1962 年, M. J. Copeland 报道了加拿大北极梅尔维尔岛中–上泥盆统梅尔维尔岛组的叶肢介化石。2014 年秋, 作者对馆藏在加拿大地质调查所的模式标本进行了观察、照相, 发现了一些新的重要的分类特征, 在此予以补充修订。1、壳瓣具有 3–5 条放射脊的翼李氏叶肢介属(Pteroleaia)是梅尔维尔岛泥盆纪最具特色的类型, 与其保存在同一层面上还发现有另一类个体十分微小的棒叶肢介属(Rhabdostichus), 该属与美国中北部及加拿大艾伯塔地区同时代的叶肢介可以对比, 之前这一类型的标本仅见于该地区中–晚泥盆世地层, 其在北极地区的出现为首次报道。同时对归于棒叶肢介科(Rhabdostichidae)的另外两属(Rhabdostichops 和 Archaeositum)的含义进行了甄别, 认为前者建属特征不成立, 后者不应属于该科。2、产于梅尔维尔岛 Dundas 半岛的叶肢介, 个体数量多且密集堆积, 曾被鉴定为 Asmussia canadensis 的那些个体的壳顶都发育有一个细小的、凸起的卵形瘤状构造, 依据这一重要鉴定特征, 其分类位置应属于锥顶叶肢介亚科(Vertexiinae)的犄叶肢介属(Cornia), 从而使这一属的地质历程从石炭纪追溯到中泥盆世。这些化石的修订, 丰富了我们对加拿大北极地区泥盆纪叶肢介面貌的认识, 为研究相关类群的起源、演化提供了新的信息。文中也讨论了李氏叶肢介亚目(Leaiina)三个早期叶肢介类群之间的相互关系, 认为 Pteroleaia (Rostroleaiidae)代表了形态特殊的一个演化分枝, 早仅见于泥盆纪并很快灭绝。壳瓣具有两条放射脊的叶肢介科(如: Leaiidae)被认为是李氏叶肢介亚目的演化主干, 兴盛于石炭纪和二叠纪。广东中泥盆世早期(Givetian)桂头群中的叶肢介组合以 Hepuleaia (Praeleaiidae)为主, 并包含壳瓣具两条放射脊的种类, 该组合是迄今已知李氏叶肢介亚目的最早代表, 这表明该亚目应起源于中国而非欧洲。

In 1962, M. J. Copeland studied and published the clam shrimp specimens from the Devonian MelvilleIsland Formation of the Canadian Arctic. The authors of this paper examined Copeland’s type specimens during their visit to the Geological Survey of Canada at Ottawa in the fall of 2014. Based on the newly discovered taxonomic characters observed on the carapace, the following revisions have been made. The multi-ribbed genus Pteroleaia (Pteroleaiinae of Rostroleaiidae) represents the most special group of clam shrimp in the Middle or Upper Devonian strata in the Canadian Arctic. In addition to Pteroleaia, the same slab from locality 24866 (8 km southwest of Kelly Point) also contains a new species, Rhabdostichus arcticensis sp. nov. (Rhabdostichidae), which represents the first record of the genus in the Canadian Arctic beyond North and Central USA and Alberta, Canada. The specimens from Dundas Peninsula that were previously identified as Asmussia canadensis should be reassigned to the genus Cornia (Vertexiinae of Lioesthriidae) based on the presence of a node on the larval valve. Thus, the geological history of this genus can be traced back to the Middle Devonian. These revisions not only have enriched our knowledge on the Devonian clam shrimp group in the Canadian Arctic but also have added new information on the origin and the early history of the leaiin clam shrimp. In addition, interrelationships among the three early groups of the Leaiina are discussed. The multi-ribbed Pteroleaia from the Canadian Artic is very different from the early forms from Estonia and China in that its carapace possesses 3–5 radial carinae and the growth lines are sinuously recurved. Pteroleaia is a special form that represents a primitive stage in the evolution of the Leaiina and became extinct shortly after its origin. The two-ribbed Leaiidae have been considered to represent the main line of the leaiin evolution. The Hepuleaia-dominated clam shrimp assemblage of the Givetian strata of Guangdong, China, which also contains tworibbed forms, represents the earliest occurrence of the Leaiina, suggesting that the suborder may have originated in China rather than Europe.

国家自然科学基金委(41772001)、现代古生物学和地层学国家重点实验室(中国科学院南京地质古生物研究所) (213120)和湖南省自然科学基金面上项目(2021JJ30816)联合资助, 本文是 IGCP735 项目“地层和奥陶纪生命的崛起”的阶段成果