Dialects of the South American language Mapudungun are claimed to display a dental-alveolar contrast across four manners of consonant articulation: stops, nasals, laterals, and fricatives. Such a full, symmetrical system of distinctions among coronals is typologically unique and, as such, is predicted to be unstable. This paper’s survey of contemporary data, however, shows that, despite lexical contrast being marginal and dentals being morphologically restricted, the distinction is highly salient to native speakers of the more vital dialects. A careful examination of the pattern’s historical roots and diachronic stability, furthermore, allows us to reconstruct it throughout the 400-year textual record. Indeed, the early descriptions and transcriptions shown that, instead of contracting, the contrast expanded, by borrowing the alveolar fricative /s/ from Quechuan and Spanish. The historical and articulatory data shows that while /t̪ n̪ l̪ θ/ are laminal, /t n l/ are apical. Incoming /s/, however, does not follow the pattern, being laminal and prompting a reorganization of featural contrasts among fricatives. As a result of erosion of native fluency under Spanish contact, loss of the dental-alveolar contrast has become commonplace, although there is much variation across speaker, dialect, and manner of articulation. Crucially, dialects which had only voiced fricatives until the borrowing of /s/ seem to have added voicing as a new contrastive feature, helping to preserve the coronal contrast among fricatives, even where vitality is reduced.