The variability of fruit stone characteristics of medieval fossil forms and modern ones was analysed in a small Prunus spinosa (blackthorn) population in the vicinity of Castle Kolno near Stare Kolnie in Opole Province. A modern putative hybrid resulting from natural crossing with cultivated plums was discovered, corresponding to some fossil morphotypes. The analysis of the correlations of features indicated developmental relationships between them. The pattern of variability of the examined stones in the ordination space confirmed the development of the Prunus spinosa-Prunus domestica complex as an element of a dynamic syngameon. The population also showed variations in leaf morphotypes, which may be used in taxonomic analysis.