I am sorry for giving you a answer that you already knew.The way you asked the question it just sounded like you didn't know.I guess I should have known you had "lots of them".You said you found out it is hard to tell the T.natans and the compressicauda apart.I will tell you what,I have caught both of them in the Amazon and they are real easy to tell them apart,even at night when they were caught.
Arapaimaman