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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。






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


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