Byttneriophyllum tiliifoliumis a leaf fossil-species of the family Malvaceae that was distributed widely throughout Eurasia from the Miocene to the Pliocene. An affinity to some Malvadendrina subfamilies has been suggested forByttneriophyllum-bearing plants, but remains to be clarified due to insufficient information on other organs. Here, we report an exceptional lower Miocene fossil locality in Japan where a monodominant forest of the wood fossil-speciesWataria parviporaflourished. Notably, the forest floor was covered by a bed consisting almost exclusively ofB. tiliifolium. We observed occurrence modes ofB. tiliifoliumin this bed that confirmed that these leaves were deposited parautochthonously. These observations imply a biological connection betweenB. tiliifoliumandW. parvipora.The wood and leaf characters together might narrow the affinity ofByttneriophyllum-bearing plants down to Helicterioideae within the Malvadendrina, although it is also possible thatByttneriophyllum-bearing plants constitutes an extinct lineage which is characterized by a combination of morphological traits found in several extant lineages. Our results suggest thatByttneriophyllum-bearing plants started to inhabit swamps no later than the end of the early Miocene when the global temperature was getting warmer.