies of the Grayson claystone in north-central Texas show that it contains a micromorph fauna. The small shell size of some of the Grayson oysters and ammonoids is probably an adaptive strategy evolved by these organisms for survival in a soft substrate environment. The small shell size of the oyster specimens Ilymatogyra arietina (Roemer) may be a consequence of progenesis, which is one mechanism whereby organisms can adapt to a soft substrate. Grayson pyritized specimens are probably a result of microreducing environments that developed within individual shells in the soft mud.