nature

当前位置: Nature » 动物学 » 正文

5亿年古老生物现新特征

摘要 : Metaspriggina walcotti是世界上已知最古老的脊椎动物之一。2012年,科学家在加拿大西南部发掘的出一批化石,更清晰地展示了这种生物的特征,并揭示了其额部的进化轨迹。原先,科学家对该生物的了解仅仅只能从两块非常零碎的化石中得来——这两块化石早在一个世纪之前就已经被发现,直到1993年科学家才第一次描述出这种生物的特征。

 metaspriggina walcotti是世界上已知最古老的脊椎动物之一。2012年,科学家在加拿大西南部发掘的出一批化石,更清晰地展示了这种生物的特征,并揭示了其额部的进化轨迹。原先,科学家对该生物的了解仅仅只能从两块非常零碎的化石中得来——这两块化石早在一个世纪之前就已经被发现,直到1993年科学家才第一次描述出这种生物的特征。

sn-gill

本次新发现的化石混杂在许多距今5亿年至5.15亿年的石块中,科学家从中提取出约100个化石样本,其中最大的有6厘米长。其中一些不完整的化石显示,该生物的体态在之后的进化过程中变得更长。

目前,最完整且保存最好的化石样本包含有该生物的眼睛、肌肉群及腮部,科学家将研究结果在线发表于近日的《自然》杂志。虽然研究者目前没有发现任何与鳍有关的线索,但他们发现这种鱼类非常好动,眼睛巨大且突出。该生物眼部周围有锋锐的圆形区域,这说明它的肉眼含有晶体,这在当时是进化的高级特征。

此外,该生物的腮部呈弧形结构,这是意义最重大的一个发现。新发现的以上两种特征预示着该生物的进化方向,而略厚的腮部则预示它进化的第一步就是下颚。科学家经过详细分析后认定,metaspriggina是最古老的鱼类之一,且是所有脊椎动物家族的始祖。

原文摘要:

A primitive fish from the Cambrian of North America

Simon Conway Morris & Jean-Bernard Caron

Knowledge of the early evolution of fish largely depends on soft-bodied material from the Lower (Series 2) Cambrian period of South China. Owing to the rarity of some of these forms and a general lack of comparative material from other deposits, interpretations of various features remain controversial, as do their wider relationships amongst post-Cambrian early un-skeletonized jawless vertebrates. Here we redescribe metaspriggina on the basis of new material from the Burgess Shale and exceptionally preserved material collected near Marble Canyon, British Columbia, and three other Cambrian Burgess Shale-type deposits from Laurentia. This primitive fish displays unambiguous vertebrate features: a notochord, a pair of prominent camera-type eyes, paired nasal sacs, possible cranium and arcualia, W-shaped myomeres, and a post-anal tail. A striking feature is the branchial area with an array of bipartite bars. Apart from the anterior-most bar, which appears to be slightly thicker, each is associated with externally located gills, possibly housed in pouches. Phylogenetic analysis places metaspriggina as a basal vertebrate, apparently close to the Chengjiang taxa Haikouichthys and Myllokunmingia, demonstrating also that this primitive group of fish was cosmopolitan during Lower–Middle Cambrian times (Series 2–3). However, the arrangement of the branchial region in metaspriggina has wider implications for reconstructing the morphology of the primitive vertebrate. Each bipartite bar is identified as being respectively equivalent to an epibranchial and ceratobranchial. This configuration suggests that a bipartite arrangement is primitive and reinforces the view that the branchial basket of lampreys is probably derived. Other features of metaspriggina, including the external position of the gills and possible absence of a gill opposite the more robust anterior-most bar, are characteristic of gnathostomes and so may be primitive within vertebrates.

来源: 中国科学报 浏览次数:18

热门文章TOP

RSS订阅 - 填写您的邮件地址,订阅我们的精彩内容: - 网站地图
网站联系电话:020-87540820 备案号:粤ICP备11050685号-8 增值电信业务经营许可证:粤B2-20120479
©2011-2015 生物帮 All rights reserved.