Srivastava Gaurav, Mehrotra R C
Birbal Sahni Institute of Palaeobotany, Lucknow, India.
PLoS One. 2014 Oct 29;9(10):e111140. doi: 10.1371/journal.pone.0111140. eCollection 2014.
The family Phyllanthaceae has a predominantly pantropical distribution. Of its several genera, Bridelia Willd. is of a special interest because it has disjunct equally distributed species in Africa and tropical Asia i.e. 18-20 species in Africa-Madagascar (all endemic) and 18 species in tropical Asia (some shared with Australia). On the basis of molecular phylogenetic study on Bridelia, it has been suggested that the genus evolved in Southeast Asia around 33±5 Ma, while speciation and migration to other parts of the world occurred at 10±2 Ma. Fossil records of Bridelia are equally important to support the molecular phylogenetic studies and plate tectonic models.
We describe a new fossil leaf of Bridelia from the late Oligocene (Chattian, 28.4-23 Ma) sediments of Assam, India. The detailed venation pattern of the fossil suggests its affinities with the extant B. ovata, B. retusa and B. stipularis. Based on the present fossil evidence and the known fossil records of Bridelia from the Tertiary sediments of Nepal and India, we infer that the genus evolved in India during the late Oligocene (Chattian, 28.4-23 Ma) and speciation occurred during the Miocene. The stem lineage of the genus migrated to Africa via "Iranian route" and again speciosed in Africa-Madagascar during the late Neogene resulting in the emergence of African endemic clades. Similarly, the genus also migrated to Southeast Asia via Myanmar after the complete suturing of Indian and Eurasian plates. The emergence and speciation of the genus in Asia and Africa is the result of climate change during the Cenozoic.
On the basis of present and known fossil records of Bridelia, we have concluded that the genus evolved during the late Oligocene in northeast India. During the Neogene, the genus diversified and migrated to Southeast Asia via Myanmar and Africa via "Iranian Route".
叶下珠科主要分布于泛热带地区。在其多个属中，土蜜树属（Bridelia Willd.）尤为引人关注，因为它在非洲和热带亚洲有间断且分布均匀的物种，即在非洲 - 马达加斯加有18 - 20种（均为特有种），在热带亚洲有18种（有些与澳大利亚共有）。基于对土蜜树属的分子系统发育研究，有人提出该属大约在33±5百万年前于东南亚演化，而物种形成和向世界其他地区的迁移发生在10±2百万年前。土蜜树属的化石记录对于支持分子系统发育研究和板块构造模型同样重要。
我们描述了一种来自印度阿萨姆邦渐新世晚期（查特期，28.4 - 23百万年前）沉积物中的土蜜树属新化石叶。该化石详细的脉序模式表明它与现存的卵形土蜜树（B. ovata）、钝叶土蜜树（B. retusa）和具托叶土蜜树（B. stipularis）有亲缘关系。基于目前的化石证据以及来自尼泊尔和印度第三纪沉积物中土蜜树属的已知化石记录，我们推断该属在渐新世晚期（查特期，28.4 - 23百万年前）于印度演化，物种形成发生在中新世。该属的茎系通过“伊朗路线”迁移到非洲，并在新近纪晚期在非洲 - 马达加斯加再次物种形成，导致非洲特有类群的出现。同样，在印度板块与欧亚板块完全缝合之后，该属也通过缅甸迁移到了东南亚。该属在亚洲和非洲的出现及物种形成是新生代气候变化的结果。
基于土蜜树属目前和已知的化石记录，我们得出结论，该属在渐新世晚期于印度东北部演化。在新近纪，该属多样化并通过缅甸迁移到东南亚，通过“伊朗路线”迁移到非洲。
