[tachyonbeam]

It doesn't have to be all about the evolutionary fitness of regenerating limbs. I think it might just be that losing a limb (and having the opportunity/time to regrow it) may have been a fairly unlikely event. Possibly, those genes mutated and were broken, right around the same time that some other genetic advantage evolved that caused a huge gain in evolutionary fitness.

The ability to regenerate limbs could have been lost in mammals simply because some old pre-mammal who happened to have broken regeneration genes was smarter and much better at finding food than the rest, and went on to pass his/her genes to all of us.

Furthermore, the evolutionary advantage of regenerating limbs may not be much in most animals. It's something we really would like to have, But imagine that some ancient primate species was able to regenerate limbs. Under what scenario do primate lose limbs? When some tiger chews them off? If that was ever to happen to a primate, it was basically fucked. Even if the tiger didn't eat it, it didn't fare very well lacking an arm for several months. The primate that is better at spotting tigers and avoiding them has much higher evolutionary fitness.