EXTREME ENDEMISM OF LATE CRETACEOUS MARINE OSTRACODS IN THE US GULF COASTAL PLAIN

Ostracods are abundant and diverse fossils in the North American coastal plain sediments. Detailed sampling of late Campanian-Maastrichtian marine deposits on the eastern flank of the Mississippi Embayment reveals extremely localized occurrences, often at only a single outcrop, of rare but diverse species of a clade of ornamented ostracods that have been assigned to the genus Anticythereis. Taxonomic studies indicate that there are more than 20 previously undescribed species in this clade in the Atlantic and Gulf coastal plains, although none have been reported from the western flank of the Mississippi Embayment and no species occurs in both the Atlantic and Gulf coastal plains. Recently, 11 of those species from the TN-MS-AL region were named and described and assigned to 4 new genera and a new Subfamily Anticytherideinae. The subfamily is characterized mainly by a distinctive muscle scar pattern and a pair of inverted platform structures on the inside of the carapace that corresponds to the inner surface of the postocular sulcus. The clade first appeared in the later Campanian Radotruncana calcarata planktonic foraminiferal taxon range zone and became extinct at the K/Pg boundary event. Several species were found at a single outcrop of the Providence Sand of eastern Alabama, whereas others occur only in the Owl Creek Formation of northern Mississippi. Although there have been many species assigned to the genus Anticythereis from continents exclusive of North America, their endemism indicates that none of them should be assigned to that genus and represent taxa that evolved in their respective areas. This endemism makes ostracods very useful tools in deciphering plate tectonic reconstructions of the past.