Advanced Cambrian hydroid fossils (Cnidaria: Hydrozoa) extend the medusozoan evolutionary history

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, Palaeodiphasia gen. nov., affiliated with the advanced monophyletic hydrozoan cl...原始刺胞动物（cnidarians）对于阐明新元古代晚期至古生代时期多细胞动物躯体构型与生活史的早期演化至关重要。刺胞动物类群中演化复杂度最高的类群当属现生水螅虫类（hydrozoans）。诸多被覆几丁质外骨骼（chitinous exoskeletons）的群居水螅虫类具备形成化石的潜力，但目前仅报道了少量疑似水螅类化石，且仍需进一步厘定。本文报道一件产自中国北方上寒武统凤山组的保存极为完好的水螅类化石。该标本最初被解释为分类位置不明的疑难笔石（graptolite）。基于其与现生水螅类共有的三项关键形态特征：成对的水螅体室（hydrothecae）、规则的水螅茎（hydrocaulus）节间结构，以及水螅分枝（hydrocladium）特有的鞘内起源模式，我们将该化石水螅类建立为一新属——古双相螅（Palaeodiphasia gen. nov.），隶属于先进的单系水螅虫类支系（clade）……