Yoshi's saddle being homologous to Koopas' shells exapted for another use is clever, and something I suspected for years. It's not unprecedented either, with Spikes and Clubbas obviously being part of the Koopa family tree but whose shells are entirely vestigial.
Makes me wonder if my hypothesis that Bowser+his Clown Car are supposed to be equivalent to Lakitu+his cloud is correct after all...
Also, I wonder if Croco is what Yoshi was original supposed to be.