Why Spanish historical morphosyntax (badly) needs dialectology

It has long been a prevalent idea in the history of Spanish that the surge of literary monuments in the 13th led to the implantation of a solid standard superimposed on neighboring varieties as Castile expanded southwards. Recent research, however, has stressed the transfer of both Western and Eastern dialectal solutions into literary texts throughout the Middle Ages. A few key examples are offered here (mostly from the Early Modern period, whose morphosyntax has received far less attention) to show the importance of considering dialectal complexity in understanding syntactic innovation and diffusion in Spanish even far beyond the threshold of the Renaissance.