Biogeography and diversity of South Atlantic Cretaceous echinoids: implications for circulation patterns

This survey of echinoid species living on the African and South American margins of the South Atlantic Ocean in Cretaceous times is based on a review of published species and abundant unpublished material. The synthetic approach is used to ascertain how the South Atlantic was colonised by echinoids from the time it first opened in the Early Cretaceous until communications were established with the North Atlantic and Mediterranean–Tethys in the Late Cretaceous. The survey focuses on the biogeographical origin of immigrant species and their relations with Indo-Madagascan, North Atlantic and Mediterranean–Tethyan faunas. Qualitative variations in species diversity are also studied where available fossil material permits, together with the in situ evolution of certain lineages and their endemism. Finally, echinoid colonisation of the South Atlantic during the Cretaceous is viewed against palaeocurrent patterns. The survey shows, within the limits of material available, that no echinoid fauna attests to the existence of the narrow marine trough separating South America and Africa in the Barremian. In the Aptian, West African echinoids display a Mediterranean–Tethyan influence, indicating a probable link between south-east and west of Africa via the strait opened between Africa and Madagascar. Echinoid fauna on the Brazilian margin includes species found in what is now continental North America and on the Pacific seaboard of South America. During the Albian, South America separated completely from Africa and normal marine communications were established with the Western Tethys to the north and the Southern Ocean domain to the south. The echinoid fauna of the South Atlantic during the Albian and early Cenomanian was particularly rich and diversified, with the earliest ubiquitous species on both margins of the ocean. In the late Cenomanian–early Turonian, a Tethys–South Atlantic Ocean link probably formed in West Africa through the Sahara and Nigeria, favouring the influx of Mediterranean–Tethyan species to the South Atlantic. During the mid Turonian–Coniacian, the trans-Saharan channel closed, slowing down colonisation of the South Atlantic by Mediterranean–Tethyan echinoids, as the only connection was via the Central Atlantic. This period was marked by the stability of a number of lineages that survived without major morphological changes in areas colonised during Cenomanian times. In the late Senonian, the South Atlantic was largely open to the neighbouring oce