近日,《国际植物科学期刊》(International Journal of Plant Sciences)刊登了一项对北美第三纪银杏胚珠器官及其伴生叶片的研究。该项研究由中国科学院南京地质古生物研究所周志炎院士及其博士后全成等人完成。这是迄今为止首次也是唯一一例对新生代银杏繁殖器官化石的细致研究,也是继周志炎等报道距今约1.7亿年的最早的银杏属化石和距今约1.2亿年的银杏演化史上的缺失环节之后的又一项重要研究工作。
作为裸子植物中一支独特的类群,“活化石”银杏具有非常悠久的演化历史。和现生银杏相似的叶化石较为常见,广泛分布于南北半球自侏罗纪至第三纪的各个时期。但是,具有更高分类价值的银杏属繁殖器官化石并不多见,在新生代地层中尤其稀少,这使得人们对该属在这一时期的演化历程知之甚少。
这项研究的化石材料产自美国北达科他州的晚古新统,包括有三维立体保存的银杏胚珠器官(雌性繁殖器官),压扁的种子以及叶片压型化石。这些化石最初为Peter Crane教授(FRS)等人发现和报道。为了纪念他的这项工作,新发现的银杏胚珠器官被命名为克伦银杏Ginkgo cranei。克伦银杏的胚珠器官和现代银杏非常接近,它的胚珠不具有珠柄,直接着生于总柄顶端的珠领上。每个总柄上仅有一枚胚珠最终发育成熟。珠被角质层的气孔器为双环式,保卫细胞深度下陷,副卫细胞稍隆起。区别于现代银杏的是,克伦银杏的种子较小(10-19mm×12-17mm;现生种的胚珠长宽达30mm×20mm),气孔器较为稀少,珠被和珠领角质层普通表皮细胞外面观呈穹顶状,平周壁增厚,垂周壁角质突缘较为发育。相伴生的银杏叶化石的角质层结构与胚珠器官的大致相似,气孔器密度也比现生叶片稍低。
长期以来,银杏属的演化趋势被认为是一个退缩的过程,但一直缺乏化石实证。直至近二十多年来,随着中侏罗世和早白垩世银杏属繁殖器官的发现,我们对该属演化历史的认识才逐渐清晰起来。化石证据显示,银杏属的演化趋势为胚珠器官的珠柄逐渐退缩直至最终消失,胚珠体积不断增加,成熟胚珠数目逐渐减少直至仅有一枚种子直接着生于较粗强的总柄上,叶片不断扁化、蹼化和融合。
克伦银杏的发现和研究为新生代银杏演化提供了确凿的证据,进一步证实了退缩的假说。克伦银杏的胚珠器官和叶片都已经与现生银杏比较相似,它的种子比现生银杏小,但比侏罗纪的银杏大。在胚珠器官角质层微观构造方面,克伦银杏和侏罗纪的义马银杏等比较,其珠被角质层较厚,表皮细胞却较小,这些特征可能都与当时不利的环境条件有关。(生物谷Bioon.com)
doi:10.1086/662651
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Tertiary Ginkgo Ovulate Organs with Associated Leaves from North Dakota, U.S.A., and Their Evolutionary Significance
Zhiyan Zhou, Cheng Quan and Yu-Sheng (Christopher) Liu
The evolutionary history of Ginkgo is poorly understood for the Cenozoic Era because of the rarity of fossil reproductive organs. We here describe a new species, Ginkgo cranei sp. nov., on the basis of well-preserved ovulate organs and associated leaves from the Upper Paleocene Sentinel Butte Formation of North Dakota, USA. The ovulate organ is of the modern type, which lacks a pedicel supporting each of the two ovules. The ovules are seated in separate collars directly attached to the peduncle, but only one of them is mature. Stomatal complexes are mostly amphicyclic, with deeply sunken guard cells and slightly raised subsidiary cells. They are sparsely distributed among epidermal cells characterized by domelike, strongly bulging periclinal walls and developed anticlinal wall flanges in integument and collar cuticles. The associated leaves are generally similar to the ovulate organ in cuticular structure. Ginkgo cranei is the only Tertiary species of the genus described in which the ovulate organs are studied in some detail. The study further corroborates the hypothesis that modern Ginkgo evolved from its ancestors by reduction and is helpful to classify Cenozoic ginkgos in a natural system.