Cenozoic Exploration in the Rukwa Rift

After the end-Cretaceous extinction event, it has been suggested that Afro-Arabia entered a period of relative isolation that lasted throughout much of the Paleogene (65 – 24 million years ago), with only occasional small-scale faunal exchange with other landmasses. As this endemic African fauna radiated in isolation, certain mammal groups dominated the landscapes. Many of the endemic forms later became extinct shortly after the collision between the Arabian and Eurasian plates. Our knowledge of African Paleogene faunas has largely been based upon sites from supra-equatorial Africa, particularly throughout what is now the Sahara. Until recently, virtually nothing has been known of Oligocene African terrestrial vertebrate faunas from below the equator, rendering assessments of geographic patterns in these forms all but impossible.

Since the inception of the RRBP, a wealth of new invertebrate and vertebrate forms have been recovered from Nsungwe Formation deposits in the Rukwa Rift Basin, including freshwater crabs, gastropods, fishes, frogs, turtles, crocodylians, birds, rodents, macroscelideans, hyracoids and primates (for an overview, Stevens et al., 2008). Documentation of this diverse fauna from below the equator can help to better characterize regional faunal patterns and examine the role of this paleoecological setting in preserving a broader sampling of vertebrate evolutionary history.

Hyracoids are an interesting group of mammals that appear to have achieved their apex in diversity during the Paleogene, at which time the group dominated the small-medium sized herbivorous niches in Afro-Arabian faunas. During this time, hyracoids flourished, occupying a vast array of feeding and locomotor niches and exhibiting a rabbit-to-rhinoceros range in body sizes. Faunal exchange beginning in the Miocene introduced a number of immigrant forms to the African continent, and a dramatic decline in hyracoid diversity followed soon thereafter. Today, only three hyrax genera remain.

The RRPB has recently described the first hyracoid evidence from the Paleogene of subequatorial Africa, a new species called Rukwalorax jinokitana (jinokitana means comb-tooth in the Swahili language). This new species is represented by a comb-like right lower first incisor, distinctive among Paleogene hyracoids in preserving four separate tines on the central lower incisor. Other taxa exhibit only three tines whereas one species from the Paleogene of Egypt preserves up to 10 individual tines. There is some debate about how the tines on the teeth were used by these animals, but it is generally accepted that as with the modern hyraxes, these animals fed on vegetation. Rukwa Rift Basin localities provide a fascinating glimpse of hyracoid evolutionary history below the equator prior to the Neogene faunal transition, helping to expand our knowledge of Paleogene vertebrate diversity on the African continent.