The Earliest Fossil of the African Clawed Frog (Genus Xenopus) from Sub-Saharan Africa
Although the fossil record of pipoid frogs is more extensive than for other anuran clades, crown-group genera are poorly documented throughout the Cenozoic. We report an isolated neurocranium from the Nsungwe Formation (∼25 million years ago; Oligocene) in southwestern Tanzania, providing the earliest evidence for the genus Xenopus in sub-Saharan Africa. The specimen is well preserved, allowing us to use three-dimensional geometric morphometrics to compare the shape of this neurocranium to those of all extant species of Xenopus based on microcomputed tomography scans. Analyses revealed that this small fossil resembles diminutive extant species of Xenopus such as X. longipes. The fossil neurocranium preserves well-ossified tectum nasi and septum nasi, the latter separating large ovoid olfactory foramina, contributing to a more-ossified region surrounding the prootic foramen than observed in extant species of similar size. The Nsungwe Formation pipoid fossil contributes new information to the early biogeography and body-size diversification within the genus Xenopus.Abstract
Location of the Rukwa Rift Basin containing the Nsungwe Formation fossil-bearing localities. Star notes collection locality of RRBP 13274B.
High-resolution computed tomography (CT) scan of RRBP 13274B showing the neurocranium in dorsal (A), ventral (B), rostral (C), right lateral (D), and caudal (E) views; inset shows color photograph of specimen in same view. Also shown is the left stapes in left lateral view (F), which was isolated from the left otic region in the CT scan. Abbreviations: Ec, Eustachian canal; fp, frontoparietal; mc, attachment site for m. cucullaris; oc, occipital condyle; of, olfactory foramen; pk, palatine knob; pof, prootic foramen; sn, septum nasi; sph, sphenethmoid. Scale bars equal 1 mm.
Relationship of RRBP 13274B to patterns of neurocranium shape observed among extant species in the genus Xenopus. (A) Values of PC2 scores for each species are mapped onto the mtDNA phylogeny of Evans et al. (2015), revealing both differences between subgenus Silurana and multiple trends towards negative PC2 scores within subgenus Xenopus. (B) Morphospace of neurocrania of the genus Xenopus defined by PC1 and PC2 scores showing high overlap among species groups within subgenus Xenopus and similarities of RRBP 13274B to small species such as X. longipes.
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