Professional ski instructor Aaron McLean-Foreman, 24, said the discovery left him in shock and it was some time before he could compose himself enough to go to police and report the grisly find.
A jury at the High Court in Edinburgh has heard that the remains on Corstorphine Hill were later identified as Philomena Dunleavy, 66, from Marino, Dublin.
Her son James Dunleavy, also known as Seamus, is on trial accused of beheading his mother and burying her dismembered body.
Dunleavy denies battering her to death between 30 April and 7 May last year. He also denies attempting to defeat the ends of justice by trying to cover up the alleged murder and destroy evidence.
At the time, Dunleavy, 40, was living in a flat Edinburgh’s Balgreen Road, close to where the body was unearthed.
Giving evidence at the opening of the trial yesterday, Mr McLean-Foreman said he had borrowed his father’s bike to cycle to his home in the East Craigs area of the city.
The Corstorphine Hill nature reserve was slightly out of his way, he said but “I did not mind the detour and enjoying the sunny weather”.
The day was “as good as it gets in Scotland” as he pushed his bike up the hill.
“I decided to have a break in the sunshine and relax and enjoy the fine weather,” said Mr McLean-Foreman.
Because of nettles and other vegetation, he looked for a clear place to sit and noticed a likely clearing.
“Almost immediately, the first thing I noticed was very white teeth,” he told advocate depute Alex Prentice, QC, prosecuting.
“Teeth and a skull.”
Mr McLean-Foreman said there were marks in the earth he thought had been made by a shovel. “It appeared to me, after coming to terms with what I was looking at, that the soil had been altered.”
A photo of the sight which had confronted Mr McLean-Foreman was shown in court, after Mr Prentice warned it might be “disturbing”.
Mr McLean-Foreman continued: “I have seen a fair share of sheep skulls and deer skulls while walking in the hills. I wanted to believe that was what I was looking at – but it was fairly clear that was not what I was looking at.”
There were also flies around the area, the court heard.
Mr McLean-Foreman took a photo so that he could lead police back to the spot and left.
“I believe I went into a state something like shock,” he said.
Later that afternoon, he went to his local police station, wondering if they would believe what he had seen.
The murder charge alleges that Dunleavy, in his Balgreen Road flat, inflicted “blunt force trauma” by means unknown, compressed his mother’s throat, and cut off her head and legs with a blade and something like a saw.
A second charge accuses Dunleavy of pretending that his mother was unwell and had returned to Ireland.
The charge further alleges that Dunleavy put his mother’s torso, severed legs and head into a suitcase and took the remains to Corstorphine Hill, where he buried her.
Prosecutors also claim that Dunleavy vacuumed and washed his flat to remove bloodstains, and torched a bed and mattress.
The trial continues.