I suppose it could be a shell. Perhaps it's a shell that's been flattened out because maybe the events after the first Yoshi's Island caused them to evolve to be better suited for carrying passengers. We know that Yoshis can reproduce absurdly fast because they can turn pretty much anything into an egg so, like flies, they could rapidly create enough generations between, say, the time Mario is a baby until the time he's a middle aged adult, to make this happen. This would be further reinforced in the gene pool by the subsequent adventures that Yoshi had with the other baby Mario characters in the sequels to Yoshi's Island leading years down the road to Super Mario World and beyond, leading to a model that is strikingly similar to what animal breeders use for selective breeding.
Oh god... what the hell did I just type...