Awesome frogfish video
This is one of the coolest, weirdest things I've seen in a while. I knew that frogfish existed and roughly what they were, but there is a big difference between seeing still photos and watching one of them walking around.
Weird but true: the more widely spread "posterior" limbs are actually the pectoral fins, and the medial "anterior" limbs are the pelvic fins. Lots of teleosts have the pelvic fins moved forward until they are even with or even anterior to the pectoral fins. Suppose this lineage came out onto land (say, after a mass extinction) and gave rise to a parallel group of tetrapods. Those animals would have forelimbs made from their ancestral hindlimbs! (Quick, to the Speculative-Zoology-Mobile!)
The latest stuff on the fish-tetrapod transition suggests that well-formed limbs first evolved in totally aquatic animals and were exapted for terrestrial locomotion later. Although they are very different in habitat and shape, these frogfish show how useful limbs might be underwater, especially for ambush predators.
If you watch the whole video, you'll see the thing walking quadrupedally on all four fins, and also bipedally on just its pectoral fins. It's a freakin' hadrosaur.
Hat tip to Mike.