Research Article |
Corresponding author: Tao Su ( sutao@xtbg.org.cn ) Academic editor: Nianhe Xia
© 2020 Aye Thida Aung, Jian Huang, Truong Van Do, Ai Song, Jia Liu, Zhe-Kun Zhou, Tao Su.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Aung AT, Huang J, Do TV, Song A, Liu J, Zhou Z-K, Su T (2020) Three new fossil records of Equisetum (Equisetaceae) from the Neogene of south-western China and northern Vietnam. In: Jin X-H, Xia N-H, Tan Y-H (Eds) Plant diversity of Southeast Asia-II. PhytoKeys 138: 3-15. https://doi.org/10.3897/phytokeys.138.38674
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Three fossil species of Equisetum (Equisetaceae) were reported from the Neogene of south-western China and northern Vietnam, based on well-preserved rhizomes with tubers. Equisetum cf. pratense Ehrhart from the middle Miocene of Zhenyuan County, Yunnan Province, China is characterised by a bunch of three ovate tubers with longitudinal ridges on the surface. Equisetum yenbaiense A.T. Aung, T. Su, T.V. Do & Z.K. Zhou, sp. nov. from the late Miocene of Yenbai Province, Vietnam is characterised by four bunches of elongate tubers arranged in a whorl on a node. Equisetum yongpingense A.T. Aung, T. Su & Z.K. Zhou, sp. nov. from the late Pliocene of Yunnan is characterised by fibrous roots on most nodes and two to four bunches of large cylindrical tubers arranged in a whorl on a node. Floristic assemblages suggest that these species might have grown near a riverside or lakeshore. These new fossil records improve our understanding of species richness of Equisetum and their distribution range during the Neogene in Asia.
Diversity, Equisetum, Miocene, Pliocene, rhizome tubers
The order Equisetales, including Calamitaceae, Tchernoviaceae, Gonduanostachyaceae and Equisetaceae, has a very long evolutionary history that can be dated back to the Devonian (
In Equisetum fossils, tuberous rhizomes are the most commonly preserved organ in the Cenozoic strata around the world, such as in North America (
Although some species of Equisetum still survive in Asia nowadays, such as Equisetum diffusum, Equisetum hyemale and Equisetum pratense, its fossil records remain limited, especially during the Neogene. In this study, we describe three fossil species of Equisetum from the Neogene of south-western China and northern Vietnam. Further, we discuss the ecological and biogeographic implications, based on these new fossil records.
Zhenyuan, south-western China (the middle Miocene). A fossil was found in the Dajie Formation, Sanzhangtian Village, Zhenyuan County, central Yunnan Province, south-western China (Fig.
Map showing the locations of the fossils collected in this study. 1. Sanzhangtian, Zhenyuan County, Yunnan, south-western China (the middle Miocene); 2. Hop Thanh Village, Tuy Loc Commune, Yen Bai Province, northern Vietnam (the late Miocene); 3. Longmen, Yongping County, Yunnan Province, south-western China (the late Pliocene).
Yen Bai, northern Vietnam (the late Miocene). Fossils were found in the Co Phuc Formation, Hop Thanh Village, Tuy Loc Commune, Yen Bai Province, northern Vietnam (Fig.
Yongping, south-western China (the late Pliocene). The fossils were found in the Sanying Formation, Longmen Village, Yongping County, Yunnan Province, south-western China (Fig.
Equisetum fossils were imaged to view gross morphology by using a digital camera (Nikon D700) with a Kaiser 5510 stand and oblique light. To observe morphological characters in detail, fossils were photographed by stereoscope microscopes (Leica A8APO and ZEISS Smart Zoom 5). The contrast of images was slightly adjusted using the software Adobe Photoshop (version CC 2018). Morphological characters were measured by ImageJ (version 1.52). For comparison with previously published fossil taxa, we checked fossil records from online databases, for examples, Web of Science and Google Scholar. All fossil specimens in this study are deposited in the Paleoecology Collections, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences and Vietnam National Museum of Nature, Vietnam Academy of Science and Technology.
Order Equisetales Dumortier
Family Equisetaceae A. Michaux ex Alph. De Candolle
Genus Equisetum Linnaeus
XTBGSZTF0001 (Fig.
Dajie Formation, Sanzhangtian Village, Zhenyuan County, central Yunnan Province, South-western China (24.100N, 101.216E).
The middle Miocene.
Paleoecology Collections, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences.
Only one bunch of tubers are preserved on the specimen, tubers are ovate in shape, three tubers of equal size are arranged in one row (Fig.
XTBGVNMN4001 (Fig.
XTBGVNMN4002-4004 (Fig.
Hop Thanh Village, Tuy Loc Commune, Yen Bai Province, northern Vietnam (21.725N, 104.849E).
The late Miocene.
Paleoecology Collections, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences and Vietnam National Museum of Nature, Vietnam Academy of Science and Technology.
The species name ‘yenbaiense’ means that fossils are from Yen Bai Province, northern Vietnam.
Rhizomes with internodes and nodes, node round; Four bunches of tubers arranged in a whorl on a node (Fig.
Rhizomes have both internodes and nodes (Fig.
XTBGYP0748 (Fig.
Sanying Formation, Longmen Village, Yongping County, western Yunnan Province, south-western China (25.518N, 99.519E).
The late Pliocene.
Paleoecology Collections, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences.
The species name ‘yongpingense’ means that fossils are from Yongping County, south-western China.
Rhizomes with internodes and round nodes; fibrous roots on most nodes; two to four bunches of tubers arranged in a whorl on a node (Fig.
Rhizomes have both internodes and nodes (Fig.
In this study, we reported three new fossil records of Equisetum from south-western China and northern Vietnam, based on well-preserved rhizomes with tubers. In Equisetum, tubers are the most commonly preserved organ in the fossil records. They are mainly characterised by either a single tuber or one bunch of tubers with longitudinal ridges on the surface, present on fossils we collected (Figs
Morphological comparisons between new fossil records in this study and previously reported Equisetum and Equisetites fossil species.
Species | Tuber arrangement | Number of tubers per bunch | Shape of tuber | Tuber length (cm) | Tuber width (cm) | Age | Locality | References |
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Equisetum sp. | ? | 3 | Elliptical | 1.0–1.2 | 0.6–0.8 | Miocene | Iceland |
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Equisetum cf. arcticum | Whorl | 2–3 | Elongate, ovate, round | 1.5–2.5 | 0.5–0.7 | Oligocene to Miocene | America |
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Equisetum hunchunense | Whorl | 2–3 | Elliptical, ovate | 1.5 | 0.4–0.9 | Eocene | China |
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Equisetum jiuquanense | Single to acervate | 2–3 | Elliptical, ovate, round | 0.3–0.8 | 0.4–0.7 | Early Cretaceous | China |
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Equisetites longevaginatus | Single or opposite | 2–3 | Elliptical, round | 0.5–0.8 | 0.3–0.5 | Cretaceous | China |
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Equisetum oppositum | Opposite | 6 | Round, elliptical, ovate | 1.0–1.6 | 0.8–1.2 | Early Oligocene | China |
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Equisetum cf. oppositum | Single | 5–6 | Elliptical, ovate, and nearly round | 1.0–1.6 | 0.8–1.2 | Paleocene-Eocene | China |
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Equisetum cf. pratense | ? | 3 | Ovate | 0.9–1.2 | 0.6–0.8 | Middle Miocene | China | This study |
Equisetites vittatus | Whorl | 1–2 | Elliptical, elongate | 0.8–3.0 | 0.4–1.0 | Late Miocene | Vietnam | This study |
Equisetum yongpingense | Whorl | 1–2 | Cylindrical | 1.5–3.4 | 0.6–1.2 | Late Pliocene | China | This study |
Equisetum cf. pratense from the middle Miocene of Zhenyuan, south-western China have an ovate tuber shape, which is similar to most fossil records of Equisetum, but different from some fossil species, for example, the tubers of Equisetites longevaginatus are elliptical or round in shape (Table
For Equisetum yenbaiense from the late Miocene of northern Vietnam, the tuber arrangement is a whorl on a rhizome node, which also occurs in E. hunchunense and E. cf. arcticum. However, both E. hunchunense and E. cf. arcticum have two to three tubers per bunch, whereas E. yenbaiense only has one or two tubers per bunch, which might be partly due to the preservation of the fossil. In addition, there are more than three tubers per bunch in E. oppositum and E. cf. oppositum (Table
In Equisetum yongpingense, there are two to four bunches of tubers arranged in a whorl on a node, which has the same pattern as E. hunchunense and E. cf. arcticum. The tip of the tubers is mucronate in E. yongpingense, which is also present on other previously reported fossil species (Table
The three new fossil records described in this study expand the distribution of Equisetum during the Neogene in Asia. These findings and previous fossil records indicate that Equisetum has been widely distributed in Asia since the Paleocene and become diverse in Asia since the Oligocene. According to fossil assemblages, Equisetum mainly grew under wet conditions. Other taxa that were reported from the same stratum in Zhenyuan, such as Palaeosorum ellipticum (
We thank members from Paleoecology Research Group in Xishuangbanna Tropical Botanical Garden (XTBG), Chinese Academy of Sciences (CAS) for fossil collection; the Public Technology Service Center, XTBG for imaging; Dr. X-Y Liu and Dr. J-Y Wu for reviewing this manuscript and many constructive suggestions. This work is supported by the NSFC (National Natural Science Foundation of China)–NERC (Natural Environment Research Council of the United Kingdom) joint research programme (41661134049 and NE/P013805/1), the National Natural Science Foundation of China (41922010, U1502231, 31800183), Strategic Priority Research Programme of CAS (XDA20070301 and XDB26000000), Natural Science Foundation of Yunnan Province (2019FB026), and Southeast Asia Biodiversity Research Institute, Chinese Academy of Sciences (CAS-SEABRI) (No. Y4ZK111B01).