He was a military leader turned dictator who had such a complex about his receding hairline that he perfected the Roman comb-over and liked laurel crowns that disguised his bald patch.

In flattering posthumous portraits Julius Caesar was often portrayed as a dashing, healthy-haired, divine being. But now a realistic marble bust believed to be the oldest representation taken during his lifetime has been discovered at the bottom of the river Rhône in France.

The life-sized bust, which has thrilled French archaeologists, shows a man in his fifties with the receding hair said to have given him a complex after taunts from his battlefield enemies.

He also has wrinkles and lines that reflect the war-hardened life of the man who conquered Gaul and whose quest for power was largely responsible for turning the Roman republic into a dictatorship that would later become an empire.

The bust was discovered by French archaeologist divers scouring the bottom of the Rhône in the southern town of Arles, which Caesar founded in 46BC, distributing land among his veteran legionnaires. It is believed the bust was sculpted between 49 and 46BC, when Caesar was in his 50s, a few years before he was assassinated by nobles in the senate house.

The French excavation team said this was the only bust of Caesar made during his lifetime, apart from a Turin death mask taken just before or after his death.

Even in Rome, portraits and statues of Caesar were essentially posthumous. The French culture minister, Christine Albanel, triumphantly claimed the bust "the most ancient representation of Caesar known today".

"The important thing is that this is a bust taken from the living man," said Michel L'Hour, head of the French government's team for subaquatic archaeological research. "It's done in the republican tradition of realism from real life - he looks aged, lined and balding. It's extremely rare and most likely unique."

The riddle that remains is why the bust ended up at the bottom of the river in the town dubbed "Little Rome in Gaul".

"Perhaps this bust was thrown into the river after Caesar was assassinated because that was a difficult time to be considered a follower of his," L'Hour said. "Or was it dumped because Caesar was becoming a tyrant who wanted to kill the republic and become emperor? It could have belonged to an important person, or it could have been placed on a public building."

The find was made in October 2007 but was kept quiet until now so the site would not be disturbed.

Other items found in the treasure-trove included a 1.8m marble statue of Neptune dating from the third century. Two smaller bronzes were also found, one a satyr with its hands tied behind its back.