Evolution of the Late Cretaceous oyster genus Cameleolopha Vyalov 1936 in central New Mexico

A stratigraphic sequence of middle Turonian rocks, 35 ft (11 m) thick, in Lincoln County, New Mexico, contains a record of oyster evolution unique in the Western Interior. There, four stratigraphically distinct species of the ribbed oyster Cameleolopha show clearly the morphological changes that led from a freelying ancestral species to the attached oyster that marks the end of the lineage. The oldest species, referred to C. aff. C. bellaplicata, is a small, densely ribbed, planar morpho-type that is confined to the Prionocyclus hyatti Zone. It gives rise through a transitional form to the medium-sized, more coarsely ribbed, planoconvex C. bellaplicata. Cameleolopha bellaplicata is an important guide fossil in the Four Corners states and Texas to the upper part of the P. hyatti Zone through the middle part of the overlying P. macombi Zone. Cameleolopha bellaplicata gives rise to C. lugubris through a reduction in size, an increase in rib density, and a change in mode of life from free lying to attached, but not encrusting. Cameleolopha lugubris, which has the greatest geographic range of the group, is a well-known guide fossil throughout the Western Interior to the upper part of the P. macombi Zone through at least the overlying P. wyomingensis Zone.