Evidence for the evolutionary history and diversity of fossil sweetgums: leaves and associated capitate reproductive structures of Liquidambar from the Eocene of Hainan Island, South China

Abstract

Extant species of Liquidambar L. exhibit a disjunct distribution between western and eastern Asia and eastern North America to Central America, with the highest species diversity being in China. In this study, two new species of Liquidambar are described from the Eocene of Hainan Island, South China: L. hainanensis on basis of the leaf fossils, and L. ovoidea based on compressions of infructescences. The co‐occurrences of L. hainanensis leaves, associated infructescences of L. ovoidea and dispersed pantoporate pollen grains have the combination of morphological features peculiar to the genera Liquidambar and former Semiliquidambar and Altingia. This is considered further palaeobotanical justification for the placement of these three genera into a single genus, Liquidambar, proposed earlier based on molecular markers. Our data provide evidence for the occurrence of an ancestral polymorphic group of Liquidambar species (L. maomingensis with associated capitate infructescences, L. bella, L. hainanensis and L. ovoidea) during the Eocene in South China, and corroborate the view that South China could have been a centre of Liquidambar speciation during the Eocene. The morphological similarity of L. hainanensis leaves to those of the extant species L. orientalis from Western Asia and the North American L. styraciflua may support the importance of both the North Atlantic Land Bridge and the Bering Land Bridge for the dispersal of Liquidambar. Analysis of fossil species of Liquidambar from eastern Asia also contributes to further understanding of patterns in the taxonomic diversity and evolutionary history of this genus.

Structurally Preserved Liquidambar Infructescences, Associated Pollen, and Leaves from the Late Oligocene of the Nanning Basin, South China

“…In this case, the associated leaves and pollen provide indirect evidence that the fossil inflorescences/infructescences may belong to the same taxon. The presence of visible rims of tissues between adjacent fruits seems to be a distinctive feature of altingioid capitate reproductive organs with absent or inconspicuous styles [ 30 , 32 ].…”

“…The evolutionary history of the family Altingiaceae has become increasingly clear in recent years due to the recovery of new fossil records. Most of the Liquidambar fossils are lobate leaf fossils widely represented in the Paleogene and Neogene of North America, Asia, and Europe [ 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 ]. The oldest Liquidambar pollen [ 35 , 36 , 37 , 38 ], wood [ 39 , 40 ], and leaf morphotypes [ 30 ] are known from the Paleocene.…”

“…Three-dimensionally preserved reliable infructescences of Liquidambar have been described only from the Miocene [ 33 , 42 ]. Numerous capitate infructescences assigned to Liquidambar are preserved as compressions in the Cenozoic of Asia, Europe, and North America [ 8 , 10 , 18 , 20 , 22 , 30 , 32 , 49 ]. Furthermore, co-occurrences of Liquidambar leaves and reproductive structures are of considerable interest [ 18 , 22 , 30 , 32 , 33 , 50 ].…”

“…Numerous capitate infructescences assigned to Liquidambar are preserved as compressions in the Cenozoic of Asia, Europe, and North America [ 8 , 10 , 18 , 20 , 22 , 30 , 32 , 49 ]. Furthermore, co-occurrences of Liquidambar leaves and reproductive structures are of considerable interest [ 18 , 22 , 30 , 32 , 33 , 50 ]. In China, altingioid megafossils are known from the Eocene [ 30 , 31 , 32 , 51 , 52 , 53 ], Miocene [ 7 , 28 , 33 , 50 , 54 ], and Upper Pleistocene [ 54 ].…”

“…Furthermore, co-occurrences of Liquidambar leaves and reproductive structures are of considerable interest [ 18 , 22 , 30 , 32 , 33 , 50 ]. In China, altingioid megafossils are known from the Eocene [ 30 , 31 , 32 , 51 , 52 , 53 ], Miocene [ 7 , 28 , 33 , 50 , 54 ], and Upper Pleistocene [ 54 ]. Until now, megafossils of Liquidambar from the Oligocene of China have not been reported.…”