title: A primitive fish from the Cambrian of North America.
abstract: Knowledge of the early evolution of fish largely depends on soft-bodied material from the Lower (Series 2) Cambrian period of South China. Owing to the rarity of some of these forms and a general lack of comparative material from other deposits, interpretations of various features remain controversial, as do their wider relationships amongst post-Cambrian early un-skeletonized jawless vertebrates. Here we redescribe Metaspriggina on the basis of new material from the Burgess Shale and exceptionally preserved material collected near Marble Canyon, British Columbia, and three other Cambrian Burgess Shale-type deposits from Laurentia. This primitive fish displays unambiguous vertebrate features: a notochord, a pair of prominent camera-type eyes, paired nasal sacs, possible cranium and arcualia, W-shaped myomeres, and a post-anal tail. A striking feature is the branchial area with an array of bipartite bars. Apart from the anterior-most bar, which appears to be slightly thicker, each is associated with externally located gills, possibly housed in pouches. Phylogenetic analysis places Metaspriggina as a basal vertebrate, apparently close to the Chengjiang taxa Haikouichthys and Myllokunmingia, demonstrating also that this primitive group of fish was cosmopolitan during Lower-Middle Cambrian times (Series 2-3). However, the arrangement of the branchial region in Metaspriggina has wider implications for reconstructing the morphology of the primitive vertebrate. Each bipartite bar is identified as being respectively equivalent to an epibranchial and ceratobranchial. This configuration suggests that a bipartite arrangement is primitive and reinforces the view that the branchial basket of lampreys is probably derived. Other features of Metaspriggina, including the external position of the gills and possible absence of a gill opposite the more robust anterior-most bar, are characteristic of gnathostomes and so may be primitive within vertebrates.
authors.affiliation: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK.
authors.forename: Simon Conway
authors.initials: SC
authors.lastname: Morris
authors.affiliation: 1] Department of Natural History (Palaeobiology), Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario M5S 2C6, Canada [2] University of Toronto, Department of Ecology and Evolutionary Biology, 25 Willcocks Street, Toronto, Ontario M5S 3B2, Canada.
authors.forename: Jean-Bernard
authors.initials: JB
authors.lastname: Caron
mesh_terms: D000818:Animals; D001955:British Columbia; D005399:Fishes / Q000033:anatomy & histology*; D005580:Fossils; D005880:Gills / Q000033:anatomy & histology; D009144:Museums; D009864:Ontario; D010802:Phylogeny