Ecological and Biogeographic Implications of Asian Cenozoic Fossil Floras

Abstract

This collection of articles highlights current paleobotanical research on the Paleogene and Neogene of Asia, following on a symposium held at the International Botanical Congress in Shenzhen, China in 2017, “Ecological and biogeographic implications of Asian Oligocene and Neogene fossil floras.” Fossil floras of this age, scattered across Asia, provide important data for assessing plant community response to changing topography and climate. During this time interval, from approximately 33.9 Ma to 2.58 Ma, the effects of rising mountains, oscillations of climate, and correlated changes in sea level have influenced development of the modern spatial pattern of plant diversity. The authors of this issue have used a variety of approaches to investigate different kinds of fossil plant remains including fruits, seeds, leaves, wood, and pollen to address questions of systematics, paleobiogeography, paleoecology, and climate change. During recent years, many new fossil floras have been investigated and various taxa have been described in detail (e.g., Huang et al., 2016; Quan et al., 2016; Yabe, 2017; Yabe & Nakagawa, 2018). Now is an appropriate time to gather the latest evidence of fossil records and paleoenvironmental data to discuss their ecological and biogeographic implications, thus to better understand mechanisms of biogeographic change. Asian fossil plant records are key to understanding patterns of plantmigration, radiation, and extirpation that have shaped present-day phytogeographic patterns in the Northern Hemisphere. Investigating the past biodiversity through geological time is critical to understand how this modern botanical richness formed (Popova et al., 2013; Huang et al., 2016; Yabe, 2017; Yabe & Nakagawa, 2018). We chose to focus the symposium on Oligocene and Neogene floras of Asia. The international stratigraphic community has largely abandoned the former recognition of the “Tertiary Period” in favor of the Paleogene and Neogene Periods; however, the conventional assignment of Oligocene epoch to the Paleogene makes the distinction between Paleogene and Neogene seem artificial because Oligocene vegetation shares more in common with the Neogene (Miocene through Pliocene) than with the Paleocene and Eocene vegetation. For this issue of JSE we have augmented the scope to include some contributions on Paleocene and Eocene plants. The articles herein fall under two overlapping themes: vegetation and climate assessed from fossil plant assemblages, and taxonomic and biogeographic history. Popova et al. (2019) document floristic change and continuity in western Siberian vegetation during the early Oligocene to earliest Miocene based on well-preserved fossil fruits and seeds. Based on the taxonomic identifications, the fossils were assigned to 26 plant functional types, including but not limited to C4 herbs, broad-leaved summer green arctic shrubs, broad-leaved evergreen warm temperate shrubs, broad-leaved evergreen xeric shrubs, needle-leaved evergreen boreal/temperate cold trees, drought-tolerant needleleaved evergreen trees, broad-leaved evergreen subtropical trees, broad-leaved summer green boreal/cold temperate trees, and aquatic components. The distribution of functional types was analyzed in a database comprising 66 carpofloras of Nikitin (2006). From these data, it can be concluded that temperatemesophytic, mixed conifer broad-leaved deciduous forest persisted in western Siberia throughout the Oligocene and earliest Miocene, supporting the concept of relatively stable climate conditions, without significant drying through this time interval in this region. The authors also detected an increase in taxonomic diversity, particularly concerning mesic herbs and deciduous trees and shrubs near the end of the Oligocene thatmight be related to increased precipitation and expansion of terrestrial habitats following retreat of the Paratethys sea. Yabe et al. (2019) present an informative and insightful overview of the paleobotanical history conifer genera that are presently endemic to eastern Asia, with emphasis on Oligocene and Neogene fossil floras of Japan and Korea. Six of the genera, namely Metasequoia, Glyptostrobus, Taiwania, Keteleeria, Pseudolarix, and Cunninghamia, occupied a wider range of environmental conditions in the past than their modern representatives. Their geographic distributions have shifted and/or receded since the late Miocene, possibly through habitat partitioning. They appear to have expanded their habitat toward warmer conditions during the midMiocene Climatic Optimum but then became restricted to warmer forest vegetation by the end of Pliocene. Cryptomeria and Thujopsis were found especially in floras of cooler temperate climate and they continued to maintain their range in the northern part of Japan where they are now endemic. During the late Miocene–Pliocene, the connection of the islands with the Asian continent functioned as a corridor for the dispersal of warm-adapted genera. Wu et al. (2019) report cupressaceous foliage from the early Oligocene of Guangdong Province, South China based on fossil foliage with well-preserved cuticle. Although there is a resemblance to foliage of the extinct mesophytic species of Tetraclinis salicornioides, known from attached cones and foliage in the Oligocene–Miocene of Europe and North America (Kva cek et al., 2000), the authors conclude that their leaves represent the extant genus Calocedrus, JSE Journal of Systematics and Evolution