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.

This tiny rodent mandible represents the first mammal species described from the Paleogene of subequatorial east Africa. It preserves an evergrowing incisor, a deciduous premolar and two molars (scale bar = 1mm). The species was named, Kahawamys, which means “coffee-mouse” in the Swahili language, referring to the fact that this fossil was found in what is now a coffee growing region in Tanzania. This new species is distinctive because of the arrangement of cusps and crests on the premolar and molars. Its closest living relatives are much larger African rodents called “cane rats” and “dassie rats”.