As Andrew indicated, "spondylus" can mean either a joint or a vertebra.
While the bivalve genus Spondylus does have a very intricate and impressive
ball and socket hinge, and that "joint" may have inspired the name, I think
the alternate meaning, vertebra, is the more likely source.  A vertebra
consists of a central disk-like structure, from which processes protrude at
various angles. In human vertebrae the processes are rather short and blunt,
but in some species, especially fish,  they are quite long and slender, and
I suspect that this similarity inspired the name.