James Youman would shortly replace Urso, and Tom Fowler eventually took over again when his arm healed.
Mike played with Rare Earth from 1971 to 1974, sat in with The Mothers for at least that one date, then a group called HUB from 1975-76, and then Rare Earth again from 1977 to 1983.
A recent message from Steve Trapani, Mike Urso's brother-in-law, gives some extra info:
My understanding of Mike's time with Zappa begins when he moved to LA from Detroit with a band called Head Over Heels in the late 60's, I believe. The band didn't stay together long, and somehow Mike ended up actually living in the back of Zappa's studio sometime maybe in '71 or '72? Anyhow, it was before he joined Rare Earth in 1972. I also seem to remember Mike said they weren't the Mothers yet, or weren't officially calling it the Mothers of Invention during the time he was with Zappa.
Mike said that he did play and tour with Zappa around that time, and I want to say that he was on the road with Zappa when he got the call to join Rare Earth in 1972. At the time Rare Earth was at a high point of popularity and he had the unbelievable choice of remaining with Zappa or joining a very busy rock band in full swing.
The band broke up, like the United Mutations site says, in July of '74 and Mike, Pete Hoorelbeke, and Tom Baird formed HUB, which ended when Baird was killed in a boating accident in '75.
In any case, that is when United Mutations says Mike joined Zappa, but he had actually known, played, and toured with Frank before that.
More recently, Mike Urso has been performing / recording with Last Train South.
Track from the cowboy angel cd by Last Train South; Robby King ,vocals,guitar.Garth Hudson; keyboards.Mike Urso,bass.b. vocals.Paul Clayton,Guitar.Johnny Bredau,drums,b. vocals.Maud Hudson,b.vocals and b. vocal arrangements..buy cd @ http://www.cdbaby.com/cd/lasttrainsouth