THE EFFECT OF THE BREAKUP OF PANGEA ON THE EVOLUTION OF SHALLOW MARINE OSTRACODES

The ostracode Family Brachycytheridae is one of the most common taxa of Late Cretaceous shallow marine microfossils, and occurs in the marginal basins of North and South America, the Caribbean, Africa, India and Madagascar. Their migration pathways were controlled largely by the plate tectonic events associated with the breakup of Pangea and the formation of the North and South Atlantic Oceans, the Caribbean Plate and the Gulf of Mexico. This study is an attempt to combine specific information on the plate tectonic events as evidenced by the timings of continental-scale structural movements, sea level dynamics (largely the product of mantle processes), the creation and destruction of migration pathways, and taxonomy of the group, and thus relate deep-seated Earth processes, surficial expression of these movements, and biological evolution. Although the Late Cretaceous Brachycytheridae are very widely distributed, their use for correlation has been hampered due to their conservative external morphology, with all of them having a posteriorly-sloped dorsal margin and a swollen ventrolateral carapace, and all of them were long assigned to a single genus, Brachycythere. In 2002, Puckett defined a new genus, Acuminobrachycythere, based on a distinctive clade of North American brachycytherids. It was also recognized at that time that the members of the family from Gondwana (mainly Africa and South America) were distinct in having a single (unsplit) second adductor muscle scar, whereas all of those from North America had a split second adductor. This tiny difference holds significant paleogeographic—and presumably evolutionary—information. Two new genera were therefore described, Sapucariella, which is restricted to Gondwana, and Tricostabrachycythere, which is an early, short-lived genus that occurred around the margins of Gondwana. In all, there are six genera in the Family Brachycytheridae, which include Acuminobrachycythere, Brachycythere, Kaesleria (which is restricted to the Middle East), Opimocythere (which evolved in the Paleogene of North America and is the last of the Brachycytheridae), Sapucariella and Tricostabrachycythere. A summary of the breakup of Pangea will be presented, with precise paleogeographic reconstructions based on global magnetic anomalies.