Tom Read, 15, was stunned when he pulled the head bone from the River Stour in Dorset - which was later aged to around 430-600 AD.
Tom, of Poole, found the skull during his first river wade - having received some river waders for his 15th birthday a few days earlier.
Tom's mum, Rachel, phoned the County Coroner and the police then called and picked it up the following day.
The police needed to establish whether it was a bone of antiquity or more modern and whether a crime had been committed.
Chuffed Tom said: "Obviously I did not expect to find something like this on my first river wading trip.
"At first I thought it was an old leather boot, but after a closer look, realised it was a skull.''
Tom and Rachel met with a Dorset Police Crime Scene Investigator, three Bournemouth University BSc Archaeologists and Devon and Cornwall police dive team at White Mill a few days later.
Tom showed them where he found the skull, the divers used their aqua scopes to view the site from 100m upstream and then scuba gear to search the other side of the bridge.
