近日,，《國際植物科學期刊》（International Journal of Plant Sciences）刊登了一項對北美第三紀銀杏胚珠器官及其伴生葉片的研究,。該項研究由中國科學院南京地質古生物研究所周志炎院士及其博士后全成等人完成。這是迄今為止首次也是唯一一例對新生代銀杏繁殖器官化石的細致研究,，也是繼周志炎等報道距今約1.7億年的最早的銀杏屬化石和距今約1.2億年的銀杏演化史上的缺失環(huán)節(jié)之后的又一項重要研究工作,。
作為裸子植物中一支獨特的類群，“活化石”銀杏具有非常悠久的演化歷史,。和現(xiàn)生銀杏相似的葉化石較為常見,，廣泛分布于南北半球自侏羅紀至第三紀的各個時期。但是,，具有更高分類價值的銀杏屬繁殖器官化石并不多見,，在新生代地層中尤其稀少，這使得人們對該屬在這一時期的演化歷程知之甚少,。
這項研究的化石材料產自美國北達科他州的晚古新統(tǒng),，包括有三維立體保存的銀杏胚珠器官（雌性繁殖器官），壓扁的種子以及葉片壓型化石,。這些化石最初為Peter Crane教授（FRS）等人發(fā)現(xiàn)和報道,。為了紀念他的這項工作，新發(fā)現(xiàn)的銀杏胚珠器官被命名為克倫銀杏Ginkgo cranei,?？藗愩y杏的胚珠器官和現(xiàn)代銀杏非常接近，它的胚珠不具有珠柄,，直接著生于總柄頂端的珠領上,。每個總柄上僅有一枚胚珠最終發(fā)育成熟。珠被角質層的氣孔器為雙環(huán)式,，保衛(wèi)細胞深度下陷,，副衛(wèi)細胞稍隆起。區(qū)別于現(xiàn)代銀杏的是,，克倫銀杏的種子較?。?0-19mm×12-17mm；現(xiàn)生種的胚珠長寬達30mm×20mm）,，氣孔器較為稀少,，珠被和珠領角質層普通表皮細胞外面觀呈穹頂狀，平周壁增厚,，垂周壁角質突緣較為發(fā)育,。相伴生的銀杏葉化石的角質層結構與胚珠器官的大致相似，氣孔器密度也比現(xiàn)生葉片稍低,。
長期以來,，銀杏屬的演化趨勢被認為是一個退縮的過程，但一直缺乏化石實證,。直至近二十多年來,，隨著中侏羅世和早白堊世銀杏屬繁殖器官的發(fā)現(xiàn)，我們對該屬演化歷史的認識才逐漸清晰起來?；C據顯示,，銀杏屬的演化趨勢為胚珠器官的珠柄逐漸退縮直至最終消失，胚珠體積不斷增加,，成熟胚珠數目逐漸減少直至僅有一枚種子直接著生于較粗強的總柄上,，葉片不斷扁化、蹼化和融合,。
克倫銀杏的發(fā)現(xiàn)和研究為新生代銀杏演化提供了確鑿的證據,，進一步證實了退縮的假說?？藗愩y杏的胚珠器官和葉片都已經與現(xiàn)生銀杏比較相似,，它的種子比現(xiàn)生銀杏小，但比侏羅紀的銀杏大,。在胚珠器官角質層微觀構造方面,，克倫銀杏和侏羅紀的義馬銀杏等比較，其珠被角質層較厚,，表皮細胞卻較小,，這些特征可能都與當時不利的環(huán)境條件有關。（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.
