Late Cretaceous rudist bivalves of the Caribbean Province

The 200-m thick Guinea Corn Formation forms the marine portion of a 1,000 m thick transgressive-regressive tectonic cycle (Kellits Synthem). The complete Guinea Corn succession is exposed in the bed and banks of the Rio Minho, and the alternations of limestone, shale, mudstone, and sandstone, form small-scale cycles, which can be correlated from section to section along the river. Rudist bivalves occur throughout the limestone and also in some of the shale, with specimens preserved in growth position, toppled, or reworked. A highly diverse assemblage of rudist bivalves is present including at least 34 species belonging to 18 genera. Distinct assemblages are present including: 1, very small radiolitids (Trechmannites); 2, small radiolitids (Parabournonia, Guanacastea and Thyrastylon) and antillocaprinids (Antillocaprinia, Rotacaprina), 3, larger radiolitids (Chiapasella); 4, giant radiolitids (Macgillavryia) and hippruitids (Praebarrettia); and 5, various antillocaprind associations. The barrel-shaped Macgillavryia and Praebarrettia reach sizes of 43 cm and 35 cm high, respectively, whereas the giant Titansoarcolites can reach up to 1.3 m across (making it amongst the largest bivalves that every lived). This high-diversity rudist fauna occurs in uppermost Maastrichtian strata and is restircted to the American biogeoprovince. The bivalve community was likely terminated by the K-Pg extinction event.