Today I just randomly thought of Yoshi, and then suddenly have a feeling that the red thingy on the back of Yoshi looks like a shell. A simple Google search did result in some random links of people arguing whether that is a shell or a saddle.
However, Google search didn't get anything conclusive. Are there any strong evidences or canonical answers (e.g. from Nintendo) which perhaps unluckily didn't show up in Google search?
Here is an image on the Yoshi page on Wikipedia which I think that red thingy on his back really seems to resemble a shell: