Is it possible my elephant nose killed my irridescent shark

What else do you have in the tank? Is it possible that the "bite" is actually an injury caused by a parasite similar to "hole in the head" (which doesn't always manifest on the head of a fish). Could it have been caused by something else in the tank like a sharp decoration? Unfortunately, the Pangasius is FAMOUS for darting into things like the side of the tank hard enough to do lethal damage.

My personal experience with these fish is that 99% of the time they suffer some kind of lethal injury it is self inflicted. They are powerful, fast swimmers and the bigger they get the easier it is for them to injure themselves in a tank. How big is your tank? Was it over 100 gallons as suggested for this species?

I once lost a fish sold under the common name "Crocodile Fish" by not paying attention. They, like the Pangasius were famous for injuring themselves. I was having no issues until I had the incredibly stupid idea of putting my live food in the tank with him, but in a clear breeder cage. The "Crocodile Fish" has a fragile mouth that shoots forward like a spring trap to grab prey. In less than 24hours my fish had struck the clear breeder cage trying to get at the feeder fish. He destroyed his mouth beyond repair and I had to put him down. :o(

Sometimes we have to pay attention to what our fish need more than what fish we want to keep or the *way* we want to keep them. If your tank was the right size, then maybe you need to revisit the decorations and make sure that they are neither too rigid or sharp?