Vowel Loss in Tirax and the History of the Apicolabial Shift

There has been speculation in the literature as to whether the shift from bilabial to apicolabial articulation (and often further to dental) is a sound change inherited by all of those northern Malakula–southern Santo languages that manifest it, or whether it developed in only one language and was borrowed into others. An examination of the phonological history of Tirax, a northern Malakula language, shows that the shift was a relatively late development in that language, and occurred after a rule deleting low vowels. This low vowel deletion rule did not apply in any other language showing the shift. We therefore assume that apicolabials were borrowed into Tirax (where they subsequently became dentals), and we make some proposals concerning the direction of this borrowing in the region.