Monday 5 May 2014

Fossil Legumes from the Oligocene of Guangxi Province, South China.

The Legumes are one of the most successful groups of modern plants, with over 19 000 described species; over 9% of all known Dicots are Legumes. The group owe their success largely to the presence of symbiotic nitrogen-fixing Bacteria in their roots, which enable Legumes to thrive in nutrient-poor soils. The group dates back to at least the Palaeocene, with most of the modern groups established by the middle Eocene.

In a paper published in the journal BMC Evolutionary Biology on 24 April 2014, Qi Wang of the State Key Laboratory of Systematic and Evolutionary Botany at the Institute of Botany of the Chinese Academy of Sciences, Zhuqiu Song and Yunfa Chen of the Natural History Museum of Guangxi and Si Shen and Zhenyu Li, also of the State Key Laboratory of Systematic and Evolutionary Botany, describe two new species of Legume from the Ningming Formation of Guangxi Province, South China. The dating of the of Ningming Formation is imprecise, since it contains neither stratographically useful fossils nor volcanic layers which could be used for isotope dating, but it is believed to be at least Oligocene in age.

Both the two new species are placed in the extant genus Bauhinia. This genus is known to be paraphyletic, as it has been used for all tropical members of the family Cercideae and does not reflect the relationships of the plants, but members of the genus are often hard to tell apart without molecular data, and in addition a number of popular ornamental plants are found within it, so it is unlikely to be reformed in the near future. More interestingly the Cercideae are thought to be the first extant Legume family to have diverged from the rest of the group, making understanding its origins important in the study of Legumes, but to date it has had no fossil record older than the Miocene.

The first new species described is named Bauhinia ningmingensis, meaning 'from Ningming'. This species is named from two small leaves, 4.0-5.3 cm in length.

The leaves of Bauhinia ningmingensis. Scale bars are 1 cm. Wang et al. (2014).

The second new species is named Bauhinia cheniae, after Chen Dezhao of the South China Botanical Garden, 'for her important contribution to the taxonomy of Cercideae'. The species is described from 18 leaves, 2.0-6.0 cm in length.

Leaf of Bauhinia chenae. Red arrow indicates a short spine in the sinus. Green arrow shows a semicircular laminar joint at the leaf base. Scale bar is 1 cm. Wang et al. (2014).

A number of pre-Miocene Cercideae fossils have been reported before, but all of these are to poorly preserved for confident diagnosis, or apparently completely erroneous. The Ningming fossils are, in contrast, well preserved and can be placed confidently within the family. The group is thought to have originally arisen in east Asia, and spread first along the shores of the (now vanished) Tethys Ocean (as are Legumes in general), so the Ningming fossils appear to come from close to the source of the group, though it is likely that the Cercideae originated much earlier than these fossils, and that older members of the group will come to light in the future.

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