Continental break up and the distribution of fishes of Western Gondwana during the Early Cretaceous

Abstract

Many Early Cretaceous fishes were endemic to western Gondwana, making their fossils potentially useful tools in biogeographic reconstruction. These occur in a variety of tectonic settings; for example, in rift basins of the East Brazilian Rift System (EBRIS) and corresponding sites in western Africa, in non-rift basins within the EBRIS region, and in interior localities far removed from this area in both Africa and Brazil. Furthermore these occurrences span much of the Early Cretaceous and include syn-rift and post-rift (transitional and marine) deposits. Development of a permanent equatorial seaway in the late Aptian is identified as a crucial event in the biogeographical history of western Gondwana, but several earlier tectonic events are recognized that could have played an important role in pre-drift intercontinental speciation. Different patterns of distribution are noted among non-marine fishes, some taxa occurring within areas of rifting while others are more widespread. The earliest Cretaceous marine fishes in NE Brazil are late Aptian and early Albian endemic circum-Gondwanan taxa. Their arrival coincides with the formation of the equatorial seaway but they occur in strata that were deposited under conditions of anoxia and abnormally high salinity. A more widespread marine invasion (probably from Caribbean Tethys) occurred in the middle and late Albian as oxygen and salinity levels were normalized and surface water exchange began in the equatorial Atlantic.