Pikaia gracilens Walcott, a stem-group chordate from the Middle Cambrian of British Columbia
Pikaia gracilens Walcott, a stem-group chordate from the Middle Cambrian of British Columbia
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DOI：
10.1111/j.1469-185x.2012.00220.x
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发表时间：
2012-05-01
影响因子：
10
通讯作者：
Caron, Jean-Bernard
中科院分区：
文献类型：
--
作者：
Morris, Simon Conway;Caron, Jean-Bernard
中寒武世Pikaia gracilens（Walcott）作为寒武纪脊索动物具有标志性的地位，但直到现在还没有详细的描述。在这里的114个可用标本的基础上，我们回顾其解剖，确认其在脊索动物的位置，并探索不同程度的信心，其关系，现存和灭绝的脊索动物和其他后口动物。Pikaia的身体是梭形的，横向压缩，拥有约100个肌节。头部较小，有两个裂片，并有两个狭窄的触角。没有眼睛的证据。除了一个薄背鳍（没有鳍条）和一系列至少9个两侧排列的附属物，在前端可能有咽孔，没有其他外部特征。除了肌肉组织外，内部解剖结构还包括消化道，其前部形成一个突出的透镜状单位，几乎总是以正浮雕形式保存。该腔被解释为咽腔，这意味着嘴本身几乎是终端。虽然肛门似乎是末端，但肠道的后部延伸不清楚。最突出的内部结构是一个反射性保存的单位，可能是中空的，这里称为背器官。虽然以前被解释为脊索，但它的位置和大小使其不太可能。它的原始功能仍然不确定，但它可能已经形成了一个存储器官。在背侧器官附近，一条较窄的组织被解释为代表神经索和脊索。除了这些结构外，还有证据表明存在血管系统，包括腹侧血管。
The Middle Cambrian Pikaia gracilens (Walcott) has an iconic position as a Cambrian chordate, but until now no detailed description has been available. Here on the basis of the 114 available specimens we review its anatomy, confirm its place in the chordates and explore with varying degrees of confidence its relationships to both extant and extinct chordates and other deuterostomes. The body of Pikaia is fusiform, laterally compressed and possesses about 100 myomeres. The head is small, bilobed and bears two narrow tentacles. There is no evidence for eyes. Apart from a thin dorsal fin (without finrays) and a series of at least nine bilaterally arranged appendages with possible pharyngeal pores at the anterior end, there are no other external features. In addition to the musculature the internal anatomy includes an alimentary canal, the anterior of which forms a prominent lenticular unit that is almost invariably preserved in positive relief. The cavity is interpreted as pharyngeal, implying that the mouth itself was almost terminal. The posterior extension of the gut is unclear although the anus appears to have been terminal. The most prominent internal structure is a reflectively preserved unit, possibly hollow, termed here the dorsal organ. Although formerly interpreted as a notochord its position and size make this less likely. Its original function remains uncertain, but it could have formed a storage organ. Ventral to the dorsal organ a narrower strand of tissue is interpreted as representing the nerve chord and notochord. In addition to these structures, there is also evidence for a vascular system, including a ventral blood vessel.