Dr Martin Sage, a forensic pathologist who carried out a post-mortem examination on the body parts, said bleeding on the brain after Hayden slipped into a coma after the beating was the most likely cause of death.
He was certain Hayden was dead before Gosnell started dismembering him.
Dr Sage said Hayden suffered a broken nose and a fractured right arm. He could not say what bruising or swelling had occurred, because of the advanced state of decomposition.
But he said Hayden must have been alive for a few hours after the beating stopped, for the "spectacular" swelling seen on his face by earlier witnesses to emerge.
Hayden was reported missing after going to see his friend Nicolette Vaux-Phillips at the Linwood house she shared with her then partner Gosnell.
His remains were found after a four-month missing persons inquiry.
The jury was yesterday shown hours of police interview footage in which Gosnell talked about assaulting the teen.
He snapped after Hayden told him Vaux-Phillips was just "using him" and didn't even like him.
Gosnell kicked and punched the boy as he tried to protect himself.
On several occasions, he made the boy clean himself up, before launching the attack again.
It ended only when Hayden was "breathing funny" and he was dumped on the couch for the night.
In the morning, Gosnell and Vaux-Phillips found him dead.
The Crown closed its case yesterday.
Defence counsel Craig Ruane asked to wait until Monday morning before deciding on whether to call any evidence.
