New observations on extinct Malvaceae from the lower Miocene of Japan: Fruit morphology, taphonomy, biogeography and paleocology of Banisteriaecarpum and associated foliage, pollen and wood

Abstract

Byttneriophyllum tiliifolium is a Malvaceae leaf that occurs from the late Oligocene to early Pliocene in Eurasia. A potential biological connection between By. tiliifolium and “samaras” assigned to Banisteriaecarpum giganteum has also been proposed. In this study, we report Ba. giganteum attached to a peduncle from the lower Miocene Nakamura Formation, exposed in Gifu Prefecture, central Japan. Notably, four scars were observed on the peduncle alongside a single Ba. giganteum, indicating that the schizocarp likely consists of five samaroid mericarps (i.e., Ba. giganteum), similar to the extant genus Triplochiton (Helictereae, subfamily Helicterioideae). This finding provides additional evidence supporting the placement of By. tiliifolium in Helictereae. Furthermore, we conducted palynological analyses of beds containing monodominant By. tiliifolium in the Nakamura Formation. Malvaceous pollen Intratriporopollenites instructus was recovered from these beds, with no other malvaceous pollen detected. This result supports a biological connection between By. tiliifolium and I. instructus, as previously noted in European localities. Our study suggests that the distribution of Helictereae may have expanded into the mid-latitude regions during the early to middle Miocene, a period characterized by a global climate warmer than today.