Allomorphy in the Hittite common gender accusative plural

It has recently become generally accepted that Hittite possessed two non-low back vowel phonemes denoted by means of plene spelling with ⟨u⟩ and ⟨ú⟩, representing /o/ and /u/, respectively. Upon reviewing all available evidence of plene spelling in the Hittite common gender accusative plural ending it is observed that the choice of vowel sign alternates. This forces us to reckon with at least two allomorphs of the ACC.PL.C. ending, -/us/ and -/os/, which in turn demand explanations. The choice of ending appears to be largely contingent on the stem type of the nominal to which it is attached, forming a complementary distribution. For example, ablauting u-stems take -/os/ and i-stems take -/us/. Building on this observation, a diachronic scenario is formulated to account for all observable ending allomorphy. It is argued that the endings of the non-ablauting i-stems and the barytone a-stems were analogically introduced. In the i-stems, the inherited ending -/us/ of the ablauting i-stems was generalised to all i-stems, whereas the source of the ending in the a-stems remains unknown. The sound laws resulting from this analysis indicate that the vocalic outcomes of final *-(V)m# are symmetrical to those of *-(V)ms#, leading to an improved economy in Hittite historical phonology.