Eocene Podocarpium (Leguminosae) from South China and its biogeographic implications

Abstract

Podocarpium A. Braun ex Stizenberger is one of the most common legumes in the Neogene of Eurasia, including fossil fruits, seeds, leaves, and possible flower and pollen grains. This genus is not completely consistent with any extant genera according to gross morphological characters and poorly preserved cuticular structures reported in previous studies. The fossil pods collected from the coal-bearing series of the Changchang Basin of Hainan Island and Maoming Basin of Guangdong, South China, are examined by morphologically comparative work, with special reference to venation patterns and placental position. These distinctive features, as well as the ovule development of pods from different developmental stages and the epidermal structure of the pods, as distinguished from previous records lead to the conclusion that these fossils can be recognized as a new species of Podocarpium, P. eocenicum sp. nov. This new discovery indicates that Podocarpium had arrived in South China by the Eocene. Investigation on the fossil records of this extinct genus shows that P. eocenicum is the earliest and lowest latitude fossil data. The possible occurrence pattern of this genus is revealed as follows: Podocarpium had distributed in the South China at least in the middle Eocene, and then migrated to Europe during the Oligocene; in the Miocene this genus reached its peak in Eurasia, spreading extensively across subtropical areas to warm temperate areas; finally, Podocarpium shrank rapidly and became extinct in Eurasia during the Pliocene.

Keywords: Eocene; Leguminosae; Podocarpium; South China; phytogeography.

FIGURE 1

Geographic map of Changchang Basin, Hainan Island and Maoming Basin, Guangdong Province, and Stratigraphics of fossil localities. (A) Locations of Changchang Basin and Maoming Basin (red stars), drawn by QQX. (B) Lithostratigraphic column of Changchang Basin, modified from Spicer et al. (2014). Specimens were collected from the layers marked by red arrows. (C) Lithostratigraphic column of Maoming Basin, modified from Aleksandrova et al. (2012). Specimens were collected from the layer marked by red arrow.

FIGURE 2

Fossil pods of Podocarpium eocenicum sp. nov. (A) Dehisced pod with distinct prominent base (arrowhead) in the stipe. CC254a. (B) Counterpart of (A). CC254b. (C) Dehisced pod. CC1298. (D–H) Indehisced pod. (D) CC1162b. (E) CC1216a. (F) Indehisced pod wih distinct venations on the valve. CC1100a. (G) Counterpart of (F). CC1100b. (H) Incomplete pod with a long stipe and clear venation. CC1217. (I) Dehisced pod with venation structures. CC1164. (J,K) Indehisced pod. (J) MMJ1–002. (K) MMJ1–003a. Scale bar = 1 cm.

FIGURE 3

Different developmental stages of the seed of Podocarpium eocenicum sp. nov. (A–E) Different types of seed located near the apex of the placental suture in the juvenile phase. (A) Obovate seed. CC256a. (B) Counterpart of (A). CC256b. (C) Elliptical seed. CC1168. (D) Oblong seed. CC1300. (E) Obovate seed. CC1297a. (F–I) Seed located near the middle of valve in the maturing phase. (F) Elliptical seed with a short funiculus (arrowhead). CC1163a. (G) Elliptical to oblong seed situated close to the placental suture. CC1188a. (H) Elliptical seed close by the placental suture. CC1184. (I) Ellipsoidal seed in the center of the pod. CC698a. (J) Seed split out from the valve after maturity. CC1223. (K) Dehisced pod without seed. CC1171. Scale bar = 1 cm.

FIGURE 4

Cuticular structures of Podocarpium eocenicum sp. nov. (A) Cuticle from the outside of valves shows irregular arrangement of the epidermal cells and stomatal complexes. CC1100a. 10X. Scale bar = 50 μm. (B) Cuticle from the inner side of valves shows numerous crystals. CC1100a. 10X. Scale bar = 50 μm. (C) Cuticle from the outside of valves shows irregular arrangement of the epidermal cells. MMJ1–003a. 10X. Scale bar = 50 μm. (D) Details of the epidermal cells enlarged from (A). CC1100a. 40X. Scale bar = 50 μm. (E) Details of the epidermal cells enlarged from (C). MMJ1–003a. 40X. Scale bar = 50 μm. (F) Details of the epidermal cells and crystals enlarged from (B). CC1100a. 60X. Scale bar = 25 μm. (G) An open stomatal complex with clear outer ledges. CC1300. 60X. Scale bar = 25 μm. (H) A closed stomatal complex with seven subsidiary cells. CC1100a. 60X. Scale bar = 25 μm. (I) A closed stomatal complex with clear outer ledges. CC1222b. 60X. Scale bar = 25 μm. (J) The inner side of cuticle shows irregular arranged epidermal cells and stomatal complex; some cells have patty-like ornamentation. CC1100a. 500X. Scale bar = 50 μm. (K) Details of the inner side of a stomatal complex enlarged from (J). CC1100a. 2000X. Scale bar = 10 μm. (L) The inner side of cuticle shows irregularly arranged epidermal cells with unevenly thickened anticlinal walls. CC1175. 500X. Scale bar = 50 μm. (M) Details of the inner side of the stomatal complex enlarged from (L). CC1175. 2000X. Scale bar = 10 μm.

FIGURE 5

The distribution of megafossil records of Podocarpium on the modern world map [data from Liu et al. (2001b) and Wang et al. (2007)].

FIGURE 6

The climate zones [revised from Sun and Wang (2005)] and distribution of Podocarpium megafossils from the Eocene to Pliocene in China. (A) Eocene. (1) Changchang Basin, Hainan (the present paper). (2) Maoming Basin, Guangdong (the present paper). (B) Oligocene. (C) Miocene. (3) Chifeng, Inner Mongolia (Tao, 2000). (4) Linqu, Shandong (Hu and Chaney, 1940; Wang et al., 2007). (5) Sihong, Jiangsu (Li and Guo, 1982). (6) Nanjing, Jiangsu (Li and Guo, 1982). (7) Shengzhou, Zhejiang (Liu et al., 1996). (8) Ninghai, Zhejiang (Liu et al., 1996). (9) Tiantai, Zhejiang (Li, 2010). (10) Tiandong, Guangxi (Guo and Zhou, 1992). (11) Tianyang, Guangxi (Guo and Zhou, 1992). (12) Kaiyuan, Yunnan (Zhou, 1985). (13) Zeku, Qinghai (Guo, 1980). (14) Wulan, Qinghai (Liu et al., 1996). (D) Pliocene. (15) Taigu, Shanxi (Tao, 2000). (16) Yushe, Shanxi (Tao, 2000). (17) Nanjing, Jiangsu (Li and Guo, 1982). (18) Yuanmou Basin, Yunnan (Liu et al., 2002). Arid area is shown in white and humid areas is shown in gray.