State Key Laboratory of Systematic and Evolutionary Botany, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, P,R, China.
BMC Evol Biol. 2014 Apr 24;14:88. doi: 10.1186/1471-2148-14-88.
The 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.
Three 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.
Bauhinia 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.
泛热带槐蓝属（Bauhinia）与北温带紫荆属（Cercis）和几个热带属一起，具有二回羽状、二回三出复叶或有时为单叶，基于分子系统发育学，这些叶子构成了紫荆族（Cercideae），是豆科其余部分的姐妹群。因此，紫荆族的化石记录对于理解豆科植物的早期进化和生物地理历史至关重要。
在中国广西的渐新世宁明组中描述了三种槐蓝属的化石种。新种宁明槐蓝（Bauhinia ningmingensis sp. nov.）的特征为二回三出复叶，具心形叶，具基部互生的一级脉和叉状二级脉。Cheniae 槐蓝（B. cheniae sp. nov.）具有中度或深二回羽状、心形叶，具基部直出的一级脉和弯弓状二级脉。张殿芳和陈永福修正的 Larsen 槐蓝（B. larsenii D.X. Zhang et Y.F. Chen emend.）具有浅度或中度二回羽状、心形叶，具基部直出的一级脉和叉状二级脉，以及椭圆形、具柄、无翅和少种子的果实。同时，还回顾了以前报道的槐蓝属化石，由于缺乏可靠的证据证明其具叶枕或/和基部直出或互生的脉序，因此，世界上早于渐新世的槐蓝属叶化石要么是可疑的，要么已经被拒绝。除了这里展示的渐新世叶片和果实外，槐蓝属的叶片和/或木材也已在泰国、印度、尼泊尔、乌干达和厄瓜多尔的中新世-上新世被记录下来。
槐蓝属至少在渐新世就已经在宁明这样的低纬度地区表现出一定的多样性，具有二回羽状和二回三出复叶的物种，这表明中国南方的热带地区可能是该属早期多样化的中心之一。槐蓝属和紫荆属的可靠大化石在从中低纬度的始新世-渐新世植物群中首次出现，而在同期高纬度植物群中似乎缺乏，这表明豆科植物可能起源于特提斯海，并向外扩散。然而，槐蓝属及其近缘属的历史生物地理学的详细情况仍需要未来的古植物学和分子系统发育学研究提供更可靠的数据集。
