Revision of the Devonian clam shrimp (Diplostraca: Spinicaudata) from the Canadian Arctic and discussion on the early morphotypes of the leaiins
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Abstract:
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.
