Oligocene Primates from the Nsungwe Formation of Tanzania
Summary Field and laboratory studies of fossils from the western branch of the East African Rift System in southwestern Tanzania document a complex basin history with multiple Paleozoic-Cenozoic tectonic and depositional events. The Cenozoic depositional history of the Rukwa Rift, a key segment of the western Branch, is characterized by proximal alluvial fan systems that transitioned into a complex, volcanically-influenced landscape of fluvial, alluvial and lacustrine depositional environments. Late Oligocene fossil-bearing facies of the Nsungwe Formation are dated at ~26-24 Ma via high-precision U-Pb and Ar/Ar geochronology of intercalated volcanic tuffs. Localities in the late Oligocene Songwe Member of the Nsungwe Formation have revealed two new primate taxa documenting the earliest evidence of the split between cercopithecoids and hominoids. These localities also preserve a diverse non-primate mammalian fauna including rodents, macroscelideans, hyracoids and anthracotheres. Other significant discoveries include the oldest African boid snake, the earliest evidence of venomous elapid snakes on the African continent, and the oldest colubroid-dominated snake fauna in Africa. The Nsungwe Formation fauna also contains the earliest evidence of the endemic African ranoid frog family Ptychadenidae in addition to a rare xenopodinomorph pipid frog from the Late Oligocene. Other vertebrate clades are preserved in the fauna, represented by articulated turtle and fish materials and avian cranial and postcranial elements, in addition to crocodylian and lepidosaurian remains. Nsungwe Formation discoveries offer a glimpse at the evolutionary history of late Oligocene terrestrial and freshwater habitats in eastern Africa, providing data on the complex tectonosedimentary history of the Rukwa Rift Basin. Continued exploration offers a refined perspective on the Paleogene-Neogene transition on continental Africa, with expanded opportunities for recognizing trends in paleobiological diversity across habitat types and through time.
The goals of this research project were to: 1) Conduct fossil and geological/paleoenvironmental data collection from proven vertebrate-bearing fluvial depositional settings in the ~25Ma Nsungwe Formation of Tanzania, 2) Expand data collection in newly discovered Nsungwe Formation low-energy wetland depositional settings that preserve a broader size-range of vertebrate fossil specimens, 3) Explore the surrounding region for additional fossil-bearing localities, with an interdisciplinary team providing anthropological, paleontological, and geological research perspectives.
Fig. 1: Images from Rukwa Rift (Oligocene) A – Geological map placing Oligocene Nsungwe Formation (in black circle) into regional context; B – Paleoenvironmental reconstruction of Nsungwe Formation; C –Nsungwe Formation abbreviated faunal list.
Grant funded activities and discoveries Leakey Foundation funding was used to support a 4-week field effort in Tanzania during the dry season of 2011. Drawing upon an interdisciplinary, international crew, we explored dozens of fossiliferous localities, collected geological and paleontological data, and identified new areas for exploration. Many of the field discoveries made possible by this grant have undergone mechanical and/or digital preparation and are under study. Specimens recovered during this grant have contributed to and continue to contribute to publications and scientific conference presentations, such as those listed in the publications section. Three examples are appended to this report. Importantly, discoveries from this field season include RRBP 11178, Type specimen of Nsungwepithecus gunnelli, and earliest described stem cercopithecoid primate.
Analyses and results that pertain to specific hypotheses listed in the grant proposal Results/findings to date for Goal 1: Field work in proven fossiliferous localities has produced wellpreserved vertebrate and invertebrate fossil materials. The field season funded by this grant produced fossils featured in two published manuscripts documenting the primate and snake faunas respectively (appended at the end of this proposal). An additional manuscript is in press documenting the oldest evidence of the endemic African anuran family Ptychadinidae. Work continues on materials from three exceptionally productive localities. Results/finding to date for Goal 2: The majority of the field effort was dedicated to excavations at the Nsungwe 2B locality, a low-energy wetland depositional environment, preserving a greater size-range of specimens than previously explored fluvial localities. Through intensified screenwashing efforts at Nsungwe 2B, a number of hitherto unrecognized faunal elements were discovered (prior to this effort, the site had produced only larger specimens). These included fossil fishes, amphibians and micromammals (e.g., rodents, macroscelideans), vastly expanding the fauna and informing our paleoecological comparisons among Nsungwe Formation sites. Results/finding to date for Goal 3: Field efforts placed existing localities into geological better geological context. Findings published in Nature Geoscience established the groundwork for our understanding of East African Rift System (EARS) development, extending the age for the Western Branch of the EARS millions of years earlier in time. Preliminary work indicates a significant role for this region by producing fossils at a critically undersampled interval in primate evolutionary history. Lab research assembled highprecision U-PB CA-TIMS geochronology for the critical Nsungwe 2B primate-bearing locality. Results published in Nature provided the most precise dates for any Paleogene fossil site from Afro-Arabia. Geological analyses offered a critical part of the story for documenting the earliest evidence of the split between Old World monkeys and apes, a finding that would not have been possible without strong geological context.
Conclusions This project seeks to improve our understanding of primate evolutionary history through focused research in the late Oligocene Nsungwe Formation of southwestern Tanzania. These deposits represent the only terrestrial vertebrate assemblage documenting the close of the Paleogene in sub-equatorial Africa, offering the potential to address a range of questions significant to paleoanthropology. Deposits in the study area have yielded a diverse fauna represented by mammals, birds, reptiles, amphibians, fish, and invertebrates. Leakey Foundation-funded field research in the Nsungwe Formation has helped to refine age estimates for the western branch of the East African Rift System (Roberts et al., 2012), and produced the earliest stem cercopithecoid primate (Stevens et al., 2013), together with other significant vertebrate finds (McCartney et al., 2014, Blackburn et al., in press). A site of this age permits comparison with contemporary faunas in northern Africa and Asia, and contributes to understanding Cenozoic faunal dynamics on the African continent as a whole. Funding from the Leakey Foundation has been pivotal in establishing the significance of these fossiliferous deposits in the Nsungwe Formation of southwestern Tanzania.
List of peer-reviewed publications resulting in part from this award: In Press. D Blackburn, EM Roberts, NJ Stevens. Anurans from the late Oligocene Nsungwe Formation, southwestern Tanzania. Journal of Vertebrate Paleontology. 2014. J McCartney, NJ Stevens, PM O'Connor. The Earliest Colubroid-Dominated Snake Fauna from Africa: Perspectives from the Late Oligocene Nsungwe Formation of Southwestern Tanzania. PLOS-ONE. 2013. NJ Stevens et al. Palaeontological evidence for an Oligocene divergence between Old World monkeys and apes. Nature. 497: 611-614. 2012. EM Roberts, NJ Stevens, PM O’Connor, PHGM Dirks, MD Gottfried, WC Clyde, RA Armstrong, AIS Kemp, S Hemming. Initiation of the western branch of the East African Rift coeval with the eastern branch. Nature Geoscience. 5:289-294.
List of scientific presentations related to this award: 2014. NJ Stevens, ER Seiffert, PM O’Connor, EM Roberts. Primates from the Nsungwe Formation of southwestern Tanzania: Fossil evidence of strepsirrhines, anthropoids, and an Oligocene divergence between Old World monkeys and apes. American Journal of Physical Anthropology. 2014. B Benefit, NJ Stevens. Fossil evidence for the emergence of Old World monkeys: When, where and why. American Journal of Physical Anthropology. 2013. NJ Stevens, PM O’Connor, EM Roberts. Continental vertebrates from the late Oligocene Nsungwe Formation (Southwestern Tanzania) and faunal dynamics in sub-Saharan Africa during the PaleogeneNeogene transition. Geology. 2013. NJ Stevens, E Seiffert, PM O’Connor, EM Roberts. Primate diversity in the late Oligocene Nsungwe Formation of southwestern Tanzania. J. Vert Paleo. 33(S33): 219. 2013. EM Roberts, PM O’Connor, NJ Stevens. Dating Mesozoic-Paleogene continental vertebrate assemblages in Africa: An interdisciplinary approach. J. Vert Paleo. 33(S33): 200. 2013. E Naylor, C Krause, NJ Stevens. Phylogenetic and functional cues in micromammal tarsal bones from the late Oligocene Nsungwe Formation of southwestern Tanzania. J. Vert Paleo. 33(S33): 184. 2013. H. Hilbert-Wolf, EM Roberts, R Brown, PM O’Connor, NJ Stevens. Deciphering the links between landscape change and vertebrate evolution using U/Pb geochronology and detrital thermochronology: A case study from the Rukwa Rift Basin. J. Vert Paleo. 33(S33): 143. 2013. D Blackburn, EM Roberts, NJ Stevens. A Late Oligocene anuran fauna from the Nsungwe Formation, southwestern Tanzania. J. Vert Paleo. 33(S33): 88. 2013. H Hilbert-Wolf, EM Roberts, C Mtelela, PM O’Connor, NJ Stevens. Constraining the timing of Cenozoic rifting and basin development in the Rukwa Rift Basin of the East African Rift System via detrital zircon geochronology. Geology. 2012. JA McCartney, NJ Stevens. A new ophidiofauna from the late Oligocene Nsungwe Formation of Tanzania and the rise of colubroid snakes (Reptilia: Serpentes). J. Vert Paleo. 32(S32) 2012. WN Stevens, K Claeson, NJ Stevens. Alestid fishes from the late Oligocene Nsungwe Formation of Tanzania. J. Vert Paleo. 32(S32)