Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada.
Am J Bot. 2009 Feb;96(2):487-97. doi: 10.3732/ajb.0800307.
The evolutionary history of diatoms is only constrained partially by the fossil record. The timing of several key events, such as initial colonization of freshwater habitats by marine taxa, remains poorly resolved. Numerous specimens of the genera Cyclotella, Discostella, and Puncticulata (Ochrophyta: Thalassiosirales) have been recovered in Middle Eocene lacustrine sediments from the Giraffe Pipe locality in the Northwest Territories, Canada. These diatoms extend the fossil record of the family Stephanodiscaceae to at least 40 million years before present (Ma) and thus provide a new evolutionary milepost for the thalassiosiroid diatoms, an important clade of centric diatoms with global representation in both marine and freshwater environments. The quality of the fossil material enables detailed investigations of areolae, fultoportulae, and rimoportulae, revealing direct morphological affinities with a number of extant taxa. These observations extend the antiquity of several characters of phylogenetic importance within the thalassiosiroid diatoms, including the fultoportula, and imply that the entire lineage is considerably older than prior constraints from the fossil record, as suggested independently by several recent molecular phylogenies.
硅藻的进化历史仅部分受到化石记录的限制。一些关键事件的时间，例如海洋分类群最初向淡水生境的殖民化，仍然没有得到很好的解决。在加拿大西北地区长颈鹿管地点的中始新世湖相沉积物中，已经发现了 Cyclotella、Discostella 和 Puncticulata（黄藻门：Thalassiosirales）属的许多标本。这些硅藻将 Stephanodiscaceae 科的化石记录至少向前推到了 4000 万年前，因此为 thalasiosiroid 硅藻提供了一个新的进化里程碑，thalasiosiroid 硅藻是中心硅藻的一个重要分支，在海洋和淡水环境中都有全球代表性。化石材料的质量使得对类囊体、副孔和边缘孔的详细研究成为可能，揭示了与许多现存分类群的直接形态相似性。这些观察结果扩展了 thalasiosiroid 硅藻中几个具有系统发育重要性的特征的古老性，包括副孔，并且意味着整个谱系比以前的化石记录所暗示的要古老得多，这是由几个最近的分子系统发育独立提出的。
