While the topic does have me worried this could turn into a discussion about butts, I have to say the conclusions from this article seem to have more to do with speculation than they have to do with science. Based on one skeleton of one fish there is claimed evidence of quirks in evolution. The scientists here seem to be doing what proponents of intelligent design are so often accused of. Namely, they are seeing what they want to see. While a quirk in evolution is certainly a possible explanation for these butt fins that sounds like an extremely unlikely explanation.
There are a number of possible explanations. An isolated mutation or deformity, for instance. That is much more likely. In all lab cases where a species has shown some sort of change or deformity like this the result has not been evolution. The result has been all others of the species refusing to mate with it, or if they do mate then things return to normal within a few generations.
Bigger than that, however, is the claim that evolution is somehow experimenting. Exactly how can it be explained that evolution would be capable of doing that? Evolution is supposedly mindless. How could evolution decide to create such a mutation just to see if it would be useful? In order for something to be able to experiment something must exhibit intelligence. A blind watchmaker simply wouldn't be able to conduct such experiments. We, as intelligent creatures, conduct experiments. A natural process simply wouldn't be able to.
My point is that the conclusion that this one skeleton, "suggest that early in primitive vertebrates' history, evolution experimented with a number of wacky body plans, only some of which survived..." is quite a stretch. Just because someone wants evidence for evolution does not mean it is there in this fossil.