Nancy J. Stevens

Palaeontological evidence for an Oligocene divergence between Old World monkeys and apes

Apes and Old World monkeys are prominent components of modern African and Asian ecosystems, yet the earliest phases of their evolutionary history have remained largely undocumented. The absence of crown catarrhine fossils older than ∼20 million years (Myr) has stood in stark contrast to molecular divergence estimates of ∼25–30 Myr for the split between Cercopithecoidea (Old World monkeys) and Hominoidea (apes), implying long ghost lineages for both clades. Here we describe the oldest known fossil ‘ape’, represented by a partial mandible preserving dental features that place it with ‘nyanzapithecine’ stem hominoids. Additionally, we report the oldest stem member of the Old World monkey clade, represented by a lower third molar. Both specimens were recovered from a precisely dated 25.2-Myr-old stratum in the Rukwa Rift, a segment of the western branch of the East African Rift in Tanzania. These finds extend the fossil record of apes and Old World monkeys well into the Oligocene epoch of Africa, suggesting a possible link between diversification of crown catarrhines and changes in the African landscape brought about by previously unrecognized tectonic activity in the East African rift system.

The evolution of mammal-like crocodyliforms in the Cretaceous Period of Gondwana

Fossil crocodyliforms discovered in recent years have revealed a level of morphological and ecological diversity not exhibited by extant members of the group. This diversity is particularly notable among taxa of the Cretaceous Period (144–65 million years ago) recovered from former Gondwanan landmasses. Here we report the discovery of a new species of Cretaceous notosuchian crocodyliform from the Rukwa Rift Basin of southwestern Tanzania. This small-bodied form deviates significantly from more typical crocodyliform craniodental morphologies, having a short, broad skull, robust lower jaw, and a dentition with relatively few teeth that nonetheless show marked heterodonty. The presence of morphologically complex, complementary upper and lower molariform teeth suggests a degree of crown–crown contact during jaw adduction that is unmatched among known crocodyliforms, paralleling the level of occlusal complexity seen in mammals and their extinct relatives. The presence of another small-bodied mammal-like crocodyliform in the Cretaceous of Gondwana indicates that notosuchians probably filled niches and inhabited ecomorphospace that were otherwise occupied by mammals on northern continents.

An anthropoid primate humerus from the Rukwa Rift Basin, Paleogene of southwestern Tanzania

African early Tertiary paleontological sites are notoriously patchy, both spatially and temporally. The vast majority of Paleogene primate fossils have been recovered from sites in the northern portion of the continent (e.g., Thomas et al., 1991; Godinot and Mahboubi, 1992; Hartenberger and Marandat, 1992; Godinot, 1994), with the most extensive record from the Fayum Depression of Egypt (e.g., Osborn, 1908; Simons and Kay, 1983; Simons et al., 1994; Simons, 1995; Simons and Rasmussen, 1995; Simons et al., 2001). Sub-equatorial deposits are relatively sparse, and no primate postcranial remains have been described from Paleogene deposits in sub-Saharan Africa. The rarity of Paleogene strata from much of Africa poses problems not only for understanding the geographical extent and evolutionary significance of faunas occupying the majority of the continental landmass, it also provides obstacles to reconstructing biogeographic histories of a variety of clades, including primates (Stevens and Heesy, 2000; Ducrocq, 2001; Gunnell and Miller, 2001; Heesy et al., in press). In contrast, numerous sites in East Africa have provided a wealth of information concerning the Neogene primate record (e.g., Fleagle and Simons, 1978; Leakey et al., 1995; Harrison, 1997; MacLatchy and Pilbeam, 1999; Kingston et al., 2002). Until relatively recently, the earliest Tertiary faunas reported from East Africa were Miocene in age. New finds from older Tertiary sites suggest that this region also holds keys to understanding issues deeper in paleontological history (e.g., Leakey et al., 1995b; Murray, 2000; Harrison et al., 2001; Gunnell et al., 2002; Kappelman et al., 2003; Stevens et al., 2004). Here we describe a diminutive anthropoid primate humerus recovered from Paleogene deposits in southwestern Tanzania. In many features this specimen resembles basal anthropoids described from the Paleogene Jebel el Qatrani Formation of Egypt, constituting the first primate postcranial record from the Paleogene of sub-Saharan Africa.

Comparisons of Suspensory Behaviors Among Pygathrix cinerea, P. nemaeus, and Nomascus leucogenys in Cuc Phuong National Park, Vietnam

In our study at the Endangered Primate Rescue Center of Cuc Phuong National Park, Vietnam, we aimed first to assemble a positional behavioral profile of captive gray-shanked (Pygathrix cinerea) and red-shanked (P. nemaeus) doucs that relates to the use of forelimb suspensory postures and arm-swinging locomotion. The profile is of interest because researchers have documented that red-shanked doucs more frequently use suspensory postures and locomotions than other colobines do. We confirmed that red-shanked doucs commonly use suspensory positional behaviors and also that gray-shanked doucs use suspensory behaviors at similar or even higher frequencies than those of red-shanked doucs. Our second goal was to assemble a preliminary kinematic profile of suspensory locomotion in Pygathrix within the context of the arm-swinging locomotion exhibited by northern white-cheeked gibbons, Nomascus leucogenys. Mean forelimb angles at initial contact and release of arm-swinging behaviors were remarkably consistent among gibbons and doucs despite the fact that gibbons typically used more continuous brachiation. Doucs also exhibit a greater range of forelimb angles than gibbons do. In addition, trunk orientation tends to be less vertical at initial contact for doucs than for gibbons, perhaps owing to the frequent use of quadrupedal sequences directly before or after forelimb suspension. Our behavioral and kinematic analyses add to the emerging realization that Pygathrix is capable of, and frequently expresses, a range of suspensory positional behaviors, including brachiation.

The basal titanosaurian Rukwatitan bisepultus (Dinosauria, Sauropoda) from the middle Cretaceous Galula Formation, Rukwa Rift Basin, southwestern Tanzania

ABSTRACT Whereas titanosaurians represent the most diverse and cosmopolitan clade of Cretaceous sauropod dinosaurs, they remain rare components of Cretaceous African faunas. Currently recognized continental African titanosaurians include Aegyptosaurus baharijensis and Paralititan stromeri from early Upper Cretaceous deposits near Bahariya Oasis, Egypt, and Malawisaurus dixeyi and Karongasaurus gittelmani from the Lower Cretaceous (∼Aptian) Dinosaur Beds of Malawi, in addition to several undesignated and fragmentary forms across the continent. Here, we describe a new titanosaurian taxon, Rukwatitan bisepultus, on the basis of a partial, semiarticulated postcranial skeleton recovered from the middle Cretaceous Galula Formation in southwestern Tanzania. Unique to Rukwatitan are carotid processes on posterior cervical vertebrae, a deep coracobrachialis fossa and subquadrangular cross-section of the humerus, and a slender, curved, teardrop-shaped pubic peduncle on the ilium. Parsimony and Bayesian phylogenetic analyses of 35 sauropod taxa congruently place Rukwatitan as a non-lithostrotian titanosaurian, a relationship supported by cervical vertebrae with undivided pleurocoels and strongly procoelous anterior caudal vertebrae. Rukwatitan differs from the potentially penecontemporaneous and geographically proximate Malawisaurus by exhibiting weakly developed chevron articulations and posteriorly inclined neural spines on the middle caudal vertebrae, a proximally robust and distally unexpanded humerus, and an anteroventrally elongated coracoid. Similar to biogeographic patterns identified in certain crocodyliform clades (e.g., small-bodied notosuchians), titanosaurians on continental Africa appear to exhibit a regional (e.g., southern versus northern Africa), rather than a continental- or supercontinental-level signal.

A new freshwater crab (Decapoda: Brachyura: Potamonautidae)from the Paleogene of Tanzania, Africa

Discovery of numerous fragmentary remains of freshwater crab in Paleogene, probably Oligocene, sediments in Tanzania, Africa, permits the description of a new genus and species, Tanzanonautes tuerkai. The fossils represent the oldest freshwater crabs known.

The Earliest Colubroid-Dominated Snake Fauna from Africa: Perspectives from the Late Oligocene Nsungwe Formation of Southwestern Tanzania

The extant snake fauna has its roots in faunal upheaval occurring across the Paleogene - Neogene transition. On northern continents, this turnover is well established by the late early Miocene. However, this transition is poorly documented on southern landmasses, particularly on continental Africa, where no late Paleogene terrestrial snake assemblages are documented south of the equator. Here we describe a newly discovered snake fauna from the Late Oligocene Nsungwe Formation in the Rukwa Rift Basin of Tanzania. The fauna is small but diverse with eight identifiable morphotypes, comprised of three booids and five colubroids. This fauna includes Rukwanyoka holmani gen. et sp. nov., the oldest boid known from mainland Africa. It also provides the oldest fossil evidence for the African colubroid clade Elapidae. Colubroids dominate the fauna, comprising more than 75% of the recovered material. This is likely tied to local aridification and/or seasonality and mirrors the pattern of overturn in later snake faunas inhabiting the emerging grassland environments of Europe and North America. The early emergence of colubroid dominance in the Rukwa Rift Basin relative to northern continents suggests that the pattern of overturn that resulted in extant faunas happened in a more complex fashion on continental Africa than was previously realized, with African colubroids becoming at least locally important in the late Paleogene, either ahead of or as a consequence of the invasion of colubrids. The early occurrence of elapid snakes in the latest Oligocene of Africa suggests the clade rapidly spread from Asia to Africa, or arose in Africa, before invading Europe.

Kahawamys mbeyaensis (n. gen., n. sp.) (Rodentia: Thryonomyoidea) from the Late Oligocene Rukwa Rift Basin, Tanzania

[Extract] Paleogene micromammal-bearing deposits from Afro-Arabia have until recently been largely restricted to a limited number of localities in Saharan Africa and Oman (e.g., Osborn, 1908; Wood, 1968; Jaeger et al., 1985; Fejfar, 1987; Thomas et al., 1989; Holroyd, 1994; Seiffert et al., 2008; but see also Gunnell et al., 2002). Research in the Rukwa Rift Basin of Tanzania has begun to reveal a diverse late Paleogene vertebrate fauna below the equator. This work has produced evidence of primate (Stevens et al., 2005), macroscelidean (Stevens et al., 2006a) and hyracoid mammals, and in particular, an interesting array of rodent taxa (Stevens et al., 2008). Teeth attributed to the phiomorph rodent Metaphiomys have been recorded in the study area (Stevens et al., 2006b), along with a number of smaller thryonomyoid rodent specimens, many of which are severely worn, hampering precise taxonomic assessment. The recent discovery of a fairly complete, modestly worn thryonomyoid mandible allows us to recognize the presence of a new taxon from the Rukwa Rift Basin deposits. This find is significant in that it represents the first novel Paleogene rodent genus and species described from East Africa, and documents a critically under-represented temporal gap in African faunal evolution (e.g., Seiffert, 2006).

Oligocene Termite Nests with In Situ Fungus Gardens from the Rukwa Rift Basin, Tanzania, Support a Paleogene African Origin for Insect Agriculture.

Based on molecular dating, the origin of insect agriculture is hypothesized to have taken place independently in three clades of fungus-farming insects: the termites, ants or ambrosia beetles during the Paleogene (66–24 Ma). Yet, definitive fossil evidence of fungus-growing behavior has been elusive, with no unequivocal records prior to the late Miocene (7–10 Ma). Here we report fossil evidence of insect agriculture in the form of fossil fungus gardens, preserved within 25 Ma termite nests from southwestern Tanzania. Using these well-dated fossil fungus gardens, we have recalibrated molecular divergence estimates for the origins of termite agriculture to around 31 Ma, lending support to hypotheses suggesting an African Paleogene origin for termite-fungus symbiosis; perhaps coinciding with rift initiation and changes in the African landscape.

A Hyracoid from the Late Oligocene Red Sandstone Group of Tanzania, Rukwalorax jinokitana (gen. and sp. nov.)

[Extract] A striking array of fossil hyracoids has been described from northern Africa and the Arabian Peninsula, with several taxa recognized from Paleogene strata not only in the Fayum Depression of Egypt, but also in Morocco, Algeria, Tunisia, Libya and Oman (e.g., Sudre, 1979; Rasmussen, 1989 and references therein; Thomas et al., 1989; Gheerbrant et al., 2005). Hyracoids were abundant in these faunas, comprising up to 90% of the mammalian fauna recovered from the L-41 locality in the Jebel Qatrani Formation of Egypt (Rasmussen and Simons, 1991). Hyracoids appear to have achieved their apex in diversity during the Paleogene, at which time the group dominated the small-medium sized herbivorous niches in known faunas (Schwartz et al., 1995). During this time, they spanned a rabbit to rhinoceros range in body size, exhibiting a diversity of locomotor and dietary morphologies (Rasmussen et al., 1996). From the bunodont Geniohyus, to the common lophoselenodont Thyrohyrax, hyracoids flourished, assuming a vast array of niches that would later be occupied by immigrant artiodactyls and perissodactyls (Schwartz et al., 1995). Indeed, specializations for limb stabilization attributed to cursoriality in Antilohyrax pectidens suggest that in some ways it converged upon modern springboks in aspects of its locomotor habits (Rasmussen and Simons, 2000), whereas at the other extreme, postcranial specializations of the hind limb later emerged in some procaviids to permit extreme rotation for rock and tree climbing (Fischer, 1986).