What's so ancient about propping up fish, anyway? Pectoral props seem to work pretty well for mudskippers, walking catfish, and batfish, although their limbs are far removed from the legs of tetrapods.

Methinks the article makes quite an unsubstantiated leap from "fish fingers" to a tetrapodal "proto-humerus", or is it the other way around?

As I understand it, tetrapods (four-legged veterbrates) evolved from fish about 360 million years ago:

The early tetrapods were the first vertebrates to truly walk the land. Before tetrapods existed, vertebrates were all confined to living in aquatic habitats.

The Independent's article explains that the new bit of tetrapod bone (is it the only example?) is more like a crutch than a leg:

...the bone - a prototype humerus - can help to explain how ancient fish with primitive lungs grew rudimentary limbs from their fins which allowed them to make the first tentative steps towards a terrestrial life.

...the first tetrapod limbs were used to prop up the heads of air-breathing fish and only afterwards did the fish begin to use these legs to clamber out of water. The fossilised bone is thick and flat and would not have allowed much movement between the limb and the shoulder, indicating that it was of little use other than to prop up the creature's front half.

"When this humerus is compared to those of closely related fish, it becomes clear that the ability to prop the body is more ancient than we previously thought. This bone is a lot more robust than a humerus from any of the ancient species. Relative to other tetrapods, this is almost over-engineered. There's a massive space for the attachment of substantial muscle going to the chest."

"This means that many of the features we thought evolved to enable life on land originally evolved in fish living in aquatic ecosystems."

While being "robust" might limit a limb's relative range of motion, does it reduce the animal's ability to survive within its habitat? Crocodiles and elephants come to mind.

And I am somewhat confused by that last paragraph: are there fish that don't live in aquatic ecosystems of some kind?

The article then jumps to the conclusion that:

...the fossilised bone found in Pennsylvania helped the forelimb fulfil an intermediate function between the braking and steering of a fish's fin and the walking movements of an early amphibian.

Thus we have a rather counterclockwise circular argument that evidence of a bigger-boned early tetrapod (which evolved from aquatic vertebrates, i.e., fish) demonstrates that fish developed walking limbs.

Is it not possible that we're looking at a more highly-developed tetrapod, one even further removed from fish? Maybe he was the biggest TP on the block (at the time), a better digger for food or shelter, or propelled himself mostly with two of his four legs.

Another point. The term "tetrapod" encompasses a wide range of four-legged veterbrates, including those living in aquatic environments:

Tetrapods include all land-living vertebrates, but also include a number of animals that have returned to life in the water, such as sea turtles, sea snakes, whales and dolphins, seals and sea lions, and extinct groups such as plesiosaurs, ichthyosaurs, and mosasaurs. Some tetrapods, like whales and snakes, have lost some or all of the four limbs that their ancestors had, but because of their ancestry they are still grouped as tetrapods.

Aquatic tetrapods possess fishlike characteristics -- fins, flippers, tails, streamlining, etc. -- conducive to living in water. Would anyone say that their particular musculoskeletal adaptations indicate that fish developed rudimentary limbs?