WINONA, Minn. — It’s hard to overstate how rare and exciting a dinosaur mummy is. And yes, the fossil called “Medusa” could be a dinosaur mummy — the remains of an Edmontosaurus about 66 million years old that researchers believe contains a significant amount of skin and tendon tissue. It’s so rare that only about a dozen such fossils exist worldwide — and it’s come to Winona. “We don’t know …