Song Xikun, Ruthensteiner Bernhard, Lyu Mingxin, Liu Xi, Wang Jian, Han Jian
State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen, People's Republic of China.
SNSB-Zoologische Staatssammlung München, München, Germany.
Proc Biol Sci. 2021 Feb 10;288(1944):20202939. doi: 10.1098/rspb.2020.2939. Epub 2021 Feb 3.
Primitive cnidarians are crucial for elucidating the early evolution of metazoan body plans and life histories in the late Neoproterozoic and Palaeozoic. The highest complexity of both evolutionary aspects within cnidarians is found in extant hydrozoans. Many colonial hydrozoans coated with chitinous exoskeletons have the potential to form fossils; however, only a few fossils possibly representing hydroids have been reported, which still require scrutiny. Here, we present an exceptionally well-preserved hydroid found in the Upper Cambrian Fengshan Formation in northern China. It was originally interpreted as a problematic graptolite with an uncertain systematic position. Based on three characteristic morphological traits shared with extant hydroids (with paired hydrothecae, regular hydrocaulus internodes and special intrathecal origin pattern of hydrocladium), we propose this fossil hydroid as a new genus, gen. nov., affiliated with the advanced monophyletic hydrozoan clade Macrocolonia typically showing loss of the medusa stage. More Macrocolonia fossils reviewed here indicate that this life strategy of medusa loss has been achieved already as early as the Middle Devonian. The early stratigraphical appearance of such advanced hydroid contrasts with previous molecular hypotheses regarding the timing of medusozoan evolution, and may be indicative for understanding the Ediacaran cnidarian radiation.
原始刺胞动物对于阐明新元古代晚期和古生代后生动物身体结构和生活史的早期演化至关重要。刺胞动物这两个进化方面的最高复杂性存在于现存的水螅虫纲动物中。许多覆盖着几丁质外骨骼的群体水螅虫纲动物有形成化石的潜力；然而，仅有少数可能代表水螅体的化石被报道，这些化石仍需仔细审查。在此，我们展示了在中国北方上寒武统凤山组发现的一个保存异常完好的水螅体化石。它最初被解释为一种分类位置不确定的疑难笔石。基于与现存水螅体共有的三个特征形态特征（具成对的水螅鞘、规则的水螅茎节间以及水螅体鞘内特殊的起源模式），我们将这个化石水螅体定为一个新属，即新属，隶属于进化程度较高的单系水螅虫纲分支大群体水螅虫类，该类群通常表现为水母体阶段缺失。这里回顾的更多大群体水螅虫类化石表明，早在泥盆纪中期就已经实现了这种水母体缺失的生活策略。这种高等水螅体在早期地层中的出现与之前关于水螅虫类演化时间的分子假说形成对比，可能有助于理解埃迪卡拉纪刺胞动物的辐射演化。
