Department of Geology and Geophysics, Yale University, New Haven, CT 06520, USA.
Biol Lett. 2011 Dec 23;7(6):929-32. doi: 10.1098/rsbl.2011.0592. Epub 2011 Jul 6.
The oldest annelid fossils are polychaetes from the Cambrian Period. They are representatives of the annelid stem group and thus vital in any discussion of how we polarize the evolution of the crown group. Here, we describe a fossil polychaete from the Early Cambrian Sirius Passet fauna, Pygocirrus butyricampum gen. et sp. nov., with structures identified as pygidial cirri, which are recorded for the first time from Cambrian annelids. The body is slender and has biramous parapodia with chaetae organized in laterally oriented bundles. The presence of pygidial cirri is one of the characters that hitherto has defined the annelid crown group, which diversified during the Cambrian-Ordovician transition. The newly described fossil shows that this character had already developed within the total group by the Early Cambrian.
最古老的环节动物化石是来自寒武纪的多毛类。它们是环节动物主干群的代表，因此对于讨论冠群的进化如何两极分化至关重要。在这里，我们描述了来自早期寒武纪 Sirius Passet 动物群的一种化石多毛类，即 Pygocirrus butyricampum 属和种，其结构被鉴定为尾节刚毛，这是首次在寒武纪环节动物中记录到的结构。该生物体细长，具有双叉的附肢，刚毛排列成侧向定向的束。尾节刚毛的存在是迄今为止定义环节动物冠群的特征之一，该冠群在寒武纪-奥陶纪过渡期间多样化。新描述的化石表明，这个特征在早寒武世就已经在总群中发展起来了。
