In another world, there wasn't a first tetrapod dragging itself on the mud. Or, if there was, it faded into obscurity.

Rather, another group of animals took to the land. And they did it by the air.

Somewhere in the mid-Devonian, a placoderm similar to Stensioella acquired a pelagic lifestyle. A relatively weak swimmer, it nonetheless could easily escape predators thanks to its proportionally massive pectoral fins, allowing it to flutter in a manner similar to butterfly fish, hatchetfish and, perhaps more aptly, manta rays. This defense strategy was soon augmented by the evolution of a remarkable organ: a 'lung', derived from the animal's own heart. This two halved organ was connected to the spiracles and latter extended into the animal's wings, allowing the constant passage of air in the animal's body. Furthermore, being part of the heart, all blood running through the animal's veins is oxygenated, with carbon dioxide being expelled either through still functional gills or alongside other bodily waste.

Not only did this allow this placoderm to breathe air, but it also allowed for an endurant metabolism, and so flight developed quickly. Flying placoderms quickly spread unto both marine and pelagic environments, colonising inland freshwater rivers and lakes where even other fish couldn't reach. Several secondarily flightless species arose, both as "placoderm penguins" and as simple pond fish with no predators, but the sophisticated lung still gave them a competitive advantage over other fish.

At some point, however, some placoderms began to forage on land. First they hawked on terrestrial insects, then they began to land. Some became secondarily flightless and lizard-like, others kept refining their wings and became powerful terrestrial stalkers, still retaining powered flight much as terrestrial bats and pterosaurs did in other timelines.

By the Late Devonian, wingspans could reach up to nine meters, casting shadows on the land and on the seas. And this was before the Devonian ended...