Jeff from Possessed also comes to mind first. Possessed was probably my introduction to death metal back in the mid-80's. Check out Seven Churches and Beyond the Gates to see death metal in its early stages.
The possessed demo was 1984. There was a track from the demo ("Swing Of The Axe") on a Metal Massacre compilation (number 5 or 6..I'll check my albums).
good question but i recall it was from a somehow reliable source...still i believe mantas was earlier than almost anything thats why a lot of people venture to say chuck created death metal...i dont i was 4 years old at that time