Modeling diverging diphthongization in esse reflexes from Latin to Spanish and Portuguese

It is known that the default diphthong structure in Latin was falling (vowel + semivowel) and that the most common diphthongs in the language were aj aw ej ew oj uj. Portuguese maintains this default diphthong structure, while Spanish has tended toward rising diphthongs. This divergence in diphthongization patterns affects the preterite, imperfect subjunctive, and future subjunctive paradigms of ser and ir in both languages. This paper proposes that a combination of changes in morphophonological parameters was responsible for the changes in the perfect reflexes of ESSE from Latin to Spanish and Portuguese. In Spanish, a stress shift to the thematic vowel, combined with a preference for rising diphthongs, caused the preservation and diphthongization (fue, fuimos) of the Latin sequences. In Portuguese, stress is assigned to the root, causing falling diphthongs in some reflexes (foi) while monophthongizing in others (fomos). The optimality-theoretic analysis herein harmonically models the divergences of Spanish and Portuguese from Latin and from each other.