This paper gives an example of how enriched diachronic treebank data can shed new light on an old and conflicted topic, even when that topic is morphological and semantic in nature rather than syntactic. The topic is the rise of the Russian po delimitatives, a change seen as crucial in most accounts of the history of Russian aspect, since it represents a major step in generalising the derivational aspect system. Earlier accounts concur that the po delimitatives spread fairly recently, too recently for the development to be connected to the loss of the aorist tense, which also had delimitative readings with atelic verbs. Using treebank data from the Tromsø Old Russian and OCS Treebank, enriched with tags for derivational morphology and semantics, I show that the po delimitatives were not marginal even in the earliest Slavic sources, neither in terms of frequency nor semantics, and that they first complemented and then competed with the delimitative aorists. It can thus be claimed that the exotic po delimitatives grew organically out of the old Indo-European inflectional aspect system. The data set includes the data files and R scripts necessary to replicate the study.