Without Smallville star Tom Welling, there would be no "Crisis on Infinite Earths" -- according to Arrow's Stephen Amell, that is.On Twitter, Amell reacted to the news that Welling has joined the upcoming five-show crossover between The CW's Arrowverse shows. "There’d be no crossover without Tom Welling. So Tom Welling is in the crossover," he wrote. He also included an Anchorman gif of Ron Burgundy saying, "It's science."RELATED: Arrowverse's Monitor Prepares for Crisis on Infinite Earths in New Photo

In August, longtime Arrow executive producer Marc Guggenheim revealed he was actively seeking a Smallville alum to join "Crisis on Infinite Earths," saying, "It's such a big part of DC TV history. I would love to have at least one Smallville character in the crossover... the whole reason we started talking about the crossover as early as we did this year was to give us time to have all those conversations." Today, he made good on that promise, as an official announcement confirmed Welling is indeed on board.

RELATED: Tom Welling and Smallville Join Arrowverse's Crisis on Infinite Earths

"Crisis on Infinite Earths" already has a star-studded list of guest stars. Batman: The Animated Series star Kevin Conroy, Black Lightning's Cress Williams, Batman 1966 star Burt Ward, Legends of Tomorrow's Jonah Hex, The Flash triple-threat John Wesley Shipp and "Elseworlds" alums Tyler Hoechlin and Elizabeth Tulloch are among those appearing the crossover. Routh has been confirmed to appear as Kingdom Come Superman.

"Crisis on Infinite Earths" begins Sunday, December 8 at 8 pm ET/PT on Supergirl, then continues in Batwoman on Monday, December 8 at 8 pm ET/PT and in The Flash on Tuesday, December 10 at 8 pm ET/PT. After the winter hiatus, the crossover will conclude on Tuesday, January 14 in Arrow at 8 pm ET/PT and in DC's Legends of Tomorrow at 9 pm ET/PT.