The billionaires were among nearly two dozen of America's biggest tech tycoons - with a combined net worth of over $380 billion - who were grilled about the future of artificial intelligence at a behind-closed-doors hearing in front of the US Senate. 

Longtime AI critic Musk was in a playful mood, joking with reporters and speaking off the cuff despite the release of a bombshell biography earlier this week that focused on the Tesla CEO's erratic behavior.

The private meeting was a crash course for legislators on how best to regulate AI: a technical achievement which some of these same industry leaders likened to the 'extinction'-level risk of nuclear weapons in a joint 'open letter' to Congress.

Democrat Senate Majority Leader Chuck Schumer, who spearheaded the effort for today's 'AI Insight Forum,' described the all-day debate on the legal and ethical implications of artificial intelligence as an 'all-hands-on-deck moment for Congress.'

The forum was the first time that feuding industry titans Elon Musk and Meta Platforms CEO Mark Zuckerberg were in a room together since a summer of online gamesmanship on a proposed MMA 'cage match' between the two billionaires. 

Early photos from today's forum showed the two combative executives segregated across the dais of the Senate office building's Kennedy Caucus Room.

Their cage match, which at one point was pitched as a charity bout to be held at the Colosseum in Rome, appears to be postponed indefinitely, an official with the Ultimate Fighting Championship (UFC) said last week.