Hi Andrew,
It certainly could be a "early defect" but this is usually pretty rare compared to later accidents (especially in wild caught animals - CB Axolotls can more easily survive with handicaps). Since Pachys can be especially agressive, I guess it's pretty likely that the leg was torn off.
Since only a single leg is affected I'm pretty sure that it's not due to a genetic defect. So if the development of a leg can be compromised by some factor in larval caudates, I see no reason why it should be unpossible to mess up limb regeneration, too. It sems to be rare - that's why I'm asking for more observations. What size was your axolotl when you first saw the defective leg?
Best wishes,
kai
