Leaves and fruits of Bauhinia (Leguminosae, Caesalpinioideae, Cercideae) from the Oligocene Ningming Formation of Guangxi, South China and their biogeographic implications

Abstract

BackgroundThe pantropical genus Bauhinia, along with the northern temperate Cercis and several tropical genera, bear bilobate, bifoliolate, or sometimes unifoliolate leaves, which constitute the tribe Cercideae as sister to the rest of the family Leguminosae based on molecular phylogenetics. Hence, the fossil record of Cercideae is pivotal to understand the early evolution and biogeographic history of legumes.ResultsThree fossil species of Bauhinia were described from the Oligocene Ningming Formation of Guangxi, South China. Bauhinia ningmingensis sp. nov. is characterized by its bifoliolate, pulvinate leaves bearing basal acrodromous primary veins and brochidodromous secondary veins. B. cheniae sp. nov. bears moderately or deeply bilobate, pulvinate leaves, with basal actinodromous primary veins and eucamptodromous secondary veins. B. larsenii D.X. Zhang et Y.F. Chen emend. possesses shallowly or moderately bilobate, pulvinate leaves bearing basal actinodromous primary veins and brochidodromous secondary veins, as well as elliptic, stipitate, non-winged, and oligo-seeded fruits. Meanwhile, previously reported Bauhinia fossils were reviewed, and those pre-Oligocene foliage across the world are either questionable or have been rejected due to lacking of reliable evidence for their pulvini or/and basal actinodromous or acrodromous venations. Besides Oligocene leaves and fruits presented here, foliage and/or wood of Bauhinia have been documented from the Miocene–Pliocene of Thailand, India, Nepal, Uganda, and Ecuador.ConclusionsBauhinia has exhibited a certain diversity with bifoliolate- and bilobate-leafed species in a low-latitude locality–Ningming since at least the Oligocene, implying that the tropical zone of South China may represent one of the centres for early diversification of the genus. The reliable macrofossils of Bauhinia and Cercis have made their debut in the Eocene–Oligocene floras from mid-low latitudes and appeared to lack in the coeval floras at high latitudes, implying a possible Tethys Seaway origin and spread of legumes. However, detailed scenarios for the historical biogeography of Bauhinia and its relatives still need more robust dataset from palaeobotany and molecular phylogeny in future research.

Bilobate leaves of Bauhinia (Leguminosae, Caesalpinioideae, Cercideae) from the middle Miocene of Fujian Province, southeastern China and their biogeographic implications

“…These records, however, have been either rejected or questioned due to lack of evidence for the pulvinus and/or basal actinodromous or acrodromous venation [30]. Paleobotanical evidence indicates that Cercis [75] and Bauhinia [30] (Table 2) had first appeared in the Eocene to Oligocene of mid-low latitudes in the Northern Hemisphere.…”

“…In addition, we examined the previously reported Bauhinia leaf fossils from the Oligocene Ningming Formation of Guangxi, South China [29, 30] and the Neogene of India and Nepal [38, 39, 42, 44]. The specimens are housed at Natural History Museum of Guangxi, Nanning (NHMG) and Birbal Sahni Institute of Palaeobotany, Lucknow (BSIP) (see Additional file 3).…”

“…The morphological interpretation and terms specifically for leaves of the Cercideae follow the literature [24, 27, 30]. Time calibrations and geological terms referred to the latest International Chronostratigraphic Chart [58].…”

“…2.0–6.0 × 2.2–6.5 cmFig. 2CIII hereNingming County, Guangxi Zhuang Autonomous Region, ChinaPossibly late Eocene or Oligocene[29, 30] B. larsenii D.X. Zhang et Y.F.…”

Cheniella gen. nov. (Leguminosae: Cercidoideae) from southern China, Indochina and Malesia

“…The fossil species Bauhinia cheniae Qi Wang, Z.Q.Song, Y.F.Chen, S.Shen & Z.Y.Li (Wang et al 2014) from the late Eocene-Oligocene Ningming Formation in Ningming County (Guangxi Zhuang Autonomous Region, South China) was likewise named for Prof. Chen Dezhao.…”