Maybe you never know, when I raised my tadpoles I found a couple had weird abberations like that, the one had an oversized lower jaw that gave him an under bite. One of his siblings grew wrong and developed front legs then back legs but the back ones didn't grow at all they were just stumps He passed away...
Yeah I think it's chemicals for these guys, the pond where I got them is inbetween four railroad tracks. The stuff from the trains prolly gets in the water.