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南京科学家发现最古老银杏木化石 距今约1.6亿年

2016-12-21 04:25:00   现代快报

由中科院南京地质古生物研究所研究员王永栋领衔的研究小组,在辽宁西部北票地区侏罗纪地层中发现了1.6亿年前的银杏木化石,命名为“辽宁银杏木”。这棵1.6亿岁的银杏,树木的细胞结构和现在的银杏细胞几乎一致。“这对研究银杏植物的演化以及恢复侏罗纪时期的古环境,有着重要意义。”王永栋说。

近日,这一研究成果发表在英国《自然》出版集团旗下的学术期刊《科学报告》上。

银杏木化石最少,非常稀有

在地球上,算得上“活化石”的少之又少。银杏就是其中之一。

王永栋介绍,之前,古生物学家在距今2.5亿年左右的地层中发现了不少类似的银杏叶片化石;而与现代银杏有关的银杏属叶子和果实化石,可以追溯到1.7亿年前。但是,银杏木化石,也就是茎干化石之前的记录是1.2亿年的白垩纪早期。“相比于保存较多的叶片化石,银杏的树干化石发现得最少,所以非常稀有。”

不过,这次王永栋他们的发现,刷新了纪录。他们在辽宁西部发现的银杏木化石距今约1.6亿年,往前推了4000万年。

距今约1.6亿年的银杏木化石标本。王永栋供图

王永栋的发现之旅始于6年前。当时,王永栋的团队在辽宁西部北票地区考察,在侏罗纪地层中发现了很多木化石。他们把这些木化石采集后拿到南京的实验室切片,然后放在显微镜下逐一研究,发现这些化石大多数是松柏类和苏铁类的木化石,尤其难得是还发现了一块银杏木化石。“由于所在地层距今大约1.6亿年,所以,这棵银杏木化石的时间距今约1.6亿年,是目前发现的最古老的银杏木化石。”

细胞结构和现在的几乎一致

中国地质科学院高级工程师蒋子堃博士介绍,现在的银杏叶片像小扇子,分裂不深,而约1.6亿年前的银杏叶像手指,叶子窄而细……虽然叶子的长相和现在不太相同,但细微的表皮结构却差不多。

显微镜下,化石切片被放大了两三百倍,科学家们对银杏木细胞结构进行了仔细的观察、比对。他们发现,古老的银杏木化石管胞结构和现在的银杏茎干的管胞构造十分相似。“侏罗纪时期的银杏木化石就已经拥有了和现代银杏相同的构造。”蒋子堃说。

既然和恐龙生活在一起,那么,它们会不会是恐龙口中的美食?王永栋说,这完全有可能。恐龙分为食肉类和食草类,高个子的恐龙脖子长,完全有可能去吃银杏叶。“不过,这还需要化石证据,比如在恐龙化石的胃中找到银杏叶或者果实化石的话,就可以推断其食性关系。但这样的化石很难发现。”

“很多人喜欢吃白果。事实上,不是所有的银杏都会结果子。我们发现,不管是古老的银杏,还是现在的银杏,雌树结果,雄树只长叶而不结果。”王永栋介绍。

此外,侏罗纪时期的银杏叶气孔的密度比现在的小很多。这是因为,侏罗纪时期二氧化碳浓度特别高,是现在二氧化碳浓度的4倍,“气孔的密度越小,反映了当时的古大气二氧化碳浓度就越高。”

王永栋说,恐龙称霸时期,也是银杏最繁盛的时期之一。那时候气候适宜,森林里古木参天。但6500万年前的生物大灭绝,也给银杏植物的发展演化带来了影响,其多样性大大减少。“6500万年前,银杏至少有十个属种以上,现在的银杏只有一个属。”王永栋说。

1.6亿年前南京的银杏不多

说起南京的秋天,自然会想起那满地金黄的银杏叶。但是1.6亿年前的侏罗纪中晚期,南京地区生长的银杏不太多。“不仅是南京,整个华南地区由于气候环境的原因,发现的银杏相关化石不是很丰富,但是蕨类和苏铁类较为丰富。”王永栋分析,银杏喜欢温凉又相对潮湿的环境,太干热和太寒冷,都不适合银杏的生长。

责任编辑:穆启春 CN063
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