A New Species of Comptonia (Myricaceae) from the Early Miocene of Central Inner Mongolia, China, and Phytogeographic History of Sweet-Fern

Simple Summary The deciduous shrub Comptonia is a monotypic genus of Myricaceae, which currently is distributed only in eastern North America, with a smaller range than that in other periods of the Cenozoic. By analyzing the macroscopic and microscopic characteristics of the leaves, we describe a new species of Comptonia (i.e., Comptonia hirsuta) from the Hannuoba Formation in Zhuozi, Inner Mongolia, North China. The co-occurring fruits were also studied based on their morphological characteristics, which were assigned to Comptonia tymensis. Variation in the distribution range of Comptonia indicates the influence of global cooling on the expansion of this plant. Furthermore, the Bering Land Bridge played an important role in the migration from North America to East Asia. The Thulean route may have provided an opportunity for plant exchange between western Europe and eastern North America. Moreover, the reason for the disappearance of Comptonia from China according to the analysis of the changes in both the global climate and the distribution of Comptonia fossils is also discussed. It is suggested that the climatic changes after the late Miocene and the progenitive pattern of Comptonia together caused the disappearance of Comptonia in China. Comptonia (Myricaceae) is well known as a monotypic genus living only in eastern North America; however, fossils show that the genus occurred extensively in the Northern Hemisphere during the Cenozoic. We observed dozens of Comptonia leaf fossils from the early Miocene in Zhuozi, China. The leaf architecture characteristics and epidermal features of the fossil specimens are described in detail here for the first time, and they were assigned to a new species: Comptonia hirsuta. The fruit fossils collected simultaneously from the same layer were assigned to Comptonia tymensis. The global fossil records indicate that the spatial distribution range of Comptonia reached its peak in both the Eocene and Miocene as two warm periods and then gradually decreased in the Oligocene, as well as after the late Miocene, because of the cooling global climate. Furthermore, the Comptonia taxon in East Asia may have migrated from North America via the Bering route in the late Paleocene or Eocene. Plant exchange between western Europe and eastern North America possibly occurred during the Eocene via the Thulean route. Phytogeographic variation in the Comptonia fossils from China also indicates that the reason for the disappearance of Comptonia from China may not only be due to the prolonged cooling and drying after the late Miocene, but also due to its progenitive pattern.