Mild spoilers ahead. What is read may never die. 

As we await the aftermath of Tyrion’s controversial mission on last week’s episode of Game of Thrones, it seems only fitting to highlight the show’s scaly beasts. We watched in awe as The Imp entered Rhaegal and Viserion’s holding cell, braving the odds to free the pair of dragons from their shackles. It was a valiant display of BAMF-ery no doubt, but something interesting caught my eye during the scene: Dany’s dragons walk on their wings, indicating that they aren’t dragons at all, but wyverns.

To understand the difference, we have to take a dive into mythozoology, which much to my excitement, is a real thing. In medieval mythology and heraldry – the practice of devising coats of arms – wyverns are described as serpent-like creatures with wings, often having spiked tails. The word “wyvern” is derived from the French “wyvere,” which means both “viper” and “life.” As a symbol, the animal represents war, and is a sign of strength to those who bear it.

It all sounds pretty dragon-esque, but here is where things start to diverge: unlike their pop-culture-pervasive kin, wyverns only have two legs. The forelimbs are absent, replaced instead by bat-like wings with claws that can be used to make contact with the ground. Exhibit A:

European dragons, on the other hand, traditionally have four limbs and wings. Think Toothless:

Or Sean Connery:

By morphology alone, Drogon, Rhaegal, and Viserion are more like wyverns than anything else, but modern fantasy has been breaking the old rules for many years. Tolkien’s Smaug is a great example. Smaug is clearly described as being a “dragon,” but his on-screen self is depicted with wyvern-like characteristics. George R.R. Martin was certainly aware of the difference between the two when he conjured up Westeros’ fauna.

“According to the rules of heraldry, dragons have four legs and wyverns two, yes,” he wrote on his blog. “But have you ever seen a heraldic ‘seahorse?’ Heralds didn’t know crap about biology. Now, there are no actual dragons, to be sure. But there are bats, and there are birds, and once upon a time there were pterodactyls. Those are the models to use when designing a dragon. No beast in nature has four legs and wings.”

He’s right. And there’s a reason for that: wings evolved from walking limbs. We know that birds, for example, evolved from small two-legged, meat-eating dinosaurs with fluffy feathers. The fuzz on their arms eventually gave way to flight feathers, and their arms became wings. It’s thought that bats, on the other hand, evolved from small gliding mammals of the order Insectivora. In both cases, winged structures arose from existing limbs. To make four-legged flight possible, an animal would have to possess three muscle-anchoring bone girdles, and that body plan simply doesn’t exist. Your body has two: the collarbone-breastbone combo, and the hips, or pelvic girdle. That basic blueprint is shared with all the vertebrates on Earth.

Some have pointed out that there were four-winged dinosaurs, and while this is true, those animals did not possess an additional set of limbs. To our knowledge, a quadrupedal flying animal has never walked the planet.

“Besides,” adds Martin, “the best dragon ever shown on film, Vermithrax Perjorative, has two legs and two wings. My dragons have two legs.”

Fair enough. And like Vermithrax Pejorative, the GOT dragons also breathe fire, something that – in most accounts of medieval lore – wyverns do not do. Plus, “the mother of wyverns,” doesn’t quite roll off the tongue.

IMAGES: HBO,  DreamWorks Animation, Universal Pictures