Department of Ecology and Evolutionary Biology and Biodiversity Institute, University of Kansas, Lawrence, KS 66044, USA.
Proc Natl Acad Sci U S A. 2012 Dec 18;109(51):20971-4. doi: 10.1073/pnas.1218879109. Epub 2012 Dec 3.
Our understanding of the evolution of life on Earth is limited by the imperfection of the fossil record. One reason for this imperfect record is that organisms without hard parts, such as bones, shells, and wood, have a very low potential to enter the fossil record. Occasionally, however, exceptional fossil deposits that preserve soft-bodied organisms provide a rare glimpse of the true biodiversity during past periods of Earth history. We here present an extraordinary find of a fossil ciliate that is encased inside the wall layer of a more than 200 Ma leech cocoon from Antarctica. The microfossil consists of a helically contractile stalk that attaches to a main body with a peristomial feeding apparatus and a large C-shaped macronucleus. It agrees in every aspect with the living bell animals, such as Vorticella. Vorticellids and similar peritrichs are vital constituents of aquatic ecosystems worldwide, but so far have lacked any fossil record. This discovery offers a glimpse of ancient soft-bodied protozoan biotas, and also highlights the potential of clitellate cocoons as microscopic "conservation traps" comparable to amber.
我们对地球生命演化的认识受到化石记录不完善的限制。造成这种不完整记录的原因之一是，没有硬体部分（如骨骼、壳和木材）的生物体进入化石记录的潜力非常低。然而，偶尔会有保存软体生物的特殊化石沉积物，为我们提供了过去地球历史时期真正生物多样性的罕见一瞥。我们在这里展示了一个非凡的发现，即在南极洲的一条 2 亿多年前的水蛭茧的壁层内发现了一个化石纤毛虫。这个微体化石由一个螺旋状收缩的柄组成，该柄附着在一个具有口围取食器官的主体上，还有一个大的 C 形大核。它在各个方面都与活的钟形动物，如旋口虫相符。旋口虫和类似的缘毛类是全球水生生态系统的重要组成部分，但迄今为止，它们缺乏任何化石记录。这一发现让我们得以一窥古代软体原生生物群，也凸显了环节动物茧作为微观“保护陷阱”的潜力，可与琥珀相媲美。
