Published online by Cambridge University Press: 14 July 2015
The Protocardiinae is represented by 11 valid species of Protocardia and one species of Nemocardium in the Lower Cretaceous of the Gulf of Mexico. These taxa appeared in the Caribbean Province through a combination of immigration and evolution from the local stock. One species group, Protocardia texana-Protocardia salinaensis-Protocardia subspinigera, developed as geographically and environmentally distinct populations that migrated in space and time. Another group, Protocardia roanokensis-Protocardia prosogyrata-Protocardia nutans-Protocardia denisonensis, differentiated through time by dichotomous branching. These species occupied sand to mud substrates. The coarseness of their shell ornament is positively related to substrate grain size. The geographic and stratigraphic distributions of these species illustrate that diversity is positively correlated with the extent of distribution and that endemism develops during transgression.