Department of Geology and Geophysics, Yale University, New Haven, CT 06520, USA.
Evol Dev. 2010 Mar-Apr;12(2):201-9. doi: 10.1111/j.1525-142X.2010.00404.x.
Dickinsonia is one of the most recognizable forms in the Ediacaran fauna, but its phylogenetic position has been contentious, and it has been placed in almost every kingdom of life. Here, it is hypothesized that the affinities of Dickinsonia lie with the Placozoa (Metazoa), an understudied phylum that is widespread in tropical seas worldwide. Modern placozoans show obvious differences in size and axial organization compared with Dickinsonia, but these differences can be accounted for by the stem-group/crown-group distinction. The affinity with placozoans is evidenced primarily by the unique feeding mode of Dickinsonia, which is demonstrated by a series of feeding traces. These traces indicate that Dickinsonia moved over the Ediacaran matgrounds, and digested the mat using its entire lower sole. The ability of Dickinsonia to move negates an algal, fungal, or sponge affinity, while the feeding mode, external digestion with a ventral sole, rules out placement within any sponge or eumetazoan lineage. The only organisms that both move and feed in this manner are placozoans. Recent molecular phylogenetic studies have demonstrated that placozoans lie above sponges but below Eumetazoa. We hypothesize that Dickinsonia and other externally digesting Ediacaran forms are either stem-placozoans, or a series of extinct lineages above sponges and below eumetazoans on the metazoan tree. We discuss the potential evolutionary transitions between the main metazoan feeding modes in the context of the emerging molecular phylogeny, and suggest that aspects of the sponge and placozoan feeding strategies are relicts of nonuniformitarian Proterozoic ocean conditions.
狄更逊水母是埃迪卡拉纪动物群中最具代表性的形态之一，但它的系统发育位置一直存在争议，曾被归入几乎所有的生命王国。在这里，我们假设狄更逊水母与扁盘动物门（后生动物）有关，这是一个研究较少的门，在全世界的热带海域广泛分布。现代扁盘动物在大小和轴向组织上与狄更逊水母明显不同，但这些差异可以通过祖先进化枝和现代进化枝的区别来解释。与扁盘动物的亲缘关系主要是由狄更逊水母独特的摄食方式来证明的，这一方式通过一系列摄食痕迹来体现。这些痕迹表明，狄更逊水母在埃迪卡拉纪垫状地物上移动，并利用其整个下底面来消化垫状地物。狄更逊水母的移动能力否定了它与藻类、真菌或海绵的亲缘关系，而外部消化和腹面底面的摄食方式则排除了它在任何海绵或后生动物谱系中的位置。唯一以这种方式移动和摄食的生物是扁盘动物。最近的分子系统发育研究表明，扁盘动物位于海绵动物之上，但在后生动物之下。我们假设，狄更逊水母和其他外部消化的埃迪卡拉纪形态要么是原始扁盘动物，要么是一系列已经灭绝的谱系，位于海绵动物和后生动物之上，在后生动物进化树上。我们讨论了主要后生动物摄食方式之间的潜在进化转变，结合新兴的分子系统发育，提出了海绵动物和扁盘动物摄食策略的某些方面是不均匀的前寒武纪海洋条件的遗迹。
