Voiceless rhotic / retroflex consonants as an areal feature of the Atlantic languages

Voiceless rhotic or retroflex obstruents like [ʈʂ] or [tɹ̥] are rare in Africa, but the existence of a single such sound is relatively common among the Atlantic languages within Niger-Congo. The sound is not a shared retention across languages, but was repeatedly innovated through areal pressure. The sound can furthermore be reconstructed for multiple Atlantic protolanguages, and as such seems to have existed in the area for some time. This paper catalogues the occurrence of this rare sound in Atlantic languages as documented in existing sources, and suggests that due to its persistence in the area through time, it is relatively likely that the most recent common ancestor of the Atlantic languages contained a sound similar to [ʈ] or [ʈʂ], supporting existing proposals which reconstruct such a sound.