d previously: broad, alluvial valleys and flood plains that were transformed in the wake of the origin of trees. The recognition that this environment spurred tetrapod evolution has led directly to the discovery of Gogonasus and, perhaps the most famous transitional species ever, Tiktaalik.
In this environment, many of the features of the transitional species were preadaptive. Lobed fins aided maneuvering in a complex environment in the same way that limbs later did, in water or out. Muscle attachment sites in the bones of the fins worked equally well when used in legs. For paleontologists, the discoveries that filled the gaps revealed that this major transition occurred through a series of forms that were mosaic, with features added to the tetrapod repertoire in an order that's essentially random. Tiktaalik has a broad, fish-like snout, but the far end of its skull has been reordered to allow the first flexible neck seen in a tetrapod.
Two messages were clear from Daeschler's summary. The first is that there's so much left to discover; we don't know which gap will wind up filled next, just that gaps continue to be filled with rich information. He'll continue chasing road crews as they dig through Pennsylvania for as long as they'll let him. The second message is that it's time to let go of the false distinctions that are left over from Linnean times and only serve to confuse the public. In Daeschler's view, all of these animals, on both sides of the former gaps, are tetrapods. Some have limbs, some have fins, but they're clearly all part of a boundary- and gap-less transition.