I worked with bird dogs a lot so I'm not real sure what to do here. Your dog is hunting. It found the dead possum and some fox fur. What your dog doesn't understand is what you want.
I got a 6 yr old Shorthair in once that had actually been in a kennel it's entire life. Nice dog but didn't have a clue what was going on. It would not chase fly away birds nor even birds walking around on the ground. Finally in desperation I took him into the quail pen with me and just stood there. The dog just sat there next to me. Boy did I feel dumb! But then after awhile, the dog stuck out his muzzle and looked at a bird walk by. A bit later, he got up and walked over to on, sniffed it and walked back to sit by me. Then one walked by to close and he tried to grad it but it ran away. After awhile he finally went and caught one, then spit it out! Damn, that dog just didn't get it. After a good hour of standing around, he caught one and killed it. Then the light went on and he wanted to catch them all.
Back to the field and he had a new reason to live. Turned into a goog hunting dog.
Point is, your dog is hunting, just has no purpose. Try getting some rabbit's and turn them loose in an enclosure they can't escape from. Take your dog in and watch. Do not encourage him unless he actually get's a rabbit, them fuss over him. Then if he get's to wanting all of them, turn one loose in the yard where he can see it and turn him loose to get it.
Next time, plant more and slowly move them into light then heavy cover. If he's going to hunt for you, this may well show him what you want.
Remember you've got a herding dog that has been bred for years to herd animal's, not necessarily hunt them as we precieve it. Send an Austrailian Sheppard after the sheep and it will go find them (hunt them) and bring them back unscathed.
Don't give up just yet and don't become frustrated.