Why are quarks fundamental particles?

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Are quarks really considered fundamental particles that cannot be divided further? If an up quark can transmute to a down quark and release a W⁺ boson which decays to a positron and a neutrino (for example) - doesn't this mean that there is substructure to a quark?

What exactly is it that makes a particle fundamental and non divisible?

 

[haael]

Are quarks really considered fundamental particles that cannot be divided further?

In Standard Model, yes.

If an up quark can transmute to a down quark and release a W⁺ boson which decays to a positron and a neutrino (for example) - doesn't this mean that there is substructure to a quark?

No, it does not prove that.

What exactly is it that makes a particle fundamental and non divisible?

A particle is either fundamental or it is a bound state. Bound states have more degrees of freedom. A fundamental particle is completely described by its internal quantum numbers, position, momentum and spin. The internal quantum numbers must be discrete.
In case of a bound state we have additional degrees of freedom related to the position and orientation of the constituents. These degrees of freedom are always continuous.