Fossil fruits of Firmiana and Tilia from the middle Miocene of South Korea and the efficacy of the Bering land bridge for the migration of mesothermal plants

Abstract

Determining whether the high-latitude Bering land bridge (BLB) was ecologically suitable for the migration of mesothermal plants is significant for Holarctic phytogeographic inferences. Paleobotanical studies provide a critical source of data on the latitudinal positions of different plant lineages at different times, permitting assessment of the efficacy of the BLB for migration. Here we report exceptionally preserved fossils of Firmiana and Tilia endochrysea from the middle Miocene of South Korea. This represents a new reliable record of Firmiana and the first discovery of the T. endochrysea lineage in the fossil record of Asia. The occurrence of these fossils in South Korea indicates that the two lineages had a distribution that extended much farther north during the middle Miocene, but they were still geographically remote from the BLB. In light of the broader fossil record of Asia, our study shows that, in the middle Miocene, some mesothermal plants apparently inhabited the territory adjacent to the BLB and thus they were possibly capable of utilizing the BLB as a migratory corridor. Some other mesothermal plants, such as Firmiana and the T. endochrysea lineages, however, are restricted to more southern regions relative to the BLB based on current fossil evidence. These lineages may have been ecologically unable to traverse the BLB, which raises questions about the efficacy of the BLB as a universal exchange route for mesothermal plants between Asia and North America during the middle Miocene.

Fruits of Firmiana and Craigia (Malvaceae) from the Eocene of the Central Tibetan Plateau with emphasis on biogeographic history

“…Despite these previous findings of Firmiana and Craigia fossils, especially in East Asia (Xie et al, 2014; Jia et al, 2021; Wang et al, 2021a), no occurrence has been reported from what is now the Tibetan Plateau. The Tibetan Plateau, also called the “Third Pole” of the world (Qiu, 2008), is the highest plateau worldwide.…”

“…Because the specimens studied here are fin-winged fruits, we follow the venation terminology developed previously for leaves (Ellis, 2009), as well as terminology in prior relevant literature relating to these types of fruit (Manchester & O'Leary, 2010;Jia et al, 2021;Wang et al, 2021a). Initial comparisons were made with existing literature describing fin-winged fruits (Manchester & O'Leary, 2010) and herbarium sheets consulted online (Appendix I), including the Queensland Department of Environment and Science (2020), the New York Botanical Garden (NYBG), the National Herbarium of the Netherland (NHN), the Muséum National d'Histoire Naturelle, Herbier National (P), the Harvard University Herbaria (HUH), the Kew Herbarium (K), the Guangxi Institute of Botany (IBK) and the Chinese National Herbarium (PE).…”

“…There are only a few leaf fossil records of Firmiana and they are reported from the late Oligocene of India, and the Miocene of China and South Korea (Wolfe, 1977; Srivastava & Mehrotra, 2013; Xie et al, 2014; Jia et al, 2021). Fossil follicles are represented by only two species, Firmiana sinomiocenica Hu et Chaney and Firmiana yunnanensis Xie et Manchester from the Neogene in Asia (Table 1).…”

“…There are 17 modern species of Firmiana (POWO, 2019) ranging from South to East Asia and Malesia, and the diversity center today is in East Asia (Appendix ). The fruit is highly conservative among modern species with only few differences in the size, general shape, apex shape, and the midvein course (Tang et al, 2007; Xie et al, 2014; Jia et al, 2021). However, Xie et al (2014) show that these differences are often overlapping due to natural intraspecific variation of living species in China (Tang et al, 2007; Huang et al, 2011).…”

“…In the fossil record, foliaceous valves corresponding to Firmiana are found in Neogene deposits in China and South Korea, and leaves apparently representing the genus occur in the Oligocene of India (Sun, 1999;Srivastava and Mehrotra, 2013;Xie et al, 2014;Jia et al, 2021).…”