When nothing does something: The proliferation and triumph of the third person plural preterite variant ending -eron in Old and Early Modern Spanish

It is well known that the Spanish third person plural of and verbs has two different preterite endings: primary , and the variant , which appears after stem-final , e.g., , , , etc., and that arose from through absorption of the semiconsonant [j] into the Old Spanish stem-final palatal [ʃ], e.g., > . What is not so well known is that OSp. , , like > , likewise reduced to , , but unlike , did not survive. It is also to date unknown that the reduction of > occurred in other grammatical categories, e.g., OSp. > , before reverting to . The present study will show when first arose and became the predominant variant of the two after , the extent to which it arose in other grammatical categories, and will explain why vertical vocalic symmetry between OSp. , , , later , , , was ulimately broken, and how the loss of , on the one hand, and the triumph of on the other, are completely interconnected, ironically tied to the same historical linguistic phenomenon, and actually two sides of the same coin.