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Evolution of Chevrons

I recall reading a conversation about a week ago regarding the homology of 
chevrons to ribs and such.  That got me thinking about the evolution of 
chevrons as a whole.

Exactly where in the vertebrate fossil record are chevrons first noted (okay, 
generally where)?

I ask because I have been reading some work by Cowen, in which he describes the 
anatomy of cephalochordates, a living group of soft-bodied chordates related to 
all vertebrates.  Cowen suggests that cephalocordates may be the closest living 
animal to the vertebrate ancestor.

He describes the notochord being surrounded by packs of body muscle arranged in 
V-shaped chevrons (not bones in this case, but muscle).  Could chevrons (the 
bones) have originally evolved to protect these muscles, much as the vertebrae 
evolved to enclose and protect the notochord??  

In living vertebrates with chevrons, these bones protect a series of nerves and 
blood-vessels running down the spine (if I'm not mistaken).  Do these bones 
also anchor or enclose muscles, such as the ones Cowen described??

Any input appreciated.


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