The early Cambrian Emu Bay Shale radiodonts revisited: morphology and systematics

Two species of Radiodonta (stem-group Euarthropoda) from the Emu Bay Shale (Cambrian Series 2, Stage 4), Kangaroo Island, South Australia, are revised based on new field collections and insights from recent phylogenetic analyses and advances in knowledge of radiodonts globally. Anomalocaris briggsi Nedin, 1995, the most common Emu Bay Shale radiodont, is designated the type species of a new monotypic genus of Tamisiocarididae, Echidnacaris gen. nov. The less common species, previously identified as Anomalocaris aff. canadensis Whiteaves, 1892, is formally named Anomalocaris daleyae sp. nov. Oral cones are assigned to both Echidnacaris briggsi comb. nov. and A. daleyae based on that of the latter species being found in association with pairs of frontal appendages. The Echidnacaris briggsi oral cone is the best preserved for the family Tamisiocarididae; it is triradial, with three large plates and a more pervasive ornament of nodes than in any other known radiodont. Shared characters of the Echidnacaris and Anomalocaris oral cones add support for a sister group relationship between Tamisiocarididae and Anomalocarididae. Unique eye characters documented in E. briggsi, such as being sessile and encircled by an eye sclerite, are unknown in the other tamisiocaridids, Tamisiocaris and Houcaris, and are tentatively regarded as diagnostic for Echidnacaris. An ovate head element resembling that of Tamisiocaris borealis is assigned to E. briggsi, informed by the sister group relationship between these taxa. Isolated radiodont body flaps and sets of setal blades in the Emu Bay Shale cannot be confidently assigned to a species, although relative abundance suggests that many or most are likely E. briggsi. The inner attachment margin of the body flaps is sharply defined and may represent a suture at which flaps are shed in moulting. http://zoobank.org/urn:lsid:zoobank.org:pub:AEFDB294-AE8F-426D-9805-FC701798A986

基于新的野外采集成果、近期系统发育分析的研究进展以及全球放射齿目（Radiodonta）类群的研究认知提升，对产自澳大利亚南澳州袋鼠岛鸸鹋湾页岩（Emu Bay Shale，寒武系第二统第4阶）的两种放射齿目（干群真节肢动物（stem-group Euarthropoda））进行了修订。最常见的鸸鹋湾页岩放射齿类物种布里格斯奇虾（Anomalocaris briggsi Nedin, 1995），被指定为筛虾科（Tamisiocarididae）下一新单型属——针状奇虾属（Echidnacaris）gen. nov.的模式种。此前被鉴定为近似加拿大奇虾（Anomalocaris aff. canadensis Whiteaves, 1892）的较稀见物种，被正式命名为戴利奇虾（Anomalocaris daleyae）sp. nov.。基于戴利奇虾的口锥（oral cones）与成对前附肢（frontal appendages）共生的化石证据，布里格斯针状奇虾（Echidnacaris briggsi comb. nov.）与戴利奇虾均被归入对应物种的口锥化石。其中布里格斯针状奇虾的口锥是筛虾科已知保存最完好的类群：其呈三辐射对称，具有三块大型骨板，且覆盖的瘤状纹饰比其他所有已知放射齿类都更为广泛致密。针状奇虾属与奇虾属（Anomalocaris）口锥共有的特征，为筛虾科与奇虾科（Anomalocarididae）互为姊妹群的系统发育关系提供了支持。布里格斯针状奇虾（E. briggsi）具有独特的眼部特征，例如无柄且被眼骨片（eye sclerite）环绕，这类特征在其他筛虾科类群（筛虾属Tamisiocaris和Houcaris）中均未被发现，因此暂被视为针状奇虾属的鉴别特征。参考该类群与北方筛虾（Tamisiocaris borealis）的姊妹群关系，一件形态与北方筛虾相似的卵形头部结构被归入布里格斯针状奇虾。鸸鹋湾页岩中孤立的放射齿类躯体瓣（body flaps）和成组刚毛叶（setal blades）无法被可靠地归属到具体物种，尽管相对丰度表明其中多数或大多数可能属于布里格斯针状奇虾。躯体瓣的内侧附着边缘轮廓清晰，可能代表蜕皮过程中躯体瓣脱落的缝合线。http://zoobank.org/urn:lsid:zoobank.org:pub:AEFDB294-AE8F-426D-9805-FC701798A986