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Cambrian fish from British Columbia reveals origin of jaws

Ben Creisler

A new paper of interest:

Simon Conway Morris & Jean-Bernard Caron (2014)
A primitive fish from the Cambrian of North America.
Nature  (advance online publication)

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 Columbia6, 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.

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